![• Initially, the PC has stored the address of the instruction about to be executed and the SC is cleared to 0.
• With each clock pulses the SC is incremented and the timing signals go through the sequence T0 , T1 , T2 , etc.
• It is necessary to load the AR with the PC’s address (it is connected to memory address inputs):
• T0 : AR ¬ PC Fetch and Decode
• Subsequently, as we fetch the instruction to be executed, we must increment the program counter so that it
points to the next instruction:
• T1 : IR ¬ M[AR], PC ¬ PC + 1
• In order to carry out the instruction, we must decode and prepare to fetch the operand
• During time T3 , the control unit determines if this is a memory-reference, register-reference or input/output
instruction
Instruction Cycle](https://image.slidesharecdn.com/unit3-computerorganization-250110134934-7ef5baed/85/Unit-3-Computer-Organization-pdf-15-320.jpg)
Unit 3 - Computer Organization .pdf
- 1. Computer Organization Chapter 3 BCA , Sem - II
- 2. • Computer Organization is realization of what is specified by the computer architecture . • It deals with how operational attributes are linked together to meet the requirements specified by computer architecture. • Some organizational attributes are hardware details, control signals, peripherals. Computer Organization
- 3. Computer Architecture Vs Organization
- 4. Instruction Codes • An instruction code is a group of bits that instruct the computer to perform a specific operation. • The operation code of an instruction is a group of bits that define operations such as addition, subtraction, shift, complement, etc. • An instruction must also include one or more operands, which indicate the registers and/or memory addresses from which data is taken or to which data is deposited. • The instructions are stored in computer memory in the same manner that data is stored. • The control unit interprets these instructions and uses the operations code to determine the sequences of microoperations that must be performed to execute the instruction. • The simplest design is to have one processor register (called the accumulator) and two fields in the instruction, one for the opcode and one for the operand. Opcode Operands
- 6. Computer Register • Registers are a type of computer memory used to quickly accept, store, and transfer data and instructions that are being used immediately by the CPU. • The registers used by the CPU are often termed as Processor registers. • A processor register may hold an instruction, a storage address, or any data • The computer needs processor registers for manipulating data and a register for holding a memory address. • The register holding the memory location is used to calculate the address of the next instruction after the execution of the current instruction is completed. Memory Location Instruction 0001 H 0002 0003 0004 0005 H Call Next 0006 0007 PC Memory Location Instruction 001C 001D 001E 001F RET Next :
- 9. Computer Instructions • Computer instructions are a set of machine language instructions that a particular processor understands and executes. • A computer performs tasks on the basis of the instruction provided. • An instruction comprises of groups called fields. These fields include: ✓ The Operation code (Opcode) field which specifies the operation to be performed. ✓ The Address field which contains the location of the operand, i.e., register or memory location. ✓ The Mode field which specifies how the operand will be located. A basic computer has three instruction code formats which are: 1. Memory - reference instruction 2. Register - reference instruction 3. Input-Output instruction
- 10. Computer Instructions Memory - reference instruction In Memory-reference instruction, 12 bits of memory is used to specify an address and one bit to specify the addressing mode 'I'.
- 11. Register - reference instruction The Register-reference instructions are represented by the Opcode 111 with a 0 in the leftmost bit (bit 15) of the instruction.
- 12. Input-Output instruction Just like the Register-reference instruction, an Input-Output instruction does not need a reference to memory and is recognized by the operation code 111 with a 1 in the leftmost bit of the instruction. The remaining 12 bits are used to specify the type of the input-output operation or test performed.
- 13. Timing & Control • The timings for all the registers is controlled a master clock generator. • Its pulses are applied to all flip-flops and registers, including in the control unit. • There are two types of control: ➢ Hardwired – control logic is implemented with gates, flip-flops, decoders and other digital circuits. ➢ Microprogrammed – control information is stored in a control program, which is programmed to perform the necessary steps to implement instructions.
- 14. Instruction Cycle • The instructions of a program are carried out by a process called the instruction cycle. • The instruction cycle consists of these phases: 1. Fetch an instruction from memory 2. Decode the instruction 3. Read the effective address from memory / Fetch the operand 4. Execute the instruction.
- 15. • Initially, the PC has stored the address of the instruction about to be executed and the SC is cleared to 0. • With each clock pulses the SC is incremented and the timing signals go through the sequence T0 , T1 , T2 , etc. • It is necessary to load the AR with the PC’s address (it is connected to memory address inputs): • T0 : AR ¬ PC Fetch and Decode • Subsequently, as we fetch the instruction to be executed, we must increment the program counter so that it points to the next instruction: • T1 : IR ¬ M[AR], PC ¬ PC + 1 • In order to carry out the instruction, we must decode and prepare to fetch the operand • During time T3 , the control unit determines if this is a memory-reference, register-reference or input/output instruction Instruction Cycle
- 16. • Machine language is a collection of binary digits or bits that the computer reads and interprets. • Machine language is the only language a computer is capable of understanding. • It is also called as machine code or object code • Machine language is a low-level language that machines understand but that humans can decipher using an assembler. • A compiler plays an important role between humans and computers as it converts machine language into other code or language that is understandable by humans. Machine Language
- 17. • An assembly language is a low-level programming language. • Sometimes referred to as assembly or ASM • Programs written in assembly languages are compiled by an assembler. • Every assembler has its own assembly language, which is designed for one specific computer architecture. • Using ASM, programmers can write human-readable programs that correspond almost exactly to machine language. • The disadvantage is that everything the computer does must be described explicitly, in precise detail. • The advantage is that the programmer has maximum control over what the computer is doing. • An assembler is a program that takes basic computer instructions and converts them into a pattern of bits that the computer's processor can use to perform its basic operations Assembly Language