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GNU Assembler not making sense
Posted: Sun Aug 29, 2010 2:00 pm
by scanish
For reasons unknown I have to rewrite my startup assembly code for my kernel compatible for the GNU toolchain. I used the Netwide before, with Intel syntax, and my chums told me they want to stick strictly to the GNU chain. So I thought of it as a very nice opportunity to get familiar with AT&T syntax. But now it drives me mad.
I worked through a small reference on Memory Reference in Intel and in AT&T, but the hints there just give me errors I cannot work out.
This is the code I need to translate:
Code: Select all
mov eax, _isr%1
mov word [idt + 8 * %1], ax
mov word [idt + 8 * %1 + 2], OS_CODE_DESC
mov word [idt + 8 * %1 + 4], 0x8e00
shr eax, 16
mov word [idt + 8 * %1 + 6], ax
And my tries look like this:
Code: Select all
movl isr\num, %eax
movl idt, %ebx
movw %ax, 0(%ebx, 8, \num)
movw $OS_CODE_DESC, 2(%ebx, 8, \num)
movw $0x8e00, 4(%ebx, 8, \num)
shrl $16, %eax
movw %ax, 6(%ebx, 8, \num)
So. The problem is basically: Any position I try for the references either makes AS tell me it's a wrong syntax or a wrong scale.
Please help...
Re: GNU Assembler not making sense
Posted: Sun Aug 29, 2010 2:07 pm
by jrepan
You can use .intel_syntax directive to use Intel syntax with GNU assembler.
Re: GNU Assembler not making sense
Posted: Sun Aug 29, 2010 2:11 pm
by scanish
You can? But the dotted directives are the same as with AT&T? I didn't know as supports Intel, sorry.
Re: GNU Assembler not making sense
Posted: Sun Aug 29, 2010 2:16 pm
by jrepan
scanish wrote:But the dotted directives are the same as with AT&T?
I am quite sure they are (but I may be wrong, because I use NASM too). The simplest way to know for sure, is to just try and see if it works.
Re: GNU Assembler not making sense
Posted: Sun Aug 29, 2010 2:43 pm
by scanish
Thanks for the help. The .intel_syntax is not quite the same as with nasm, it still expects % on registers and $ on who knows what, but there was no quick documentary on it
It turned out the entire problem was that the gas does not like to have direct values in memory references. It wants them in a register, unlike nasm. But no documentary or translation site had any hint on that.
Sorry to have made that thread.
Re: GNU Assembler not making sense
Posted: Sun Aug 29, 2010 2:48 pm
by Owen
It would have helped if you told us specifically where the error was
Re: GNU Assembler not making sense
Posted: Mon Aug 30, 2010 1:15 am
by Solar
scanish wrote:Thanks for the help. The .intel_syntax is not quite the same as with nasm, it still expects % on registers and $ on who knows what, but there was no quick documentary on it
There's always ".intel_syntax no_prefix" to try.
Yes, GAS is somewhat under-documented.
Re: GNU Assembler not making sense
Posted: Mon Aug 30, 2010 1:47 am
by qw
This is incorrect:
GAS syntax is
So it should be
I haven't tested this, additional errors may occur.
Re: GNU Assembler not making sense
Posted: Mon Aug 30, 2010 10:18 am
by scanish
Yes, thanks, that works, because \num is a constant.
Works perfectly... thanks a lot!
Re: GNU Assembler not making sense
Posted: Mon Aug 30, 2010 12:40 pm
by qw
You're welcome!