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assemblyx86sign-extensionzero-extension

Weird results with movzwl, %ax and negative values

Alright, so I am dealing with the following snippet of code:

push   %ebp
mov    %esp,%ebp   
push   %ebx
mov    0x8(%ebp),%eax 
movzwl %ax,%edx

So this behaves as expected when dealing with positive values. The value copied into %edx is the trailing 16 bits of %eax (or %ax).

However, if you put a negative number in, everything starts getting weird and it does not seem to be behaving as expected.

For example, if the value of %eax is -67043552, then the value copied into %edx is 65312.

I'm fairly new to assembly, sorry if this is an obvious misinterpretation on my part. Any help would be greatly appreciated.

Solution

  • Remember that movzwl copies only the bits in %ax into %edx filling in the high 16 bits of %edx with zeros.

    So %edx always ends up with a positive number less than or equal to 65535.

    In detail: -67043552 in hex is fc00ff20. So if that is in %eax, then %ax contains ff20. If you move that into %edx with zero-extension, then %edx gets 0000ff20. That's 65312.