Lec 7

Lecture 7: Instruction Execution Cycle
By Noor Kadhum
Architecture For 2nd class- Computer Dep.
Babylon University -Sciences College For Women
Execution cycle
The execution steps are different from one instruction to another. This depends on the nature of the executed instruction. We can classify the instruction to :





The unconditional branching instruction is a change in the
Unconditional branch instruction changes the sequence of execution of the instructions by jumping x steps. This requires to update the value of (PC) by using the following equation:
PC=PC+X
X is available by instruction itself.




As any unconditional instruction, it is all about updating PC by a certain value offered by the instruction using the equation:
PC=PC+X
To do this job, the entries of ALU must be filled by values:
- We don’t need the value (1) so, the carry line remains unset.
- Pc and x is distributed on and the direct line











Using ALU to implement unconditional branch instruction
Conditional branch instruction implemented as if it was unconditional in the case of the condition and carried out as if it was sequential if not achieved condition




After decoding this instruction the CPU concludes that:
- The content of R1 is already available in the CPU because it is register.
- The content of X is resident in the memory , this mean that the value of X should be brought to CPU (i.e. read it),


Execution Cycle:
1- Reading the value of address X:







2- Implementing the operation :













After decoding this instruction the CPU concludes that:
- The required operation is moving the content of register R1 into memory address (x) i.e. this is a write operation.

- The content of R1 is already available in the CPU because it is register.