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I have the following disassembly:

[dest] = d5 cd e8 ca 68

movzx   eax, [ebp+dest]
# value of edx at this point is: F7FBB898
movsx   edx, al
# value of edx after this is: FFFFFFD5

# [ebp+var_E] stores 0
movzx   eax, [ebp+var_E]
movsx   eax, al
# eax = 0 here
add     eax, edx
# eax becomes FFFFFFD5
cmp     eax, 0D5h
jnz     short loc_565564E6

I have given the explanation and flow for each instruction below:

  1. It reads a byte from the [dest] and stores it in eax.
  2. Value of edx initially is: F7FBB898. After, movsx edx, al instruction it becomes FFFFFFD5. How can I make sure the value of edx will be 0x000000d5 at this point?

What should be my initial value in [dest] so that after these operation, the final value in eax is 0xd5 and not 0xFFFFFFD5

Peter Cordes
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Neon Flash
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    In other words, you don't want sign extension, you want zero extension. – Rudy Velthuis Apr 02 '19 at 09:49
  • Note that your first instruction, `movzx eax, [ebp+dest]`, is ambiguous: it doesn't specify a size for the memory source, so it could be zero-extending a byte or a word into EAX. And since EAX is clobbered in the next instruction, it's entirely pointless to zero-extend first into EAX before sign-extending the low byte into EDX. `movsx edx, byte [ebp+dest]` / `movsx eax, byte [ebp+var_E]` would be exactly equivalent to the first 4 instructions. This is probably compiler-generated code with no or minimal optimization, resulting in braindead code like this. – Peter Cordes Apr 02 '19 at 12:45

1 Answers1

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You should use movzx (Move with Zero-Extend) instead of movsx (Move with Sign-Extension) if you want to extend the value with zeros without looking at its sign.

When you use movsx to move 0xd5 to edx, it will copy the lower to bytes into edx and fill the remaining with the MSB of the copied value (0xd5 = 0b11010101, the MSB is 1), which fills the 6 remaining bytes with 0xFFFFFF. With movzx, the remaining bytes are filled with 0x000000 regardless of the MSB.

Simon Doppler
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