Needed Example of while loop in Nasm

Question

Could someone please explain me how to write a While loop in NASAM assembly language , example which registers are involved what mov operations are needed and how the counter is kept, i'm trying to grasp assembly but it's a little tricky at the begining...

Thanks Guys

Btw : I'm trying to learn this language for a course i'm taking

Solution

I guess you ask about x86 assembler? In assembler there is no while loop (maybe you can use special macros for that sometime, depends on the Assembler). You have to construct by your own. You can use assembler commands like

jne, jnz, je, jz, cmp, inc, dec

or some more special outdated command

jcxz, loop

for example:

mov ecx,255

L1:

dec ecx
jne L1
ret

This example counts the value in the register ecx until 0 and then left the loop. Another example but which count above.

mov ecx,0

L2:

inc ecx
cmp ecx, 255
jne L2
ret

Comments

[Shawn ricshawnawic]

Alright Suppose if i wanted to make a hypothetical while loop that takes a # suppose 16 and divides it by two until the quotient is 0 i would need to perform a compare each time the number is divided by 2 , Also i assume " loop: " is a label and not a function correct ?

[David J]

In this case you should simply initialize a register (maybe eax) with that number you want to divide by 2. In this special case you can use shift by one bit to the right (= equivalent to integer division by 2) and leave the loop if the number in eax is already 0. And yes you are right loop is a label.

[Peter Cordes]

NASM doesn't support this kind of thing. MASM does have some high-level directives like IF, and probably loops, but this is a NASM question.

[Peter Cordes]

don't use loop: as a label: It's also an instruction mnemonic, so you'll get weird syntax errors from some assemblers.

[David J]

Yes, not a good idea to use 'loop' as a Label. I fixed the answer. Sorry for that.