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assemblyoperating-systemnasminstructions

NASM - Using several BITS directives in one file

I am writing a second stage bootloader, a part of whose responsibility will be to enter 32 bit protected mode from 16 bit real mode.

My code has some initializing real mode code, a few real mode functions, protected mode code and a few protected mode ones. Here's what I had done (the actual code is in the same order):

  • Added BITS 16 before the starting of first real mode function.

  • Added BITS 32 before the first protected mode function

  • Added BITS 16 before the initializing real mode code

  • Added BITS 32 before the initializing protected mode code (basically before mov cr0, eax after ORing eax with 1.

This didn't work, so I read NASM's manual which said its not necessary to use this directive. I removed all of them and the code worked!

So what was the mistake in my approach ? Also, am I somehow mixing up bitness and the processor mode ?

Solution

  • BITS 32 would go after mov cr0, as that is still executed in real mode. You shouldn't switch to BITS 32 until you do the JMP 0x08:ProtectedModeMain instruction,