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How to know first instruction in GAS assembler?
Posted: Sat Sep 01, 2012 7:30 am
by xiangzhc
What I want to achieve is to calculate the length of the code segment.
In Masm Compiler, we can get it just through
But how to get it through GAS assembler?
Here's my code, but when compiling, it returns
" Error: bad or irreducible absolute expression"
Code: Select all
.text
.global _start
#.code16
_start:
jmp over
os_size:
.word 0
.word 0
over:
movw $100, %ax
forever:
jmp forever
foo:
.rept . - _start
.byte 0
.endr
Re: How to know first instruction in GAS assembler?
Posted: Sat Sep 01, 2012 12:04 pm
by Owen
GAS isn't designed to generate binary files. You must use a linker script in order to guarantee the correct padding
Re: How to know first instruction in GAS assembler?
Posted: Sat Sep 01, 2012 2:10 pm
by xiangzhc
Owen wrote:GAS isn't designed to generate binary files. You must use a linker script in order to guarantee the correct padding
Thanks for your reply. Could you please add more code here? So that I can search for that.
By the way, when linking, we can specify segment location like:
Code: Select all
ld -Ttext=0x0 -o bootblock bootblock.c
The abovementioned code script is taken from our assignment. What confused me is the fact that bootblock/bootloader would be loaded into 0x07c00 by BIOS, but why we specify code segment to 0x0?
Does that imply the cs register is assigned to 0x07c0 automatically?
Re: How to know first instruction in GAS assembler?
Posted: Sat Sep 01, 2012 3:42 pm
by Combuster
...okay...
Linking
source files, asking about homework, you just broke two forum rules like really hard.
Go back to asking your teacher. You paid him after all.