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From: "J.W. Jagersma (jwjagersma AT gmail DOT com) [via djgpp AT delorie DOT com]" <djgpp AT delorie DOT com>
Date: Tue, 28 Mar 2017 18:47:06 +0200
Message-ID: <CAPLG5jZx4v-0QV+6F5xLALeQHGGiz2RUzkF3eOcYX7Y2MkiJGQ@mail.gmail.com>
Subject: Re: using fs/gs segment registers in djgpp?
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Thanks. So as long as I avoid using those headers, gs/fs are safe to
use? (can't use them anyway since I'm on -masm=intel)
The farptrgs.h header mentions movedata(), dosmemget() and dosmemput()
all use gs, but I don't see any of that in the disassembly. Really the
only functions that appear to use the extra segment registers are
dos_alloc_ok() and start(), and those only access fs.

On 28 March 2017 at 17:47, Eli Zaretskii (eliz AT gnu DOT org) [via
djgpp AT delorie DOT com] <djgpp AT delorie DOT com> wrote:
>> Date: Tue, 28 Mar 2017 08:19:11 -0700 (PDT)
>> From: "jwjagersma AT gmail DOT com [via djgpp AT delorie DOT com]" <djgpp AT delorie DOT com>
>>
>> I was wondering if gcc/djgpp ever uses the fs and gs registers. From disassembling a djgpp-compiled exe, I found some references to fs (no idea what's stored there though) but gs appears unused.
>
> Both are used, although FS is used more frequently.  See
> <sys/farptr.h> and <libc/farptrgs.h>; the latter is specifically for
> code internal to the library.