Does UASM not like the register fs?

[Gunther]

This may not be a smart question, but I'll ask it anyway. I need the following statements in a protected mode program:

Code Select Expand


    mov      al, ds:[esi]
    mov      byte ptr fs:[edi], al

The first instruction assembles JWASM/UASM without complaint. The second completely correct instruction, which should lead to the machine code 646807h, generates an error 2183 (JWASM) or 2184 (UASM).

The same happens in the reverse case:

Code Select Expand


    mov      al, fs:[esi]

This should result as machine code 648A06h, but again results in the error message. This occurs in JWASM for DOS version 2.13 and also in the latest version 2.51 of UASM.

Something has obviously not been right for a long time.

Gunther

[nidud]

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[Gunther]

Nidud,

FS and GS is assumed to ERROR in flat mode:

    assume cs:flat,ds:flat,ss:flat,es:flat,fs:ERROR,gs:ERROR

Try this:

    assume  fs:nothing
    mov     al,fs:[edi]
    assume  fs:error

fine, but that doesn't help. It is a 32-bit protected mode program. I know exactly what I am doing and what fs and gs contain for descriptors.
Other assemblers translate these instructions correctly and without complaint. Do I need to hack in these commands via db in the future?

Gunther

[jj2007]

if it's not flat, ERROR, nothing, what is the correct name then?

[nidud]

deleted

[Gunther]

Nidud,

the workaround with the assume works well, but it's a bit awkward.

It seems to me as an incapacitation through the assembler maintainers. The DJGPP has fs as the only free segment register. It's not my decision.

Gunther 

[nidud]

deleted

[Gunther]

Casually, with the latest version of JWASM for DOS (version 2.13 from July 23, 2015) maintained by Japheth, the error no longer occurs. The corresponding instructions are translated correctly
and without complaint. The described behavior can only be found very exclusively in UASM.

For the further development of JWASM please also note this post.

Gunther

[johnsa]

I think we could safely NOT assume fs/gs to ERROR if the output format is not -coff ?

[Gunther]

Johnsa,

be careful, the DJGPP requires coff. It is the 32-bit com model.

Gunther

[Gunther]

Johnsa,

what is supposed to happen when fs and gs no longer assume to ERROR? JWASM for DOS from 2015 shows that it works.

When someone in protected mode uses the segment registers with a valid selector, they know exactly what they are doing.
If a novice fiddles with the segment registers in Protected Mode, he will simply get a GPF. This is the worst thing that can happen. So what?

Gunther

[johnsa]

Agreed, you can even use fs: in win32 .. I'm inclined to say just remove the ERROR assume outright.

[Gunther]

Johnsa,

Agreed, you can even use fs: in win32 .. I'm inclined to say just remove the ERROR assume outright.

I need register fs under DJGPP (32-bit Protected Mode DOS).

Gunther