Consider the fundamental practitioner equation of computer performance:
TimeCPU = (dfrac|s e c o n d s|c omp l e t e p r o g r a m e x e c u t i o n= (dfrac|i n s t r u c t i o n s|
p r o g r am) ×(dfrac|a v e r a g e c y c l e s|i n s t r u c t i o n) × (dfrac|a v e r a g e s e c o n d s|c y c l e)
3.1 Assume you need to decide between two machines that are otherwise identical except for the actual CPU - that is, the machines have the same RAM, internal bus, I/O systems, and the like - only the CPUs change but that the throughput of the various other components balance with either CPU. One CPU is a CISC machine, and the other is a RISC machine. The average executable program of the CISC machine has 1/4 the instructions used for the same program on the RISC machine (that is, the same program source code when compiled by equivalent compilers on each of the two platforms is used to produce the executable program for each platform). What must be the relationship between the average number of cycles per instruction and the average time per cycle for the two machines to take the same TimeCPU?
Assume that the time for the optimizing compiler to run on the two platforms is five times longer on the RISC machine than on the CISC machine.
Assume that the RISC program takes 50 seconds to compile. Assume that the program will be run once. How much time must the program take for execution on the RISC platform before the total time for compilation plus execution is less on the RISC platform than the CISC platform?
As the number of runs of the executable programs grows ("goes to infinity"), prove that the difference in compilation time on the two platforms becomes irrelevant for the total time for the program.
As discussed in lecture, RISC machines use different resources (e.g., registers) compared with a CISC machine for actual execution of programs, and in particular, for programs that use modules (e.g., subroutines). What influence do these differences have on the fundamental equation of computer performance? (Hint: consider the availability of resources for such issues as starvation.) You must provide quantitative examples (either using actual values or symbols) for full credit. In other words, how realistic is this equation for real platforms?