In x86 assembly how can you set the zero flag (ZF) without doing a compare operation?

Question

I have a short piece of (x86) assembly that I am trying to figure out what it does.

...
 6:     81 ec 00 01 00 00       sub    $0x100, %esp
 c:     31 c9                   xor    %ecx  , %ecx
 e:     88 0c 0c                mov    %cl   , (%esp, %ecx, 1)
11:     fe c1                   inc    %cl
13:     75 f9                   jne    0xe
....

It looks like its looping though until the "JNE" evaluates to false, i.e. the zero flag = 0. (possibly its putting the numbers 1, 2, 3 ... into the stack??)

From my short investigation into assembly (im new at this) it seam you set the zero flag by doing a compare operation (CMP), but I dont see a compare operation.

So, under what conditions will it break out of this loop?

Answer 1

inc sets ZF if the value of cl after the increment is zero. Your loop is doing this:

sub    $0x100, %esp            // unsigned char array[256];
xor    %ecx  , %ecx            // unsigned char cl = 0;
mov    %cl   , (%esp, %ecx, 1) // e: array[cl] = cl;
inc    %cl                     //    cl += 1;
jne    0xe                     //    if (cl != 0) goto e;

The loop terminates when cl is incremented from 255 and wraps around to 0, setting ZF.