Hey there, let's break down exactly why you're hitting these errors with your mov instructions involving rbp. The root cause almost always boils down to a mismatch between your assembly code's addressing mode and the register/operand sizes you're using, and here's the breakdown:

1. You're Missing a BITS Directive (Most Likely Culprit) #

NASM defaults to 16-bit real mode if you don't explicitly tell it otherwise. The rbp register is a 32-bit extended register—and in 16-bit mode, you can't use 32-bit registers like rbp as base pointers for memory addressing. 16-bit mode only allows bx, bp, si, or di as base registers, with strict combinations (like [bx+si] but not [rbp]).

All the instructions you listed:

• mov [rbp], eax
• mov [rbp - 4], eax
• mov edi, [rbp]
• ...and the rest are fully valid in 32-bit or 64-bit protected mode. The error pops up because NASM is trying to parse them as 16-bit code, where rbp isn't a valid base register for memory access.

Fix: Add a BITS Directive at the Top of Your Code #

Specify the correct mode for your target program:

• For 32-bit x86 code:

  BITS 32
  ; Rest of your code here
  

• For 64-bit x86_64 code:

  BITS 64
  ; Rest of your code here
  

Make sure your NASM compile command matches too—use -f elf32 for 32-bit output, or -f elf64 for 64-bit.

2. Rare Case: Accidental Address/Operand Size Prefixes #

If you already have a BITS directive set, double-check for unintended prefixes that force a different address size. For example, an addr16 prefix in 32-bit mode would tell NASM to treat memory addresses as 16-bit, but using rbp (a 32-bit register) would create a conflict. Look for any stray db 0x67 (the addr16 prefix byte) in your code that might be messing things up.

3. Quick Validation Test #

To confirm it's the mode issue, try compiling a minimal test snippet:

BITS 32
section .text
global _start
_start:
    mov [rbp], eax
    mov [rbp - 4], eax
    mov edi, [rbp]
    mov esi, [rbp - 4]
    mov edi, [rbp - 8]
    mov [rbp - 8], eax

This should compile without errors using nasm -f elf32 test.asm—if it does, your original code's problem was definitely missing the BITS directive.

内容的提问来源于stack exchange，提问作者Bennett