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GAS is not very useful for writing bootsectors with as it insists on prefixing every memory access and jump/call with o32, this requires an additional 3 bytes that are unnecessary (1 for the prefix, 2 for 32bit address) which makes it hard to fit all the required code into the 512byte bootsector. I don't know whether this is just because the developers didn't think it was necessary or because LD lacks the ability to relocate 16bit addresses, in either case you will be better off just using NASM / FASM / YASM / whatever for this. GAS is otherwise ok for 16bit code provided you don't have space concerns. (Note that this was observed with an ELF Cross Compiler, an a.out cross compiler might not have this problem)

No, I'm not that incompetent, here's a code snippet:

Code: Select all

.text
.code16
ljmp $0, $start
start:
movw var1, %eax
jmp end
var1: .word 0x4A4A
end:
1:
jmp 1b

Something that is also particularly interesting: movw %ss:(%sp), %ax refuses to assemble (Quote: "Error: base/index register must be 32 bit register" - solution is to use %esp). Built with: gcc -o Test.o -c test.S; ld -o Test.bin --oformat binary --Ttext 0x7C00 Test.o
Output from the NASM Disassembler:

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00000000  66EA087C00000000  jmp dword 0x0:0x7c08
00000008  67A1117C0000      mov ax,[dword 0x7c11]
0000000E  66EB02            o32 jmp short 0x13
00000011  4A                dec dx
00000012  4A                dec dx
00000013  66EBFD            o32 jmp short 0x13

And contradictory to the ".code16" comment, it actually produces BETTER 16bit code in .code32 since the instructions have the same opcodes but it doesn't bother prefixing, however it creates a mess from the 2 NULL bytes inbetween every instruction. If your version of GAS doesn't make such a mess of it then please enlighten me to what configuration settings to use

[tt]66 ea 08 7c 00 00 00 00 67 a1 11 7c 00 00 66 eb 02 4a 4a 66 eb fd[/tt]

Changes in 2.10:

(...)

* Full 16-bit mode support for i386.