Far pointers in assembly

[chibicitiberiu]

Some bios interrupts need stored in [es:di] (like get vesa information).
How do I store a pointer like this, es = segment, di = offset?

This doesn't work in nasm. If I remove 'offset' it says that the output file doesn't support segment or something:
mov ax, seg value
mov es, ax
mov di, offset value

Note that the code is part of the 2nd stage bootloader, and it's mixed 16 and 32 bit code.

How is it correctly done? I can't find any good example online.

[Combuster]

if you know the seg and offset, you can store it to any other register or memory location, which will thus allow you to pass it around.

Basically a 16-bit far pointer is a 32-bit struct containing two 16 bit unsigned values.

[chibicitiberiu]

Okay, now the actual address in Segment:Offset looks like this, right?

Address = Segment * 0x10 + Offset;

So this means that:

Segment = (one of the many possibilities) Address / 10h;
Offset = Address % 10h;

Is this correct?

[chibicitiberiu]

Okay so I tried to do this:

Code: Select all

      xor         eax, eax
      xor         ebx, ebx
      xor         edx, edx
      mov         dword [TemporaryStorage], vbeControllerInfo
      mov         ax, word [TemporaryStorage+2]
      mov         dx, word [TemporaryStorage]
      mov         bx, 10h
      div         bx
      mov         es, ax
      mov         di, dx
      mov         ax, 0x4f00
      int         10h
      cmp         ax, 0x004F

It freezes when it reaches the div instruction. What could be the problem?

[a5498828]

quotient overflow?

[Brendan]

It freezes when it reaches the div instruction. What could be the problem?

You've got the high word and low word around the wrong way, which is probably causing an overflow error (e.g. dx:ax divided by 16 doesn't fit in 16-bits).

Also, you probably shouldn't be using division in the first place. You could use shifts instead (which is faster and easier), you may be able to get the assembler to calculate it (which is even faster), and the segment register could/should already be correct (which is extremely fast because you don't need to do anything).


Cheers,

Brendan

[gerryg400]

You should use bitwise operators for this.

[chibicitiberiu]

I searched yesterday all the net for bitwise operators in assembly, and found nothing other than AND, OR, XOR...
How do I do bit shifts? Because that was my first idea, just couldn't find the right instructions to do it.

[gerryg400]

shr etc.

[chibicitiberiu]

shr etc.

shr and shl right? Thanks a lot now it works