fasm and Grub

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flash

Joined: 11 Mar 2006
Posts: 55
Location: Cuba

flash 24 Mar 2012, 03:37

I am testing a Grub boot fasm code with a "stolen" nasm one. But i am having some troubles with a label address(i think) at "multiboot_entry" for example:

Code:

;
; Imagen de CD arrancable, para compilar con fasm
;

MULTIBOOT_HEADER_MAGIC     equ 0x1BADB002
MULTIBOOT_HEADER_FLAGS        equ 0x00010011
CHECKSUM                              equ -(MULTIBOOT_HEADER_MAGIC + MULTIBOOT_HEADER_FLAGS)
LOADBASE                              equ 0x00100000
STACK_SIZE              equ 0x4000
org LOADBASE

                    jmp             multiboot_entry
                     align   4

multiboot_header:
                      dd MULTIBOOT_HEADER_MAGIC
                   dd MULTIBOOT_HEADER_FLAGS
                   dd CHECKSUM
                 dd multiboot_header
                 dd LOADBASE
                 dd 00
                       dd 00
                       dd multiboot_entry
     
multiboot_entry:
                 mov             esp,pila + STACK_SIZE
                       push    0
                   popf
                        push    ebx

                     mov             edi,0B8000h
                 mov             cx,2000
     clrscr:     mov             [edi],byte ' '
                    add             edi,2
                       loop    clrscr
                      
                    include 'code.asm'
pila:
                   align   4
                   times (STACK_SIZE) db 0

The thing is that exactly same code works fine with nasm. What could be wrong with label address?

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24 Mar 2012, 03:37

revolution
When all else fails, read the source

Joined: 24 Aug 2004
Posts: 20335
Location: In your JS exploiting you and your system

revolution 24 Mar 2012, 04:03

Your loadbase is at 1M. 16-bit code cannot jump to an address above 0xffff. Did you forget to place use32 at the start?

24 Mar 2012, 04:03

flash

Joined: 11 Mar 2006
Posts: 55
Location: Cuba

flash 25 Mar 2012, 00:15

Thank's revolution, that's solve the problem!
But... i though fasm compiles 32bit code by default Sad

Good to know Smile

25 Mar 2012, 00:15

egos

Joined: 10 Feb 2009
Posts: 144

egos 25 Mar 2012, 20:26

Grub supports BSS so no reason to put the bits into the file to reserve space for stack.

Code:

; --------------------
; Kernel stub for GRUB
; --------------------

MBH_MAGIC equ 0x1BADB002
MBH_FLAGS equ 0x10000

  use32

  org 0x100000
load_addr:
header_addr:
  dd MBH_MAGIC
  dd MBH_FLAGS
  dd -MBH_MAGIC-MBH_FLAGS
  dd header_addr
  dd load_addr
  dd load_end_addr
  dd bss_end_addr
  dd entry_addr

entry_addr:
  jmp $

load_end_addr:
; align 4
; rb STACK_SIZE
bss_end_addr:

25 Mar 2012, 20:26

edfed

Joined: 20 Feb 2006
Posts: 4333
Location: Now

edfed 25 Mar 2012, 21:44

as x86 is real mode 16 bits by default, fasm compiles in 16bits by default too.

don't know if it's for that reason, but if somebody asks, i think it can be a good answer cause it's logical.

welcome in the flat assembly orthogonal universe. (not parallel, parallel means same direction) Smile

25 Mar 2012, 21:44

flash

Joined: 11 Mar 2006
Posts: 55
Location: Cuba

flash 28 Mar 2012, 04:53

mmmm... the conclusion is something like: put allways "usexx" directive at binary code Wink

28 Mar 2012, 04:53

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