computer system organization and architecture

CS6303-COMPUTER ARCHITECTURE M.Ramu/AP/IT
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DHANALAKSHMI SRINIVASAN ENGINEERING COLLEGE,
PERAMBALUR
Two Marks Questions with Answers
CS6303 COMPUTER ARCHITECTURE
II YEAR/ 3
rd
SEMESTER ECE
UNIT-I OVERVIEW & INSTRUCTIONS
1. What are the eight great ideas in computer architecture? (MAY-2016)
The eight great ideas in computer architecture are:
1. Design for Moore’s Law
2. Use Abstraction to Simplify Design
3. Make the Common Case Fast
4. Performance via Parallelism
5. Performance via Pipelining
6. Performance via Prediction
7. Hierarchy of Memories
8. Dependability via Redundancy
2. What are the five classic components of a computer? (NOV/DEC-2016)
The five classic components of a computer are input, output, memory, datapath,
and control, with the last two sometimes combined and called the processor.
3. Define – ISA
The instruction set architecture, or simply architecture of a computer is the
interface between the hardware and the lowest-level software. It includes anything
programmers need to know to make a binary machine language program work correctly,
including instructions, I/O devices, and so on.
4. Define – ABI
Typically, the operating system will encapsulate the details of doing I/O,
allocating memory, and other low-level system functions so that application programmers
do not need to worry about such details. The combination of the basic instruction set and
the operating system interface provided for application programmers is called the
application binary interface (ABI).
5. What are the advantages of network computers?
Networked computers have several major advantages:
■ Communication: Information is exchanged between computers at high speeds.
■Resource sharing: Rather than each computer having its own I/O devices,
computers on the network can share I/O devices.

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■ Nonlocal access: By connecting computers over long distances, users need
not be near the computer they are using.
6. Define – Response Time (APR/MAY-2016)
Response time is also called execution time. The total time required for the
computer to complete a task, including disk accesses, memory accesses, I/O activities,
operating system overhead, CPU execution time, and so on is called response time.
7. Define – Throughput (NOV/DEC-2015)
Throughput or bandwidth is the total amount of work done in a given time.
8. Write the CPU performance equation. (MAY-2016)
The Classic CPU Performance Equation in terms of instruction count (the
number of instructions executed by the program), CPI, and clock cycle time:
9. If computer A runs a program in 10 seconds, and computer B runs the same
program in 15 seconds, how much faster is A over B.
10. What are the basic components of performance? (Nov/Dec-2015)
The basic components of performance and how each is measured are:
Components of Performance Units of measure
CPU execution time for a program Seconds for the program
Instruction count Instruction executed for the program
Clock cycles per instruction(CPI) Average number of clock cycles per instruction
Clock cycle time Seconds per clock cycle

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11. Write the formula for CPU execution time for a program. (Nov/Dec-2016)
12. Write the formula for CPU clock cycles required for a program. (Apr/May-2014)
13. Define – MIPS (Apr/Nov-2015)
Million Instructions Per Second (MIPS) is a measurement of program execution
speed based on the number of millions of instructions.
MIPS is computed as:
14. What are the fields in an MIPS instruction?
MIPS fields are
op rs rt rd shamt funct
6 bits 5bits 5 bits 5 bits 5 bits 6 bits
Where,
op: Basic operation of the instruction, traditionally called the opcode.
rs: The first register source operand.
rt: The second register source operand.
rd: The register destination operand. It gets the result of the
operation. shamt: Shift amount.
funct: Function.
15. Write an example for immediate operand. (Nov/Dec-2014)
The quick add instruction with one constant operand is called add immediate or addi.
To add 4 to register $s3, we just write
addi $s3,$s3,4 # $s3 = $s3 + 4

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16. Define – Stored Program Concepts
Today’s computers are built on two key principles:
1. Instructions are represented as numbers.
2. Programs are stored in memory to be read or written, just like data.
These principles lead to the stored-program concept. Treating instructions in the same
way as data greatly simplifies both the memory hardware and the software of computer
systems.
17. Define – Addressing Modes. (Nov/Dec-2014)
The different ways in which the operands of an instruction are specified are called
as addressing modes.
The MIPS addressing modes are the following:
1. Immediate addressing
2. Register addressing
3. Base or displacement addressing
4. PC-relative addressing
5. Pseudo direct addressing

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UNIT-II ARITHMETIC OPERATIONS
1. Add 610 to 710 in binary and Subtract 610 from 710 in binary. (Nov/Dec-2016)
Addition,
Subtraction directly,
Or via two’s complement of -6,
2. Write the overflow conditions for addition and subtraction. (Nov/Dec-2014)
Operation Operand A Operand B Result Indicating overflow
A+B ≥0 ≥0 <0
A+B <0 <0 ≥0
A-B ≥0 <0 <0
A-B <0 ≥0 ≥0
3. Define – Moore’s Law. (Nov/Dec-2015)
Moore’s Law has provided so much more in resources that hardware designers
can now build much faster multiplication and division hardware. Whether the
multiplicand is to be added or not is known at the beginning of the multiplication by
looking at each of the 32 multiplier bits.

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4. What are the floating point instructions in MIPS? (Nov/Dec-2014)
MIPS supports the IEEE 754 single precision and double precision formats with
these instructions:
■ Floating-point addition
■ Floating-point subtraction
■ Floating-point multiplication
■ Floating-point division
■ Floating-point comparison
■ Floating-point branch
5. Define – Guard and Round
Guard is the first of two extra bits kept on the right during intermediate calculations
of floating point numbers. It is used to improve rounding accuracy.
Round is a method to make the intermediate floating-point result fit the floating-point
format; the goal is typically to find the nearest number that can be represented in the
format. IEEE 754, therefore, always keeps two extra bits on the right during intermediate
additions, called guard and round, respectively.
6. Define – ULP
Units in the Last Place is defined as the number of bits in error in the least significant
bits of the significant between the actual number and the number that can be represented.
7. What is meant by sticky bit?
Sticky bit is a bit used in rounding in addition to guard and round that is set whenever
there are nonzero bits to the right of the round bit. This sticky bit allows the computer to
see the difference between 0.50 … 00 ten and .... 01 ten when rounding.
8. Write the IEEE 754 floating point format.
The IEEE 754 standard floating point representation is almost
always an approximation of the real number.
9. What is meant by sub-word parallelism? (Nov/Dec-2016)
Given that the parallelism occurs within a wide word, the extensions are classified
as sub-word parallelism. It is also classified under the more general name of data level
parallelism. They have been also called vector or SIMD, for single instruction, multiple
data . The rising popularity of multimedia applications led to arithmetic instructions that
support narrower operations that can easily operate in parallel.

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For example, ARM added more than 100 instructions in the NEON multimedia
instruction extension to support sub-word parallelism, which can be used either with
ARMv7 or ARMv8.
10. Multiply 100010 * 100110.
11. Divide 1,001,010ten by 1000ten.
12. What are the steps in the floating-point addition? (Nov/Dec-2016)
The steps in the floating-point addition are
1. Align the decimal point of the number that has the smaller exponent.
2. Addition of the significands
3. Normalize the sum.
4. Round the result.

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UNIT-III PROCESSOR AND CONTROL UNIT
1. What is meant by data path element?
A data path element is a unit used to operate on or hold data within a processor. In
the MIPS implementation, the data path elements include the instruction and data
memories, the register file, the ALU, and adders.
2. What is the use of PC register?
Program Counter (PC) is the register containing the address of the instruction in
the program being executed.
3. What is meant by register file?
The processor’s 32 general-purpose registers are stored in a structure called a
register file. A register file is a collection of registers in which any register can be read or
written by specifying the number of the register in the file. The register file contains the
register state of the computer.
4. What are the two state elements needed to store and access an instruction?
Two state elements are needed to store and access instructions, and an adder is needed to
compute the next instruction address. The state elements are the instruction memory and
the program counter.

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5. Draw the diagram of portion of datapath used for fetching instruction.
A portion of the data path is used for fetching instructions and incrementing the
program counter. The fetched instruction is used by other parts of the data path.
6. Define – Sign Extend
Sign-extend is used to increase the size of a data item by replicating the high-
order sign bit of the original data item in the high order bits of the larger, destination data
item.
7. What is meant by branch target address?
Branch target address is the address specified in a branch, which becomes the new
program counter (PC) if the branch is taken. In the MIPS architecture the branch target is
given by the sum of the off set field of the instruction and the address of the instruction
following the branch.
8. Differentiate branch taken from branch not taken.
Branch taken is a branch where the branch condition is satisfied and the program
counter (PC) becomes the branch target. All unconditional jumps are taken branches.
Branch not taken or (untaken branch) is a branch where the branch condition is
false and the program counter (PC) becomes the address of the instruction that
sequentially follows the branch.
9. What is meant by delayed branch?
Delayed branch is a type of branch where the instruction immediately following
the branch is always executed, independent of whether the branch condition is true or
false.

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10. What are the three instruction classes and their instruction formats?
The three instruction classes (R-type, load and store, and branch) use two
different instruction formats.
11. Write the instruction format for the jump instruction.
The destination address for a jump instruction is formed by concatenating the
upper 4 bits of the current PC + 4 to the 26-bit address field in the jump instruction and
adding 00 as the 2 low-order bits.
12. What is meant by pipelining?
Pipelining is an implementation technique in which multiple instructions are
overlapped in execution. Pipelining improves performance by increasing instruction
throughput, as opposed to decreasing the execution time of an individual instruction.
13. What are the five steps in MIPS instruction execution?
1. Fetch instruction from memory.
2. Read registers while decoding the instruction. The regular format of
MIPS instructions allows reading and decoding to occur simultaneously.
3. Execute the operation or calculate an address.
4. Access an operand in data memory.
5. Write the result into a register.
14. Write the formula for calculating time between instructions in a
pipelined processor.

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15. What are hazards? Write its types.
There are situations in pipelining when the next instruction cannot be executed in
the following clock cycle. These events are called hazards, and there are three different
types.
1. Structural Hazards
2. Data Hazards
3. Control Hazards
16. What is meant by forwarding?
Forwarding, also called bypassing, is a method of resolving a data hazard by
retrieving the missing data element from internal buffers rather than waiting for it to
arrive from programmer visible registers or memory.
17. What is pipeline stall?
Pipeline stall, also called bubble, is a stall initiated in order to resolve a hazard.
They can be seen elsewhere in the pipeline.
18. What is meant by branch prediction?
Branch prediction is a method of resolving a branch hazard that assumes a given
outcome for the branch and proceeds from that assumption rather than waiting to
ascertain the actual outcome.
19. What are the 5 pipeline stages?
The 5 stages of instruction execution in a pipelined processor are:
1. IF: Instruction fetch
2. ID: Instruction decode and register file read
3. EX: Execution or address calculation
4. MEM: Data memory access
5. WB: Write back
20. What are exceptions and interrupts?
Exception, also called interrupt, is an unscheduled event that disrupts program
execution used to detect overflow. Eg. Arithmetic overflow, using an undefined
instruction.
Interrupt is an exception that comes from outside of the
processor. Eg. I/O device request
21. Define – Vectored Interrupts
Vectored interrupt is an interrupt in that the address to which the control is
transferred is determined by the cause of the exception.

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1. What is meant by ILP? (Nov/Dec-2016)
Pipelining exploits the potential parallelism among instructions. This parallelism
is called instruction-level parallelism (ILP). There are two primary methods for
increasing the potential amount of instruction-level parallelism.
1. Increasing the depth of the pipeline to overlap more instructions.
2. Multiple issue.
2. What is multiple issue? Write any two approaches.
Multiple issue is a scheme whereby multiple instructions are launched in one
clock cycle. It is a method for increasing the potential amount of instruction-level
parallelism. It is done by replicating the internal components of the computer so that it
can launch multiple instructions in every pipeline stage. The two approaches are:
1. Static multiple issue (at compile time)
2. Dynamic multiple issue (at run time)
3. What is meant by speculation? (Apr/May-2014)
One of the most important methods for finding and exploiting more ILP is
speculation. It is an approach whereby the compiler or processor guesses the outcome of
an instruction to remove it as dependence in executing other instructions.
For example, we might speculate on the outcome of a branch, so that instructions
after the branch could be executed earlier.
4. Define – Static Multiple Issue
Static multiple issue is an approach to implement a multiple-issue processor
where many decisions are made by the compiler before execution.
5. Define – Issue Slots and Issue Packet
Issue slots are the positions from which instructions could be issued in a given
clock cycle. By analogy, these correspond to positions at the starting blocks for a sprint.
Issue packet is the set of instructions that issues together in one clock cycle; the
packet may be determined statically by the compiler or dynamically by the processor.
6. Define – VLIW
Very Long Instruction Word (VLIW) is a style of instruction set architecture that
launches many operations that are defined to be independent in a single wide instruction,
typically with many separate opcode fields.
UNIT-IV PARALLELISM

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7. Define – Superscalar Processor. (Nov/Dec-2016)
Superscalar is an advanced pipelining technique that enables the processor to execute
more than one instruction per clock cycle by selecting them during execution. Dynamic
multiple-issue processors are also known as superscalar processors, or simply
superscalars.
8. What is meant by loop unrolling?
An important compiler technique to get more performance from loops is loop
unrolling, where multiple copies of the loop body are made. After unrolling, there is more
ILP available by overlapping instructions from different iterations.
9. What is meant by anti-dependence? How is it removed?
Anti-dependence is an ordering forced by the reuse of a name, typically a register,
rather than by a true dependence that carries a value between two instructions. It is also
called as name dependence.
Register renaming is the technique used to remove anti-dependence in which the
registers are renamed by the compiler or hardware.
10. What is the use of reservation station and reorder buffer?
Reservation station is a buffer within a functional unit that holds the operands and
the operation.
Reorder buffer is the buffer that holds results in a dynamically scheduled
processor until it is safe to store the results to memory or a register.
11. Differentiate in-order execution from out-of-order execution.
Out-of-order execution is a situation in pipelined execution when an instruction is
blocked from executing does not cause the following instructions to wait. It preserves the
data flow order of the program.
In-order execution requires the instruction fetch and decode unit to issue
instructions in order, which allows dependences to be tracked, and requires the commit
unit to write results to registers and memory in program fetch order. This conservative
mode is called in-order commit.
12. What is meant by hardware multithreading?
Hardware multithreading allows multiple threads to share the functional units of a
single processor in an overlapping fashion to try to utilize the hardware resources
efficiently. To permit this sharing, the processor must duplicate the independent state of
each thread. It Increases the utilization of a processor.
13. What are the two main approaches to hardware multithreading?
There are two main approaches to hardware multithreading. Fine-grained
multithreading switches between threads on each instruction, resulting in interleaved
execution of multiple threads. This interleaving is often done in a round-robin fashion,
skipping any threads that are stalled at that clock cycle.
Coarse-grained multithreading is an alternative to fine-grained multithreading. It
switches threads only on costly stalls, such as last-level cache misses.

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14. What is SMT?
Simultaneous Multithreading (SMT) is a variation on hardware multithreading
that uses the resources of a multiple-issue, dynamically scheduled pipelined processor to
exploit thread-level parallelism. It also exploits instruction level parallelism.
15. Differentiate SMT from hardware multithreading.
Since SMT relies on the existing dynamic mechanisms, it does not switch
resources every cycle. Instead, SMT is always executing instructions from multiple
threads, leaving it up to the hardware to associate instruction slots and renamed registers
with their proper threads.
16. What are the three multithreading options? (Nov/Dec-2016)
The three multithreading options are:
1. A superscalar with coarse-grained multithreading
2. A superscalar with fine-grained multithreading
3. A superscalar with simultaneous multithreading
17. Define – SMP. (Nov/Dec-2016)
Shared memory multiprocessor (SMP) is one that offers the programmer a single
physical address space across all processors - which is nearly always the case for
multicore chips. Processors communicate through shared variables in memory, with all
processors capable of accessing any memory location via loads and stores.
18. Differentiate UMA from NUMA.(nov/2015)
Uniform memory access (UMA) is a multiprocessor in which latency to any word
in main memory is about the same no matter which processor requests the access.
Non uniform memory access (NUMA) is a type of single address space
multiprocessor in which some memory accesses are much faster than others depending on
which processor asks for which word.

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UNIT-V MEMORY AND I/O SYSTEMS
1. What are the temporal and spatial localities of references? (Nov/Dec-2016)
Temporal locality (locality in time): if an item is referenced, it will tend to be
referenced again soon.
Spatial locality (locality in space): if an item is referenced, items whose
addresses are close by will tend to be referenced soon.
2. Write the structure of memory hierarchy.
3. What are the various memory technologies?
The various memory technologies are:
1. SRAM semiconductor memory
2. DRAM semiconductor memory
3. Flash semiconductor memory
4. Magnetic disk
4. Differentiate SRAM from DRAM. (Nov/Dec-2016)
SRAMs are simply integrated circuits that are memory arrays with a single access
port that can provide either a read or a write. SRAMs have a fixed access time to any
datum. SRAMs don’t need to refresh and so the access time is very close to the cycle
time. SRAMs typically use six to eight transistors per bit to prevent the information from
being disturbed when read. SRAM needs only minimal power to retain the charge in
standby mode.
In a dynamic RAM (DRAM), the value kept in a cell is stored as a charge in a

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capacitor. A single transistor is then used to access this stored charge, either to read the
value or to overwrite the charge stored there. Because DRAMs use only a single
transistor per bit of storage, they are much denser and cheaper per bit than SRAM. As
DRAMs store the charge on a capacitor, it cannot be kept indefinitely and must
periodically be refreshed.
5. What is flash memory?
Flash memory is a type of electrically erasable programmable read-only memory
(EEPROM). Unlike disks and DRAM, EEPROM technologies can wear out flash
memory bits. To cope with such limits, most flash products include a controller to spread
the writes by remapping blocks that have been written many times to less trodden blocks.
This technique is called wear levelling.
6. Define − Rotational Latency
Rotational latency, also called rotational delay, is the time required for the desired
sector of a disk to rotate under the read/write head, usually assumed to be half the
rotation time.
7. What is direct-mapped cache? (Nov/Dec-2016)
Direct-mapped cache is a cache structure in which each memory location is
mapped to exactly one location in the cache. For example, almost all direct-mapped
caches use this mapping to find a block,
(Block address) modulo (Number of blocks in the cache)
8. Consider a cache with 64 blocks and a block size of 16 bytes. To what block number
does byte address 1200 map?
The block is given by,

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9. How many total bits are required for a direct-mapped cache with 16 KiB
of data and 4-word blocks, assuming a 32-bit address?
10. What are the writing strategies in cache memory?
Write-through is a scheme in which writes always update both the cache and the
next lower level of the memory hierarchy, ensuring that data is always consistent between
the two.
Write-back is a scheme that handles writes by updating values only to the block in
the cache, then writing the modified block to the lower level of the hierarchy when the
block is replaced.
11. What are the steps to be taken in an instruction cache miss?
The steps to be taken on an instruction cache miss are
1. Send the original PC value (current PC – 4) to the memory.
2.Instruct main memory to perform a read and wait for the memory to
complete its access.
3. Write the cache entry, putting the data from memory in the data portion of

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the entry, writing the upper bits of the address (from the ALU) into the tag
field, and turning the valid bit on.
4. Restart the instruction execution at the first step, which will refetch the
instruction, this time finding it in the cache.
12. Define – AMAT
Average memory access time is the average time to access memory considering
both hits and misses and the frequency of different accesses.It is equal to the following:
13. What are the various block placement schemes in cache memory?
Direct-mapped cache is a cache structure in which each memory location is
mapped to exactly one location in the cache.
Fully associative cache is a cache structure in which a block can be placed in any
location in the cache.
Set-associative cache is a cache that has a fixed number of locations (at least two)
where each block can be placed.
14. Define – MTTF and AFR
Reliability is a measure of the continuous service accomplishment or,
equivalently, of the time to failure from a reference point. Hence, mean time to failure
(MTTF) is a reliability measure. A related term is annual failure rate (AFR), which is just
the percentage of devices that would be expected to fail in a year for a given MTTF.
15. Define – Availability
Availability is then a measure of service accomplishment with respect to the
alternation between the two states of accomplishment and interruption. Availability is
statistically quantified as
16. What are the three ways to improve MTTF?
The three ways to improve MTTF are:
1. Fault avoidance: Preventing fault occurrence by construction.
2.Fault tolerance: Using redundancy to allow the service to comply with the
service specification despite faults occurring.
3. Fault forecasting: Predicting the presence and creation of faults, allowing
the component to be replaced before it fails.
17. Define – TLB
Translation-Lookaside Buffer (TLB) is a cache that keeps track of recently used
address mappings to try to avoid an access to the page table.

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18. What is meant by virtual memory? (Nov/Dec-2016) (Nov/Dec-2014)
Virtual memory is a technique that uses main memory as a “cache” for secondary
storage. Two major motivations for virtual memory: to allow efficient and safe sharing of
memory among multiple programs, and to remove the programming burdens of a small,
limited amount of main memory.
19. Differentiate physical address from logical address.
Physical address is an address in main memory.
Logical address (or) virtual address is the CPU generated addresses that
corresponds to a location in virtual space and is translated by address mapping to a
physical address when memory is accessed.
20. Define – Page Fault
Page fault is an event that occurs when an accessed page is not present in main
memory.
21. What is meant by address mapping? (Nov/Dec-2016)
Address translation also called address mapping is the process by which a virtual
address is mapped to an address used to access memory.

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PART B Questions with Answers
CS6303 COMPUTER ARCHITECTURE
II YEAR/ 3
rd
SEMESTER ECE
UNIT-I OVERVIEW & INSTRUCTIONS
1. Discuss about various component of a computer system.(Nov/2014/2015/2016/2017)
Components of computer system
1.Input Unit
2.Output Unit
3.Storage Unit
4.Central Processing Unit (CPU)
5.Arithmetic and Logic Unit (ALU)
6.Control Unit
The internal architectural design of computers differs from one system model to another.
However, the basic organization remains the same for all computer systems. The following
five units (also called "The functional units") correspond to the five basic operations
performed by all computer systems.
Input Unit
Data and instructions must enter the computer system before any computation can be
performed on the supplied data. The input unit that links the external environment with the
computer system performs this task. Data and instructions enter input units in forms that
depend upon the particular device used. For example, data is entered from a keyboard in a
manner similar to typing, and this differs from the way in which data is entered through a

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mouse, which is another type of input device. However, regardless of the form in which they
receive their inputs, all input devices must provide a computer with data that are transformed
into the binary codes that the primary memory of the computer is designed to accept. This
transformation is accomplished by units that called input interfaces. Input interfaces are
designed to match the unique physical or electrical characteristics of input devices to the
requirements of the computer system.
In short, an input unit performs the following functions.
1. It accepts (or reads) the list of instructions and data from the outside world.
2. It converts these instructions and data in computer acceptable format.
3. It supplies the converted instructions and data to the computer system for further
processing.
Output Unit
The job of an output unit is just the reverse of that of an input unit. It supplied information
and results of computation to the outside world. Thus it links the computer with the external
environment. As computers work with binary code, the results produced are also in the
binary form. Hence, before supplying the results to the outside world, it must be converted to
human acceptable (readable) form. This task is accomplished by units called output
interfaces.
In short, the following functions are performed by an output unit.
1. It accepts the results produced by the computer which are in coded form and hence
cannot be easily understood by us.
2. It converts these coded results to human acceptable (readable) form.
3. It supplied the converted results to the outside world.
Storage Unit
The data and instructions that are entered into the computer system through input units have
to be stored inside the computer before the actual processing starts. Similarly, the results
produced by the computer after processing must also be kept somewhere inside the computer
system before being passed on to the output units. Moreover, the intermediate results
produced by the computer must also be preserved for ongoing processing. The Storage Unit
or the primary / main storage of a computer system is designed to do all these things. It
provides space for storing data and instructions, space for intermediate results and also space
for the final results.
In short, the specific functions of the storage unit are to store:
1. All the data to be processed and the instruction required for processing (received from
input devices).
2. Intermediate results of processing.

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3. Final results of processing before these results are released to an output device.
Central Processing Unit (CPU)
The main unit inside the computer is the CPU. This unit is responsible for all events inside
the computer. It controls all internal and external devices, performs "Arithmetic and Logical
operations". The operations a Microprocessor performs are called "instruction set" of this
processor. The instruction set is “hard wired” in the CPU and determines the machine
language for the CPU. The more complicated the instruction set is, the slower the CPU
works. Processors differed from one another by the instruction set. If the same program can
run on two different computer brands they are said to be compatible. Programs written for
IBM compatible computers will not run on Apple computers because these two architectures
are not compatible.
The control Unit and the Arithmetic and Logic unit of a computer system are jointly known
as the Central Processing Unit (CPU). The CPU is the brain of any computer system. In a
human body, all major decisions are taken by the brain and the other parts of the body
function as directed by the brain. Similarly, in a computer system, all major calculations and
comparisons are made inside the CPU and the CPU is also responsible for activating and
controlling the operations of other units of a computer system.
Arithmetic and Logic Unit (ALU)
The arithmetic and logic unit (ALU) of a computer system is the place where the actual
execution of the instructions take place during the processing operations. All calculations are
performed and all comparisons (decisions) are made in the ALU. The data and instructions,
stored in the primary storage prior to processing are transferred as and when needed to the
ALU where processing takes place. No processing is done in the primary storage unit.
Intermediate results generated in the ALU are temporarily transferred back to the primary
storage until needed at a later time. Data may thus move from primary storage to ALU and
back again as storage many times before the processing is over. After the completion of
processing, the final results which are stored in the storage unit are released to an output
device.

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The arithmetic and logic unit (ALU) is the part where actual computations take place. It
consists of circuits that perform arithmetic operations (e.g. addition, subtraction,
multiplication, division over data received from memory and capable to compare numbers
(less than, equal to, or greater than).
While performing these operations the ALU takes data from the temporary storage are inside
the CPU named registers. Registers are a group of cells used for memory addressing, data
manipulation and processing. Some of the registers are general purpose and some are
reserved for certain functions. It is a high-speed memory which holds only data from
immediate processing and results of this processing. If these results are not needed for the
next instruction, they are sent back to the main memory and registers are occupied by the
new data used in the next instruction.
All activities in the computer system are composed of thousands of individual steps. These
steps should follow in some order in fixed intervals of time. These intervals are generated by
the Clock Unit. Every operation within the CPU takes place at the clock pulse. No operation,
regardless of how simple, can be performed in less time than transpires between ticks of this
clock. But some operations required more than one clock pulse. The faster the clock runs, the
faster the computer performs. The clock rate is measured in megahertz (Mhz) or Gigahertz
(Ghz). Larger systems are even faster. In older systems the clock unit is external to the
microprocessor and resides on a separate chip. In most modern microprocessors the clock is
usually incorporated within the CPU.
Control Unit
How the input device knows that it is time for it to feed data into the storage unit? How does
the ALU know what should be done with the data once it is received? And how is it that only
the final results are sent to the output devices and not the intermediate results? All this is
possible because of the control unit of the computer system. By selecting, interpreting, and
seeing to the execution of the program instructions, the control unit is able to maintain order
and directs the operation of the entire system. Although, it does not perform any actual
processing on the data, the control unit acts as a central nervous system for the other

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components of the computer. It manages and coordinates the entire computer system. It
obtains instructions from the program stored in main memory, interprets the instructions, and
issues signals that cause other units of the system to execute them.
The control unit directs and controls the activities of the internal and external devices. It
interprets the instructions fetched into the computer, determines what data, if any,
are needed, where it is stored, where to store the results of the operation, and sends the
control signals to the devices involved in the execution of the instructions.
2. Elaborate the different types of addressing modes with a suitable
example.(2014/2015/2016/2017)
Addressing Modes
The term addressing modes refers to the way in which the operand of an instruction
is specified. Information contained in the instruction code is the value of the operand or the
address of the result/operand. Following are the main addressing modes that are used on
various platforms and architectures.
1) Immediate Mode
The operand is an immediate value is stored explicitly in the instruction:
2) Index Mode
The address of the operand is obtained by adding to the contents of the general
register (called index register) a constant value. The number of the index register and the
constant value are included in the instruction code. Index Mode is used to access an array
whose elements are in successive memory locations. The content of the instruction code,
represents the starting address of the array and the value of the index register, and the index
value of the current element. By incrementing or decrementing index register different
element of the array can be accessed.
3) Indirect Mode
The effective address of the operand is the contents of a register or main memory
location, location whose address appears in the instruction. Indirection is noted by placing
the name of the register or the memory address given in the instruction in parentheses. The
register or memory location that contains the address of the operand is a pointer. When an
execution takes place in such mode, instruction may be told to go to a specific address. Once
it's there, instead of finding an operand, it finds an address where the operand is located.
4) Absolute (Direct) Mode
The address of the operand is embedded in the instruction code.

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5) Register Mode
The name (the number) of the CPU register is embedded in the instruction. The
register contains the value of the operand. The number of bits used to specify the register
depends on the total number of registers from the processor set.
6) Displacement Mode
Similar to index mode, except instead of a index register a base register will be used.
Base register contains a pointer to a memory location. An integer (constant) is also referred
to as a displacement. The address of the operand is obtained by adding the contents of the
base register plus the constant. The difference between index mode and displacement mode is
in the number of bits used to represent the constant. When the constant is represented a
number of bits to access the memory, then we have index mode. Index mode is more
appropriate for array accessing; displacement mode is more appropriate for structure
(records) accessing.
7) Auto increment /Auto decrement Mode
A special case of indirect register mode. The register, whose number is included in
the instruction code, contains the address of the operand. Auto increment Mode = after
operand addressing, the contents of the register is incremented. Decrement Mode = before
operand addressing, the contents of the register is decrement.
3. Discuss about the various techniques to represent instruction in a
computer system. (2014/2015/2016)

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The Instruction Set Architecture (ISA) is the part of the processor that is visible to the
programmer or compiler writer. The ISA serves as the boundary between software and
hardware. We will briefly describe the instruction sets found in many of the microprocessors
used today. The ISA of a processor can be described using 5 catagories:
Operand Storage in the CPU
Where are the operands kept other than in memory?
Number of explicit named operands
How many operands are named in a typical instruction.
Operand location
Can any ALU instruction operand be located in memory? Or must all operands be
kept internaly in the CPU?
Operations
What operations are provided in the ISA.
Type and size of operands
What is the type and size of each operand and how is it specified?

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Of all the above the most distinguishing factor is the first.
The 3 most common types of ISAs are:
1. Stack - The operands are implicitly on top of the stack.
2. Accumulator - One operand is implicitly the accumulator.
3. General Purpose Register (GPR) - All operands are explicitely mentioned, they are
either registers or memory locations.
Lets look at the assembly code of
A = B + C;
in all 3 architectures:
Stack Accumulator GPR
PUSH A LOAD A LOAD R1,A
PUSH B ADD B ADD R1,B
ADD STORE C STORE R1,C
POP C - -
Not all processors can be neatly tagged into one of the above catagories. The i8086 has many
instructions that use implicit operands although it has a general register set. The i8051 is
another example, it has 4 banks of GPRs but most instructions must have the A register as
one of its operands.
What are the advantages and disadvantages of each of these approachs?
Stack
Advantages: Simple Model of expression evaluation (reverse polish). Short instructions.
Disadvantages: A stack can't be randomly accessed This makes it hard to generate eficient
code. The stack itself is accessed every operation and becomes a bottleneck.
Accumulator
Advantages: Short instructions.
Disadvantages: The accumulator is only temporary storage so memory traffic is the highest
for this approach.

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GPR
Advantages: Makes code generation easy. Data can be stored for long periods in registers.
Disadvantages: All operands must be named leading to longer instructions.
Earlier CPUs were of the first 2 types but in the last 15 years all CPUs made are GPR
processors. The 2 major reasons are that registers are faster than memory, the more data that
can be kept internaly in the CPU the faster the program wil run. The other reason is that
registers are easier for a compiler to use.
Reduced Instruction Set Computer (RISC)
As we mentioned before most modern CPUs are of the GPR (General Purpose Register) type.
A few examples of such CPUs are the IBM 360, DEC VAX, Intel 80x86 and Motorola
68xxx. But while these CPUS were clearly better than previous stack and accumulator based
CPUs they were still lacking in several areas:
1. Instructions were of varying length from 1 byte to 6-8 bytes. This causes problems
with the pre-fetching and pipelining of instructions.
2. ALU (Arithmetic Logical Unit) instructions could have operands that were memory
locations. Because the number of cycles it takes to access memory varies so does the
whole instruction. This isn't good for compiler writers, pipelining and multiple issue.
3. Most ALU instruction had only 2 operands where one of the operands is also the
destination. This means this operand is destroyed during the operation or it must be
saved before somewhere.
Thus in the early 80's the idea of RISC was introduced. The SPARC project was started at
Berkeley and the MIPS project at Stanford. RISC stands for Reduced Instruction Set
Computer. The ISA is composed of instructions that all have exactly the same size, usualy 32
bits. Thus they can be pre-fetched and pipelined succesfuly. All ALU instructions have 3
operands which are only registers. The only memory access is through explicit
LOAD/STORE instructions.
Thus A = B + C will be assembled as:
LOAD R1,A
LOAD R2,B
ADD R3,R1,R2
STORE C,R3
Although it takes 4 instructions we can reuse the values in the registers.
4. List the eight great ideas invented by computer architects(nov/dec2016)
1. Design for Moore’s Law.
2. Use Abstraction to Simplify Design

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3. Make the common case fast
4. Performance via parallelism
5. Performance via pipelining
6. Performance via prediction
7. Hierarchy of memories
8. Dependability via redundancy
5. How CPU execution time for a program is calculated?(Nov/Dec 2015)
Performance:
Response time or execution time: The total time required for the computer to complete a
task, including disk accesses, memory
accesses, I/O activities, operating system overhead, CPU execution
time, and so on.
Throughput: The total amount of work done in a given time.
Bandwidth: The amount of data that can be carried from one point to another in a given
time period (usually a second). This kind of bandwidth is usually expressed in bits (of data)
per second (bps). Occasionally, it's expressed as bytes per second (Bps).
clock cycles per instruction (CPI): Average number of clock cycles per
instruction for a program or program fragment.
Performance:

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UNIT-II ARITHMETIC OPERATIONS
1. Explain the sequential versions of multiplication algorithm and its
hardware.(2014/2016/2017)
Multiplication:

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2. Explain briefly about floating point addition algorithm and subtraction
algorithm?
Addition:

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Subtraction:
3. Discuss in details about division algorithm in details with diagram and control lines

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4. Explain in detail the principle of of carry look ahead adder. Show how
16 bit CLAs can be constructed from 4 bit adders. (2014/2016/2017)
A carry-lookahead adder (CLA) or fast adder is a type of adder used in digital
logic. A carry-lookahead adder improves speed by reducing the amount of time required to
determine carry bits. It can be contrasted with the simpler, but usually slower, ripple carry
adder for which the carry bit is calculated alongside the sum bit, and each bit must wait until
the previous carry bit have been calculated to begin calculating its own result and carry bits
(see adder for detail on ripple carry adders). The carry-look ahead adder calculates one or
more carry bits before the sum, which reduces the wait time to calculate the result of the
larger value bits of the adder.

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UNIT
1. Explain in details basic MIPS implementation with necessary
multiplexers and control lines
COMPUTER ARCHITECTURE M.Ramu/AP/IT
UNIT-III PROCESSOR AND CONTROL
multiplexers and control lines.
36

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2. Explain how the instruction pipeline works? What are the various situations where
an instruction pipeline can stall?
an instruction pipeline can stall? Illustrate with an example.(Nov/2009/2014/2015)
38
(Nov/2009/2014/2015)

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3. What are hazard? Explain the methods for dealing with data
hazards(2016/2017).
39

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1. Explain in details Flynn’s classification of
Classifications
 1.1 Single instruction stream single data stream (SISD)
 1.2 Single instruction stream, multiple data streams (SIMD)
 1.3 Multiple instruction streams, single data stream (MISD)
 1.4 Multiple instruction streams, multiple data streams (MIMD)
 1.5 Single instruction, multiple threads (SIMT)
Single instruction stream single data stream (SISD)
A sequential computer which exploits no parallelism in either the instruction or data streams.
Single control unit (CU) fetches single instruction stream (IS) from memory. Th
generates appropriate control signals to direct single processing element (PE) to operate on
single data stream (DS) i.e., one operation at a time.
Examples of SISD architecture are the traditional
computers (PCs; by 2010, many PCs had multiple cores) and

Multiple instruction streams, single data stream (MISD)
Multiple instructions operate on one data stream. This is an uncommon architecture which is
generally used for fault tolerance. Heterogeneous systems operate on the same data stream
and must agree on the result. Examples include the
UNIT-
Explain in details Flynn’s classification of parallel hardware.(2014/2016)
1.1 Single instruction stream single data stream (SISD)
1.2 Single instruction stream, multiple data streams (SIMD)
1.3 Multiple instruction streams, single data stream (MISD)
1.4 Multiple instruction streams, multiple data streams (MIMD)
tion, multiple threads (SIMT)
Single instruction stream single data stream (SISD)
Single control unit (CU) fetches single instruction stream (IS) from memory. Th
single data stream (DS) i.e., one operation at a time.
Examples of SISD architecture are the traditional uniprocessor machines like older
(PCs; by 2010, many PCs had multiple cores) and mainframe computers
Multiple instruction streams, single data stream (MISD)
and must agree on the result. Examples include the Space Shuttle flight control computer
-IV PARALLELISM
40
(2014/2016)
Single control unit (CU) fetches single instruction stream (IS) from memory. The CU then
machines like older personal
mainframe computers.
flight control computer[4]
.

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
Single instruction stream, multiple data streams (SIMD)
A computer which exploits multiple data streams against a single stream to perform
operations which may be naturally parallelized. For example, an
processing unit (GPU)

Multiple instruction streams, multiple data streams (MIMD)
Multiple autonomous processors simultaneously executing different instructions on different
data. MIMD architectures include
using either one shared memory space or a distributed memory space.
Single instruction stream, multiple data streams (SIMD)
oits multiple data streams against a single stream to perform
operations which may be naturally parallelized. For example, an array processor
Multiple instruction streams, multiple data streams (MIMD)
ta. MIMD architectures include multi-core superscalar processors, and distributed systems
using either one shared memory space or a distributed memory space.
41
oits multiple data streams against a single stream to perform
array processor or graphics
distributed systems,

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
Single instruction, multiple threads (SIMT)
Single instruction, multiple threads
where single instruction, multiple data
originally part of Flynn's taxonomy but a proposed addition.
These four architectures are shown below visually. Each processing unit (PU) is shown for
uni-core or multi-core computer:
2. Draw a neat sketch of memory hierarchy and explain the need of cache
memory.(2014/2015)
Computer memory can be classified in the below given hierarchy:
1) Internal register: Internal register in a CPU is used for holding variables and temporary
results. Internal registers have a very small storage; however they can be accessed instantly.
Accessing data from the internal register is the fastest way to access memory.
2) Cache: Cache is used by the CPU for memory which is being accessed over and over
again. Instead of pulling it every time from the main memory, it is put in cache for fast
access. It is also a smaller memory, however, larger than internal register.
Cache is further classified to L1, L2 and L3:
a) L1 cache: It is accessed without any delay.
b) L2 cache: It takes more clock cycles to access than L1 cache.
c) L3 cache: It takes more clock cycles to access than L2 cache.
Single instruction, multiple threads (SIMT)
Single instruction, multiple threads (SIMT) is an execution model used in parallel computing
single instruction, multiple data (SIMD) is combined with multithreading
originally part of Flynn's taxonomy but a proposed addition.
These four architectures are shown below visually. Each processing unit (PU) is shown for
core computer:
Computer memory can be classified in the below given hierarchy:
Internal register in a CPU is used for holding variables and temporary
Cache is used by the CPU for memory which is being accessed over and over
access. It is also a smaller memory, however, larger than internal register.
r classified to L1, L2 and L3:
It is accessed without any delay.
It takes more clock cycles to access than L1 cache.
It takes more clock cycles to access than L2 cache.
42
parallel computing
multithreading. This is not
These four architectures are shown below visually. Each processing unit (PU) is shown for a
Internal register in a CPU is used for holding variables and temporary
Cache is used by the CPU for memory which is being accessed over and over

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3) Main memory or RAM (Random Access Memory): It is a type of the computer
memory and is a hardware component. It can be increased provided the operating system can
handle it. Typical PCs these days use 8 GB of RAM. It is accessed slowly as compared to
cache.
4) Hard disk: A hard disk is a hardware component in a computer. Data is kept permanently
in this memory. Memory from hard disk is not directly accessed by the CPU, hence it is
slower. As compared with RAM, hard disk is cheaper per bit.
5) Magnetic tape: Magnetic tape memory is usually used for backing up large data. When
the system needs to access a tape, it is first mounted to access the data. When the data is
accessed, it is then unmounted. The memory access time is slower in magnetic tape and it
usually takes few minutes to access a tape.
Below given figure shows the hierarchy of computer memory:
3. Explain various mechanisms of mapping main memory address into cache memory
addresses.(2014/2015)
Three techniques can be used:
1. Direct
2. Associative
3. Set Associative.

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DIRECT MAPPING
 The simplest technique, known as direct mapping, maps each block of main memory
into only one possible cache line. The mapping is expressed as
i = j modulo m
where
i cache line number
j main memory block number
m number of lines in the cache
 Figure 1 (a) shows the mapping for the first m blocks of main memory. Each block of
main memory maps into one unique line of the cache.
 The next m blocks of main memory map into the cache in the same fashion; that is,
block Bm of main memory maps into line L0 of cache, block Bm_1 maps into line
L1, and so on.
 The mapping function is easily implemented using the main memory address. Figure
2 illustrates the general mechanism. For purposes of cache access, each main memory
address can be viewed as consisting of three fields.
 The least significant w bits identify a unique word or byte within a block of main
memory; in most contemporary machines, the address is at the byte level. The
remaining s bits specify one of the 2s blocks of main memory. The cache logic
interprets these s bits as a tag of s – r bits (most significant portion) and a line field of
r bits. This latter field identifies one of the m = 2r lines of the cache. To summarize,

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The effect of this mapping is that blocks of main memory are assigned to lines of the cache
as follows:
 Thus, the use of a portion of the address as a line number provides a unique mapping
of each block of main memory into the cache.
 When a block is actually read into its assigned line, it is necessary to tag the data to
distinguish it from other blocks that can fit into that line. The most significant s - r
bits serve this purpose.
 The direct mapping technique is simple and inexpensive to implement. Its main
disadvantage is that there is a fixed cache location for any given block.
 Thus, if a program happens to reference words repeatedly from two different blocks
that map into the same line, then the blocks will be continually swapped in the cache,
and the hit ratio will be low (a phenomenon known as thrashing).

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ASSOCIATIVE MAPPING
 Associative mapping overcomes the disadvantage of direct mapping by permitting
each main memory block to be loaded into any line of the cache Figure 1(b).
 In this case, the cache control logic interprets a memory address simply as a Tag and
a Word field. The Tag field uniquely identifies a block of main memory.
 To determine whether a block is in the cache, the cache control logic must
simultaneously examine every line’s tag for a match. Figure 3 illustrates the logic.
Note that no field in the address corresponds to the line number, so that the number of
lines in the cache is not determined by the address format. To summarize,
 With associative mapping, there is flexibility as to which block to replace when a new
block is read into the cache.
 Replacement algorithms, discussed later in this section, are designed to maximize the
hit ratio. The principal disadvantage of associative mapping is the complex circuitry
required to examine the tags of all cache lines in parallel.

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SET-ASSOCIATIVE MAPPING
 Set-associative mapping is a compromise that exhibits the strengths of both the direct
and associative approaches while reducing their disadvantages.
 In this case, the cache consists of a number sets, each of which consists of a number
of lines. The relationships are
 This is referred to as k-way set-associative mapping. With set-associative mapping,
block Bj can be mapped into any of the lines of set j.
 Figure 4 (a) illustrates this mapping for the first blocks of main memory. As with
associative mapping, each word maps into multiple cache lines.
 For set-associative mapping, each word maps into all the cache lines in a specific set,
so that main memory block B0 maps into set 0, and so on.
 Thus, the set-associative cache can be physically implemented as associative caches.
It is also possible to implement the set-associative cache a k direct mapping caches,
as shown in Figure 4 (b).
 Each direct-mapped cache is referred to as a way, consisting of lines.The first lines of
main memory are direct mapped into the lines of each way; the next group of lines of
main memory are similarly mapped, and so on.
 The direct-mapped implementation is typically used for small degrees of associativity
(small values of k) while the associative- mapped implementation is typically used for
higher degrees of associativity.

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 For set-associative mapping, the cache control logic interprets a memory address as
three fields: Tag, Set, and Word.
 The d set bits specify one of v = 2d sets. The s bits of the Tag and Set fields specify
one of the 2s blocks of main memory.
 Figure 5 illustrates the cache control logic.With fully associative mapping, the tag in
a memory address is quite large and must be compared to the tag of every line in the
cache. With k-way set-associative mapping, the tag in a memory address is much
smaller and is only compared to the k tags within a single set.
5. Explain in detail about hardware Multithreading.
multithreading is the ability of a central processing unit (CPU) or a single core in a
multi-core processor to execute multiple processes or threads concurrently, appropriately
supported by the operating system. This approach differs from multiprocessing, as with
multithreading the processes and threads share the resources of a single or multiple cores: the
computing units, the CPU caches, and the translation lookaside buffer (TLB).
Where multiprocessing systems include multiple complete processing units, multithreading
aims to increase utilization of a single core by using thread-level as well as instruction-level
parallelism. As the two techniques are complementary, they are sometimes combined in
systems with multiple multithreading CPUs and in CPUs with multiple multithreading cores.
Advantages
If a thread gets a lot of cache misses, the other threads can continue taking advantage
of the unused computing resources, which may lead to faster overall execution as these
resources would have been idle if only a single thread were executed. Also, if a thread cannot
use all the computing resources of the CPU (because instructions depend on each other's
result), running another thread may prevent those resources from becoming idle.
If several threads work on the same set of data, they can actually share their cache, leading to
better cache usage or synchronization on its values.
Disadvantages
Multiple threads can interfere with each other when sharing hardware resources such as
caches or translation lookaside buffers (TLBs). As a result, execution times of a single thread are not
improved but can be degraded, even when only one thread is executing, due to lower frequencies or
additional pipeline stages that are necessary to accommodate thread-switching hardware.
Types of multithreading:
Coarse-grained multithreading
The simplest type of multithreading occurs when one thread runs until it is blocked by an
event that normally would create a long-latency stall. Such a stall might be a cache miss that
has to access off-chip memory, which might take hundreds of CPU cycles for the data to

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return. Instead of waiting for the stall to resolve, a threaded processor would switch
execution to another thread that was ready to run. Only when the data for the previous thread
had arrived, would the previous thread be placed back on the list of ready-to-run threads.
For example:
1. Cycle i: instruction j from thread A is issued.
2. Cycle i + 1: instruction j + 1 from thread A is issued.
3. Cycle i + 2: instruction j + 2 from thread A is issued, which is a load instruction that misses in
all caches.
4. Cycle i + 3: thread scheduler invoked, switches to thread B.
5. Cycle i + 4: instruction k from thread B is issued.
6. Cycle i + 5: instruction k + 1 from thread B is issued.
Conceptually, it is similar to cooperative multi-tasking used in real-time operating systems,
in which tasks voluntarily give up execution time when they need to wait upon some type of
the event. This type of multithreading is known as block, cooperative or coarse-grained
multithreading.
The goal of multithreading hardware support is to allow quick switching between a blocked
thread and another thread ready to run. To achieve this goal, the hardware cost is to replicate
the program visible registers, as well as some processor control registers (such as the
program counter). Switching from one thread to another thread means the hardware switches
from using one register set to another; to switch efficiently between active threads, each
active thread needs to have its own register set. For example, to quickly switch between two
threads, the register hardware needs to be instantiated twice.
Additional hardware support for multithreading allows thread switching to be done in one
CPU cycle, bringing performance improvements. Also, additional hardware allows each
thread to behave as if it were executing alone and not sharing any hardware resources with
other threads, minimizing the amount of software changes needed within the application and
the operating system to support multithreading.
Many families of microcontrollers and embedded processors have multiple register banks to
allow quick context switching for interrupts. Such schemes can be considered a type of block
multithreading among the user program thread and the interrupt threads.[citation needed]
Interleaved multithreading
The purpose of interleaved multithreading is to remove all data dependency stalls
from the execution pipeline. Since one thread is relatively independent from other threads,
there is less chance of one instruction in one pipelining stage needing an output from an older
instruction in the pipeline. Conceptually, it is similar to preemptive multitasking used in
operating systems; an analogy would be that the time slice given to each active thread is one
CPU cycle.

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For example:
1. Cycle i + 1: an instruction from thread B is issued.
2. Cycle i + 2: an instruction from thread C is issued.
This type of multithreading was first called barrel processing, in which the staves of a barrel
represent the pipeline stages and their executing threads. Interleaved, preemptive, fine-
grained or time-sliced multithreading are more modern terminology.
In addition to the hardware costs discussed in the block type of multithreading, interleaved
multithreading has an additional cost of each pipeline stage tracking the thread ID of the
instruction it is processing. Also, since there are more threads being executed concurrently in
the pipeline, shared resources such as caches and TLBs need to be larger to avoid thrashing
between the different threads.
Simultaneous multithreading
The most advanced type of multithreading applies to superscalar processors. Whereas
a normal superscalar processor issues multiple instructions from a single thread every CPU
cycle, in simultaneous multithreading (SMT) a superscalar processor can issue instructions
from multiple threads every CPU cycle. Recognizing that any single thread has a limited
amount of instruction-level parallelism, this type of multithreading tries to exploit parallelism
available across multiple threads to decrease the waste associated with unused issue slots.
For example:
1. Cycle i: instructions j and j + 1 from thread A and instruction k from thread B are
simultaneously issued.
2. Cycle i + 1: instruction j + 2 from thread A, instruction k + 1 from thread B, and instruction m
from thread C are all simultaneously issued.
3. Cycle i + 2: instruction j + 3 from thread A and instructions m + 1 and m + 2 from thread C
are all simultaneously issued.
To distinguish the other types of multithreading from SMT, the term "temporal
multithreading" is used to denote when instructions from only one thread can be issued at a
time.
In addition to the hardware costs discussed for interleaved multithreading, SMT has the
additional cost of each pipeline stage tracking the thread ID of each instruction being
processed. Again, shared resources such as caches and TLBs have to be sized for the large
number of active threads being processed.

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UNIT V
1. Explain about Multicore Processors.(2016/2015)
A multi-core CPU is a computer processor which has two or more sections. Each section of
the chip executes instructions as if it was a separate computer. The actual processors are still
on one chip. On this chip every core looks mostly like the other. They are several mostly
independent cores which work together in parallel. A dual-core processor is a multi-core
processor with two independent microprocessors. A quad-core processor is a multi-core
processor with four independent microprocessors. As you might be able to tell from the
prefix, the name of the processor is based on the number of the microprocessors on the chip.
Until 2005 single-core processors outnumbered multi-core processors.[source?]
In the
years before there were only multi-core solutions used in individual cases.[source?]
In the
majority of cases they enhanced the frequency. But at a frequency about 4 GHz the CPU
would get too hot and take a lot of electricity. This was the point when multi-core processors
became more important. Therefore the demand for multi-core processors increased. In the
second half of 2006 the best processors were dual-core processors. Since 2006 the
development has gone on, so that the new processors get four or more independent
microprocessors. Today, single-core processors are not used in new personal computers, but
they remain popular in embedded systems.
Advantages
 Having a multi-core processor in a computer means that it will work faster for certain
programs.
 The computer may not get as hot when it is turned on.
 The computer needs less power because it can turn off some sections if they aren´t
needed.
 More features can be added to the computer.
 The signals between different CPUs travel shorter distances, therefore they degrade
less.
Disadvantages
 They do not work at twice the speed as a normal processor. They get only 60-80%
more speed.
 The speed that the computer works at depends on what the user is doing with it.
 They cost more than single core processors.
 They are more difficult to manage thermally than lower-density single-core
processors.
 Not all operating systems support more than one core.
 Operating systems compiled for a multi-core processor will run slightly slower on a
single-core processor.

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Operating System Support
The following operating systems support multi-core processors
 Microsoft Windows (Windows XP or newer)
 Linux
 Mac OS X
 Most BSD-based systems
 Solaris
2. What is virtual memory? Explain the steps involved in virtual memory address
translation.(2013/2014/2015)
A virtual address is a binary number in virtual memory that enables a process to use a
location in primary storage (main memory) independently of other processes and to use more
space than actually exists in primary storage by temporarily relegating some contents to a hard
disk or internal flash drive
Virtual Memory Address Translation
In a system with virtual memory the main memory can be viewed as a cache for the disk,
which serves as the lower-level store. Due to the enormous difference between memory
access times and disk access times, a fully-associative caching scheme is used. That is, the
entire main memory is a single set - any page can be placed anywhere in main memory. This
makes the set field of the address vanish. All that remains is a tag and an offset.
The offset field identifies a particular location within page or frame.
The tag field identifies a page in the logical address space.
Since the tag field just identifies a page it is usually called the page number field.

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53
Logical Addresses
With a virtual memory system, the main memory can be viewed as a local store for a
cache level whose lower level is a disk. Since it is fully associative there is no need for a set
field. The address just decomposes into an offset field and a page number field. The number
of bits in the offset field is determined by the page size. The remaining bits are the page
number.
An Example
A computer uses 32-bit byte addressing. The computer uses paged virtual memory with 4KB
pages. Calculate the number of bits in the page number and offset fields of a logical address.
Page Tables
Virtual memory address translation uses a page table. In the diagram to the left, a page table
is represented by the large box. It is a structured array in memory. It is indexed by page
number.
Each page table entry contains information about a single page. Part of this information is a
frame number (green) — where the page is located in physical memory. In addition there are
control bits (blue) for controlling the translation process. Address translation concatenates the
frame number with the offset part of a logical address to form a physical address.
A page table base register (PTBR) holds the base address for the page table of the current
process. It is a processor register that is managed by the operating system
3. Explain the use of DMA controllers in a computer system with a neat
diagram.(2014/2015/2016)
DMA stands for "Direct Memory Access" and is a method of transferring data from the
computer's RAM to another part of the computer without processing it using the CPU. While
most data that is input or output from your computer is processed by the CPU, some data
does not require processing, or can be processed by another device.
In these situations, DMA can save processing time and is a more efficient way to move data
from the computer's memory to other devices. In order for devices to use direct memory
access, they must be assigned to a DMA channel. Each type of port on a computer has a set
of DMA channels that can be assigned to each connected device. For example, a PCI
controller and a hard drive controller each have their own set of DMA channels.
DMA DIAGRAM:

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54
4. Explain the IOP Organization and communication between CPU and
IOP.(2015/2016/2014)
IOP is a processor with direct memory access capability that communicates with I/O devices.
In this configuration, the computer system can be divided into a memory unit, and a number
of processors comprised of CPU and one or more IOPs. IOP is similar to CPU except that it
is designed to handle the details of I/O processing. Unlike DMA controller which is setup
completely by the CPU, IOP can fetch and execute its own instructions. IOP instructions are
designed specifically to facilitate I/O transfers. Instructions that are read from memory by an
IOP are called commands to differ them from instructions read by CPU . The command
words constitute the program for the IOP. The CPU informs the IOP where to find
commands in memory when it is time to execute the I/O program.
fig. I/O processor

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55
The memory occupies a central position and can communicate with each processor by means
of DMA. CPU is usually assigned the task of initiating the I/O program, from the non;IOP
operates independent of the CPU and continues to transfer data from external devices and
memory.
CPU-IO Communication
Communication between the CPU and IOP may take different forms depending on the
particular computer used.Mostly, memory unit acts as a memory center where each processor
leaves information for the other.
CPU sends an instruction to test the IOP path. The IOP responds by inserting a status word in
memory for the CPU to check. The bits of the status word indicate the condition of IOP and
I/O device (“IOP overload condition”, “device busy with another transfer” etc). CPU then
checks status word to decide what to do next . If all is in order, CPU sends the instruction to
start the I/O transfer. The memory address received with this instruction tells the IOP where
to find its program. CPU may continue with another program while the IOP is busy with the
I/O program. When IOP terminates the transfer (using DMA), it sends an interrupt request to
CPU. The CPU responds by issuing an instruction to read the status from the IOP and IOP
then answers by placing the status report into specified memory location. By inspecting the
bits in the status word, CPU determines whether the I/O operation was completed
satisfactorily and the process is repeated again.

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5. Explain the used of vectored interrupts in processors. Why is priority handling
desired in interrupts controllers? How do the different priority schemes work? (n2014)
When a Process is executed by the CPU and when a user Request for another Process
then this will create disturbance for the Running Process. This is also called as the Interrupt.
Interrupts can be generated by User, Some Error Conditions and also by Software’s
and the hardware’s. But CPU will handle all the Interrupts very carefully because when
Interrupts are generated then the CPU must handle all the Interrupts Very carefully means the
CPU will also Provides Response to the Various Interrupts those are generated. So that When
an interrupt has Occurred then the CPU will handle by using the Fetch, decode and Execute
Operations.
Types of Interrupts
Generally there are three types o Interrupts those are Occurred For Example
1) Internal Interrupt
2) Software Interrupt.
3) External Interrupt.
The External Interrupt occurs when any Input and Output Device request for any
Operation and the CPU will Execute that instructions first For Example When a Program is
executed and when we move the Mouse on the Screen then the CPU will handle this External
interrupt first and after that he will resume with his Operation.
The Internal Interrupts are those which are occurred due to Some Problem in the
Execution For Example When a user performing any Operation which contains any Error and
which contains any type of Error.
So that Internal Interrupts are those which are occurred by the Some Operations or by
Some Instructions and the Operations those are not Possible but a user is trying for that
Operation.
And The Software Interrupts are those which are made some call to the System for
Example while we are Processing Some Instructions and when we wants to Execute one
more Application Programs.

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