CODEWITHSUNDEEP
assemblyx86nasm
user5673300
user5673300

Reputation:

NASM (Intel) ASM Error on pop

Every time I try to compile my code with NASM, where there is a "pop" ("pop al" in this case), it gives me an error.

20: error: invalid combination of opcode and operands

In line 20, I have 

pop al

What's the problem?

Upvotes: 0

Views: 466

Answers (1)

Ped7g
Ped7g

Reputation: 16596

The problem is literally the error message provided by the assembler: "invalid combination of opcode and operands". The POP instruction has no POP r8 encoding possible. In other words, you cannot pop an 8-bit value off the stack

The closest equivalent would be pop ax, which uses a POP r16 encoding to pop a 16-bit value off the stack into the ax register. However, this is not exactly the same as your code, because it will modify both the lower 8-bit half (al) and the upper 8-bit half (ah) of the 16-bit ax register.

Another alternative is to load the value directly from the stack, using something like mov al, BYTE PTR [(r/e)sp]. Because this does not alter the stack pointer, you will need to adjust it manually.

Next time, please also specify what your target platform is. This is important, because the 16-bit real mode has very limited possibilities for addressing modes, compared to 32-bit and 64-bit protected mode. If you don't specify which mode you're targeting, the answers you get may not work for you. Like here, I have no idea whether that mov instruction should use rsp (64-bit), esp (32-bit), or sp (16-bit).

Upvotes: 6

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