CPU and its execution of instruction

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CPU AND ITS EXECUTION OF
INSTRUCTION
Jaseena A P
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Muhammed A P
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WHAT IS CPU?
 Central Processing Unit
 Brain of every computer
Two main components:
- ALU
- Control unit

COMPONENTS OF CPU
 ALU
-Arithmetic and Logical Unit
-Performs all arithmetic and logical operations.
Control Unit
- Extracts instruction from memory, decodes and
executes them
- Calls ALU whenever necessary.

REGISTERS
 Registers are temporary memory units that
store words.
 The registers are located in the processor, instead of
in RAM.
Some registers are
PC, AC, IR, Index register, AMASK, MAR, and MDR,
General purpose register, Data register and
Process status register.

Program Counter: Stores the address of the macro
instruction currently being executed.
Accumulator: Stores a previously calculated value or a
value loaded from the main memory.
Instruction Register: Stores a copy of the instruction
loaded from main memory (Instruction being executed
currently).
REGISTERS(CONTD..)

REGISTERS(CONTD..)
Address Mask: When the CPU needs to know the
address of a target word that an instruction is using,
the AMASK is AND'ed with the instruction to
eliminate the opcode, leaving only the desired
address.
 Memory Address Register: This register contains the
address of the place the CPU wants to work with in
the main memory. It is directly connected to the
RAM chips on the motherboard.

REGISTERS(CONTD….)
 Process status register:
This register contains the various information about
the cpu.
 Data register:
A register used in microcomputers to temporarily store
data being transmitted to or from a peripheral device.
 General purpose registers:
-A general purpose register is a memory cell.
-Each general purpose register has a unique name
-It is used to store intermediate result of complex
computation

REGISTERS(CONTD…)
 MDR(Memory data register)
-It is the register that contains the data to be
stored in the computer storage or data after a fetch
from the computer storage.
-It acts like a buffer and holds anything that is
copied from the memory ready for the processor
to use it,
 Index register:
-A hardware element which holds a number that
can be added to (or, in some cases, subtracted
from) the address portion of a computer
instruction to form an effective address.
-An index register in a computer's CPU is a
processor register used for modifying operand
addresses during the run of a program.

 Pathway that interconnect CPU to all the other
components on the motherboard
Two types:
-Internal
-External
 The lines or pins of a bus are of three types:
-Data
-Address
-Control
SYSTEM BUS

Address - the components pass memory addresses
to one another over the address bus.
Control - used to send out signals to coordinate and
manage the activities of the motherboard
components.
Data - transferred between peripherals, memory and
the CPU. Obviously, the data bus can be a very busy
pathway.
SYSTEM BUS(CONTD..)

INSTRUCTION
 Single operation of a processor defined by
the processor instruction set.
 An instruction has two components:
opcode operand
Eg: ADD Ro 100

INSTRUCTION TYPES
 Instructions can be divided into six groups:
-Data transfer
eg: MOV, LOAD
-Arithmetic
eg: ADD, SUB, DIV
-Logical
eg: AND, OR, NOT
-System-control
eg: STOP
-I/O
eg: IN and OUT
-Program Control

Program control can be divided into:
-Unconditional Branch
eg:JMP NEXT % Jump to the label NEXT
-Conditional Branch
eg: JZ LABEL % jump to label if current value in
AC is ZERO
-Subroutine call
eg: CALL SUB
-Interrupt handling
eg:TRAP
INSTRUCTION TYPES

1. Load the address of next instruction in the PC into the
MAR.
So that the control unit can fetch the instruction from the right part
of the memory.
2. Copy the instruction/data that is in the memory address
given by the MAR into the MDR.
MDR is used whenever anything is to go from the CPU to main
memory, or vice versa.
3. Increment the PC by 1.
So that it contains the address of the next instruction, assuming
that the instructions are in consecutive locations.
4. Load the instruction/data that is now in the MDR into
the IR.
Thus the next instruction is copied from memory -> MDR -> IR.
1.FETCH

2.DECODE
1. Contents of IR split into operation code and address
if present e.g. store, add or jump instructions.
2. Decode the instruction that is in the IR

3.EXECUTE
• If the instruction is a jump instruction then
Load the address part of the instruction in the IR into the PC.
• If the instruction is an input / load (directly) instruction
then
take data input and place in accumulator.
• If the instruction is a load (from memory) instruction.
Copy address part of the instruction (to load from) in the IR into MAR.
Copy data from memory address held in MAR to MDR.
Copy data in MDR into accumulator

• If the instruction is a store instruction then:
Copy address part of the instruction (to store in) in the IR into
MAR.
Copy data in accumulator to MDR.
Copy data in MDR into memory address held in MAR.
• If the instruction is arithmetic then:
Copy address part of the instruction (of number to perform
arithmetic operation) in the IR into MAR.
Copy number from memory address held in MAR into MDR.
Perform operation to number in accumulator (accumulator will
now hold the result).
EXECUTE(CONTD..)

• If the instruction is an output (from memory)
instruction then:
Copy address part of part of the instruction (of data to output) in IR
into MAR.
Output contents of MDR.
EXECUTE(CONTD…)

4.STORE THE OUTPUT
Result of calculations in cpu stored in main memory
or sent to output devices.
Program Counter could be updated to a new address

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CPU and its execution of instruction

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