Computer Performance - MIPS Rate

MIPS stands for Millions Instructions Per Second when it is used as a rate for processor performance.

(MIPS is also a type of architecture for a processor - see here).

The number of instructions per second is the number of instructions divided by the time taken to fully exectute all of them

= instruction count/CPU execution time = I_c/T

That number would be a very big one, so we speak in terms of millions of them - MIPS

Remember that T = 1/f - therefore we can replace the CPU execution time (T) with the CPU frequency (f)

MIPS numbers are often very high because of how processors work, but in fact the number tells one absolutely nothing about what the processor can actually do or how it works (ie. a processor with a lower MIPS rating may actually be a better chip because its instructions are doing more work per clock cycle).

The MIPS rating of a CPU refers to how many low-level machine code instructions a processor can execute in one second. Unfortunately, using this number as a way of measuring processor performance is completely pointless because no two chips use exactly the same kind of instructions, execution method, etc.

For example:

On one chip, a single instruction may do many things when executed (CISC = Complex Instruction Set Computing), whereas on another chip a single instruction may do very little but is dealt with more efficiently (RISC = Reduced Instruction Set Computing).

Also, different instructions on the same chip often do vastly different amounts of work (eg. a simple arithmetic instruction might take just 1 clock cycle to complete, whereas doing something like floating point division or a square root operation might take 20 to 50 clock cycles).

MIPS rate is useful for comparing performance between similar processors but machines with different instruction sets and programs with different instruction mixes cannot be easily differentiated between by using it.

If a computer is going to carry out a lot of floating-point calculations a more useful indicator of performance is MFLOPS.