Some Assembly Required No. 1

lorg — Sat, 12 Apr 2008 20:17:05 +0000

I've been working on some of the instruction tests in vial, and I wanted to test the implementation of LOOP variants. My objective was to make sure the vial version is identical to the real CPU version (as discussed here). To achieve this, I had to cover all of the essential behaviors of LOOP.

Well, using the framework Gil and I wrote, I hacked up some code that should cover the relevant cases:

code_template = """
mov edx, ecx ; control the start zf
mov ecx, eax ; number of iterations
mov eax, 0 ; will hold the result, also an iteration counter
loop_start:

cmp eax, ebx ; check if we need to change zf
setz dh
xor dh, dl ; if required, invert zf
inc eax ; count the iteration
cmp dh, 0 ; set zf

loop%s loop_start
"""
for loop_kind in ['','z','nz']:
code_text = code_template % loop_kind
c = FuncObject(code_text)
for start_zf_value in [0,1]:
for num_iters in [1,4,10]:
for when_zf_changes in [1,2,15]:
c(num_iters, when_zf_changes, start_zf_value)
c.check()

Note that c(...) executes the code both on vial's VM, and on the real cpu. c.check() compares their return value (EAX) and flags after the execution. I also wanted to avoid other kinds of jumps in this test.

To check that the code ran the same number of times, I returned EAX as the number of iterations.
All the games with edx are there to make sure that I'm testing different zf conditions.

The challenge for today:
Can you write a shorter assembly snippet that tests the same thing?

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Some Assembly Required No. 1