Loading a register from a “db 0” doesn't load a 0 into EAX?

Question

I\'ve been bashing my head against the wall for over an hour and I can\'t understand why the below doesn\'t work. If I change b: db 1 to b: db 0 then i

Answer 1

You've declared b as a byte:

b: db 1

but you load it as a dword:

mov eax, dword [b]

This explains why the zero flag is unset even when b is 0: because it's loading the next 3 bytes as well.

Just change your declaration:

b: dd 1 

Alternatively, you could load a byte with zero extension: movzx eax, byte [b]

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Similarly, you load a qword from z but you only defined it as a dd dword. See Which variable size to use (db, dw, dd) with x86 assembly?

Also, use default rel so all addressing modes pick RIP-relative addressing without having to say [rel b] everywhere.