I have a 5 stage Datapath with the following steps' times:
Fetch 190ps
Decode 120ps
Alu 170ps
Memory 200ps
Writeback 120ps
It's asked to calculate how many instructions can be executed in 1us knowing that the processor is working in multi-cycle without pipeline and that the clock is optimised.
I know that if processor was pipelined and the pipeline was initially empty, the number of instructions would be 4996 by doing:
200ps (longest stage's time) -> 1 instruction
1 us -> x
x=5000
Nº of instructions = 5000-4=4996
Since there's no pipeline on this case what I did was:
190ps+120ps+170ps+200ps+120ps = 800ps
800ps -> 1 instruction
1 us -> x
x = 1250 instructions
However the correct answer is 1000 instructions. Can someone explain me why?
Thank you