Sec 2 3 Program Execution 1 Program Execution

Sec (2. 3) Program Execution 1

Program Execution n 1. 2. In the CPU we have CU and ALU, in CU there are two special purpose registers: Instruction Register Program Counter 2

The architecture of the machine described in Appendix C 3

Program Execution 1. Instruction Register Is used to holed the instruction being executed 2. Program Counter Contains the address of next instruction to be executed 4

Program Execution CU performs its job by continually repeating an algorithm that guides it through a 3 -steps process known as the Machine Cycle The steps are: 1. fetch 2. decode 3. execute n 5

: In fetch step n the CU requests that main memory provide it with the instruction that is stored at the address indicated by the program counter ¨ ¨ Each instruction in our machine is 2 bytes long, this fetch process involves retrieving the contents of 2 memory cells The CU places the instruction received from memory in its instruction register and then increments the program counter by 2 so, it contains the address of the next instruction stored in memory 6

: Decode step n The instruction new in the instruction register ¨ The CU decodes the instruction, which involves breaking the operand field a proper component based on the instructions opcode 7

: Execute step n The CU executes the instruction by activating the appropriate circuitry to perform the requested task 8

Decoding the instruction B 258 9

: Cache memory There is a cache in CPU which takes the data from memory n Small memory and faster in accessing. n It is between CPU and RAM n It contains the information and instructions which are commonly used n 10