Can I use a register as a loop counter?

Question

Since the calling convention of a function states which registers are preserved, can a register be used as a loop counter?

I first thought that the ecx register is used as a loop counter, but after finding out that an stdcall function I have used has not preserved the value of ecx, I thought otherwise.

Is there a register that is guaranteed (by mostly used calling conventions at least) to be preserved?

Note: I don't have a problem in using a stack variable as a loop counter, I just want to make sure that it is the only way.

Answer 1

You can use any general-purpose register, and occasionally others, as the loop counter (just not the stack pointer of course ☺).

Either you use one to loop manually, i.e. replace…

loop label

… with…

dec ebp
jnz label

… which is faster anyway (because AMD (and later Intel, when they caught up, MHz-wise) artificially slowed down the loop instruction as otherwise, Windows® and some Turbo Pascal compiled software crashed).

Or you just save the counter in between:

label:
    push ecx
    call func
    pop ecx
    loop label

Both are standard strategies.