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OK, here is the situation. My bootloader sets up a pmode stack, loads in a GDT and jumps to my kernel code, which is currently located at 0x10000 physical.
Then the kernel sets up a new GDT. Works fine too. But, I have an asm function called _load_gs.
The C declaration is "extern void load_gs(unsigned int newfs);

asm code is as follows:
_load_gs:
push ebp
mov ebp,esp
xor eax,eax
mov eax, [ds:ebp+4] ; 1st param passed by C according to all docs I have
mov gs,eax
pop ebp
retn

Can anybody tell me what I could possibly be doing wrong?!? From what a routine I wrote tells me, eax contains gibberish, and not the 1st parameter passed by C. BTW, ds and ss are the same.

Thanks for any help
   Hunter Seeker

The EBP should offset to a location in the stack segment, not the data segment. Thus, the line where you get the argument should read

mov eax, [ss:ebp+4]

Actually, I don't think you need the segment selector at all, as offsets of EBP default to the stack, IIRC.

Shouldn't matter, though... he said ds and ss are the same selector.

Problem is (I _think_), if ds and ss are the same, then ss is, no doubt, an expand down segment (as is a data segment).

mov eax, [ss:ebp+4]
Is notation assuming an exapand _up_ stack.  In your case, with an exapand down, you're actually accessing part of the stack that hasn't even been written to yet.

You actually want [ss:ebp-4] (is that even possible?  Negative displacements in an address reference?)

Also, "mov gs, eax" should really be "mov gs, ax" seeing as though selectors at 16-bit, and eax is 32.

Jeff

>mov eax, [ds:ebp+4] ; 1st param passed by C according to all docs I have

You don't need the override. Also, for 32-bit code, the left-most argument
on the stack frame is at ebp+8 (ebp+0 is the old EBP, and ebp+4 is the
old EIP [return address]). Try this:

mov eax, [ebp+8]

Thanks geezer, I will remember it for future reference. I solved my problem though by replacing it with inline assembler. I was wondering why the contents of [ebp+4] looked an awful lot like a memory address from somewhere in my kernel.

Thanks for quick replies
  HuntrCkr