CPU Fetch-Execute Cycle and Computer Architecture, Slides of Information Systems

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Kingdom of Saudi Arabia Ministry of Higher Education Al-Imam Muhammad Ibn Saud Islamic University College of Computer and Information Sciences Chapter 2 Notes

Ch2: Computer Hardware

Computer Organization

• (^) The structure of the components in the computer
• (^) The function of those components
  • (^) How the CPU works
  • (^) The role of memory
  • (^) The role of I/O devices
• (^) How to assemble a computer

The Structure of the Computer

The System Bus

The Bus continued

• (^) Data bus:
  • (^) Used to send data and program instructions
    • (^) from memory to the CPU
    • (^) from the CPU to memory
    • (^) between I/O device and the CPU
    • (^) between I/O device and memory
  • (^) Size of data bus is often size of computer’s word

Memory Read

Fetch-Execute Cycle

• (^) To execute a program, the CPU performs the fetch-

execute cycle

• (^) Fetch next instruction from memory
• (^) Decode the instruction
• (^) Execute the instruction
• (^) Store the result

Example: A Program

• (^) We use the following C program to better understand the

fetch-execute cycle

#include <stdio.h> // input/output library

void main( ) // start of the program

int a, b, c; // use 3 integer variables

scanf(“%d”, &a); // input a

scanf(“%d”, &b); // input b

if(a < b) // compare a to b, if a is less then b

c=a + b; // then set c to be their sum

else c=a-b; // otherwise set c to be their difference

printf(“%d”, c); // output the result, c

Program in Machine Language

• (^) Assembly language version of our C program is stored in

the computer in machine language

• (^) The first four instructions might look like this:

1000100 0000000000000000000100001 – input (from keyboard)

1000111 0010011000100101101010001 – store the datum in a

1000100 0000000000000000000100001 – input (from keyboard)

1000111 0010011000100101101010010 – store the datum in b

op code operand (datum)

Fetch-Execute Cycle

Registers Continued

• (^) IR – instruction register
  • (^) Current instruction
  • (^) Used during decoding
• (^) Status flags
  • (^) To store information about the result of the previous ALU operation
    • (^) positive, negative, zero
    • (^) even or odd parity
    • (^) carry
    • (^) overflow
    • (^) interrupt

Fetch-Execute Cycle: Details

• (^) Fetch:
  • (^) PC stores address of next instruction
  • (^) Fetch instruction at PC location
  • (^) Increment PC
  • (^) Instruction sent over data bus
  • (^) Store instruction in IR
• (^) Decode:
  • (^) Decode opcode portion in IR
  • (^) Determine operand(s) from instruction in IR Input 33 Store a Input 33 Store b Load a Subt b Jge else Load a Add b Store c Jump next else: Load a Subt b Store c next: Load c Output 2049 Halt

Fetch-Execute Cycle: Example

• (^) Assume our program starts at location 5,000,
  • (^) PC: 5,000,
  • (^) IR: -------
• (^) Fetch instruction
  • (^) PC: 5,000,
  • (^) IR: 1000100 0000000000000000000100001
  • (^) Increment PC to 5,000,
• (^) Decode instruction
  • (^) Input operation (obtain input from keyboard)
• (^) Execute:
  • (^) Take input value from keyboard
  • (^) Move to AC

Continued

• (^) Fetch instruction
  • (^) PC: 5,000,
  • (^) IR: 1000111 0010011000100101101010001
  • (^) Increment PC to 5,000,
• (^) Decode instruction
  • (^) Store datum to memory location 0010011000100101101010001

(memory location storing variable a)