If I calculate an address of a label and store it in the eax register, how can I conditionally jump (using JE) to eax?
jmp eax
compiles, but I didn't check if it works.
je eax
doesn't compile (invalid combination of opcode and operands). Why the difference? And how can I jump if equal to eax?
There simply is no such form of je. What you can do is put a relative conditional jump based on the opposite condition, followed by an unconditional register-indirect jump:
jne skip
jmp eax
skip:
You could make a macro out of this to save you from writing the same thing over and over. For example, in NASM syntax that macro could look like this:
%macro je_reg 1
jne %%skip
jmp %1
%%skip:
%endmacro
And could be used like this:
je_reg eax
je_reg ebx
The macro could be generalized to work with any condition code:
%macro jcc_reg 2
j%-1 %%skip ; %-1 expands to the inverse of the condition in the first macro argument
jmp %2
%%skip:
%endmacro
; Example usage
jcc_reg e,eax
jcc_reg no,ebx
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