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Boot Loader Setup
Posted: Tue Sep 06, 2011 10:59 am
by brodeur235
I am writing a two stage bootloader and the two modules are linking correctly, and when I jump to the second stage, the first stops executing (tested); so the jump is working, however, stage_two (which I jump to) never executes. I think it might have something to do with segment register setup; I haven't done anything with the segment registers yet, but when I try to initialize them with something like:
Code: Select all
mov ax,0x00
mov cs,ax
mov ds,ax
mov es,ax
mov fs,ax
Then code stops executing at that point; so I assume that's wrong..
Here's the source's that pertain:
stage_one.asm:
Code: Select all
extern _stage_two
[BITS 16]
global _start
_start:
; disable interrupts
cli
; clear screen
mov ah,0x07
mov bh,0x07
mov al,0x00
mov ch,0x00
mov cl,0x00
mov dh,0x19
mov dl,0x50
int 0x10
; reposition cursor
mov ah,0x02
mov bh,0x00
mov dh,0x00
mov dl,0x00
int 0x10
jmp _stage_two
; Padding and boot flags
times 510-($-$$) db 0x00
db 0x55, 0xAA
stage_two
Code: Select all
[BITS 16]
global _stage_two
_stage_two:
; Code here will not execute.
; LOOP
jmp $
Help very much appreciated,
Brodeur235
Re: Boot Loader Setup
Posted: Tue Sep 06, 2011 11:10 am
by AJ
Hi,
I'm a little confused to see the use of 'extern' and 'global' in flat binaries.
Have you checked the register dump? Where is CS:IP when stage 1 finishes? Where would you expect it to be? Is stage2 loaded from disk somewhere (the BIOS only loads the boot sector and you need to read the rest yourself, normally via BIOS calls)?
Also, what do you mean by 'stops executing'? Have you hit a hlt instruction? Are you in an infinite loop (jmp $)? Are you executing code from a random piece of uninitialised memory?
More information, please, including a Bochs register dump and a summary of what you would expect to see.
Cheers,
Adam
Re: Boot Loader Setup
Posted: Tue Sep 06, 2011 11:12 am
by AJ
Also:
You say this is tested - how?
Cheers,
Adam
Re: Boot Loader Setup
Posted: Tue Sep 06, 2011 1:16 pm
by brodeur235
I just asked a really f*cking dumb question. I haven't done OS design in a while please forgive me. I did not load it into memory... Good call
Brodeur235
Re: Boot Loader Setup
Posted: Tue Sep 06, 2011 2:23 pm
by AJ
np - we all make mistakes
Re: Boot Loader Setup
Posted: Tue Sep 06, 2011 9:19 pm
by Chandra
brodeur235 wrote: I did not load it into memory... Good call
That's not the only problem. As AJ pointed out, you need to throw out
extern and
global symbols out of your assembly file. Moreover, make sure the segment registers are initialized properly before you make any BIOS calls.
Cheers!
Re: Boot Loader Setup
Posted: Wed Sep 07, 2011 1:52 am
by mark3094
You may also need to specify where the second stage is executing from. Something like:
I think this is dependant on the assembler. Then the segments can be set to 0x0:
Code: Select all
CLI ; Disable interrupts while changing segment registers
XOR AX, AX ; Set AX to zero
MOV DS, AX ; Set DS and ES to zero. This really sets DS and ES to the location we are executing from
MOV ES, AX
MOV SS, AX ; SS:SP is the location of the stack
MOV SP, 0xFFFF ; SS:SP = 0000:FFFF
STI ; Enable interrupts