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

[Thomas Faber]

"ASSUME DS:.text" allows the assembler to use a smaller addressing
mode -- but then also (silently) refuses to use 32 bit memory operands.
E.g.
   mov ds:[01234h], ax
   mov ds:[012345678h], ax
becomes
    6:   a3 34 12                mov    %ax,0x1234
    9:   a3 78 56                mov    %ax,0x5678

I believe we used to use NASM for everything and were happy when we
could switch to using the compiler's native assembler. However I don't
remember if there was a particular reason other than eliminating a
dependency. Perhaps Timo or Amine know the details.


On 2017-01-17 10:23, Colin Finck wrote:
> 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
>
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