CODEWITHSUNDEEP
assemblyx86nasm
SamTebbs33
SamTebbs33

Reputation: 5647

x86 Access a label without including the file

I have a simple kernel in development along with a bootloader. Before the boot loader goes into protected mode, I would like to use interrupts to retrieve the amount of memory on the system (int 0x12) and then set the value held by a label to the amount of ram that I retrieved. Once the kernel has loaded (in protected mode) I would like to access the data.

At first was going to use the following structure:

; sysinfo.asm
RAM: dd 0 ; declare RAM as a 4 byte label

; boot.asm
%include "sysinfo.asm"
; bootloader code here
xor ax, ax
int 0x12
mov [RAM], ax
; go into protected mode and launch kernel

; kernel.asm
%include "sysinfo.asm"
mov ax, [RAM]
; print ax

However as I expected to happen, since the RAM label in boot1.asm and the RAM label in kernel.asm are totally different, they don't point to the same address, how could I do this?

Upvotes: 1

Views: 156

Answers (2)

Mika Lammi
Mika Lammi

Reputation: 1298

I recommend defining a structure that holds the information gathered in loading process and then pass the address of the structure to kernel in some of the registers.

sysinfo.asm:

struc BootInfo
  .ram resd 1
  ; .. some other useful information ...
endstruc

boot.asm:

%include "sysinfo.asm"

bootinfo: istruc BootInfo
  at ram, dd 0
iend

; ....
xor ax, ax
int 0x12
mov [bootinfo + BootInfo.ram], ax ; set amount of ram
; ...
mov edx, bootinfo ; pass address of BootInfo in some register
; goto kernel code

kernel.asm:

%include "sysinfo.asm"

; Address of BootInfo in edx
mov eax, [edx + BootInfo.ram] ; get ram to eax
; ...

Upvotes: 1

User.1
User.1

Reputation: 2642

Two different names in two different files; or is it three ? Whatever

The first one... 

        RAM: dd 0 ; declare RAM as a 4 byte label

Becomes...

        Boot_Dot_Asm_RAM: dd 0 ; declare RAM as a 4 byte label

The second one Becomes...

        Kernal_Dot_Asm_RAM: dd 0 ; declare RAM as a 4 byte label

You can do this with "conditional assembly".

If you need pointers on that, just ask.

Upvotes: 0

Related Questions