Mixing 32 bit and 16 bit code with nasm

Question

Hi,

This is a low-level systems question.

I need to mix 32 bit and 16 bit code because I'm trying to return to real-mode from protected mode. As a bit of background information, my code is doing this just after GRUB boots so I don't have any pesky operating system to tell me what I can and can't do.

Anyway, I use [BITS 32] and [BITS 16] with my assembly to tell nasm which types of operations it should use, but when I test my code use bochs it looks like the for some operations bochs isn't executing the code that I wrote. It looks like the assembler is sticking in extras 0x66 and 0x67's which confuses bochs.

So, how do I get nasm to successfully assemble code where I mix 32 bit and 16 bit code in the same file? Is there some kind of trick?

Terry

Answer 1

You weren't kidding about this being low-level!

Have you checked the generated opcodes / operands to make sure that nasm is honoring your BITS directives correctly? Also check to make sure the jump targets are correct - maybe nasm is using the wrong offsets.

If it's not a bug in nasm, maybe there is a bug in bochs. I can't imagine that people switch back to 16-bit mode from 32-bit mode very often anymore.

Answer 2

The 0x66 and 0x67 are opcodes that are used to indicate that the following opcode should be interpreted as a non-default bitness. More specifically, (and according to this link),

"When NASM is in BITS 16 mode, instructions which use 32-bit data are prefixed with an 0x66 byte, and those referring to 32-bit addresses have an 0x67 prefix. In BITS 32 mode, the reverse is true: 32-bit instructions require no prefixes, whereas instructions using 16-bit data need an 0x66 and those working on 16-bit addresses need an 0x67."

This suggests that it's bochs that at fault.

Answer 3

If you're in real mode your default size is implicitly 16 bits, so you should use BITS 16 mode. This way if you need a 32-bit operand size you add the 0x66 prefix, and for a 32-bit address size you add the 0x67 prefix.

Look at the Intel IA-32 Software Developer's Guide, Volume 3, Chapter 16 (MIXING 16-BIT AND 32-BIT CODE; the chapter number might change according to the edition of the book):

Real-address mode, virtual-8086 mode, and SMM are native 16-bit modes.

The BITS 32 directive will only confuse the assembler if you use it outside of Protected Mode or Long Mode.

Answer 4

The problem turned out to be that I wasn't setting up my descriptor tables correctly. I had one bit flipped wrong so instead of going to 16-bit mode I was going to 32-bit mode (with segments that happened to have a limit of one meg).

Thanks for the suggestions!

Terry