[ros-dev] MASM using longer opcode sequences than GAS

[Colin Finck]

Hi all,

In order to make our ISOs flashable to USB drives and bootable from them
(cf. CORE-12648), I'm currently overhauling our ISO boot sector
"isoboot.S". This adds a lot of code and while I stay in the 2K size
limit when assembling it with GAS, I break it with MASM.

Disassembling the assembled files shows the difference.
A line like:

   mov word ptr ds:[GetlinsecPtr], ax

is assembled by GAS to:

   A35B50            mov [0x505b],ax

while MASM does this:

   67A35B500000      mov [dword 0x505b],ax


Is there any option to enforce the shorter opcode sequence for MASM too?
The code uses a lot of similar MOVs and this seems to be the only reason
why I break the 2K size limit with MASM.

If there is no such option, what are the advantages of using MASM for
our MSVC build anyway? Can't we just use GAS for both or decide on a
common third-party assembler (NASM) for our assembly files?


Cheers,

Colin