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From: apratt@atari.UUCP (Allan Pratt)
Newsgroups: comp.sys.atari.st
Subject: Re: How can I find current DTA address w/o using TRAP #1?
Message-ID: <2318@atari.UUCP>
Date: 1 Oct 90 18:15:29 GMT
References: <5429@bdt.UUCP>
Organization: Atari Corp., Sunnyvale CA
Lines: 30

ADAM_TILGHMAN@bdt.UUCP writes:
>I need to trap all FSFIRST/FSNEXT calls (in order to substitute my own
>filenames), as well as FOPEN() calls (to change the filename to suit my
>needs)...
>I can't find the current DTA address;  is there any way that I can
>do this while processing a TRAP #1?

Since you're intercepting trap 1 calls, this should be cake.  Trap 1 is
not inherently non-reentrant, it's GEMDOS that makes it so.  Your code
could make a "real" trap 1 call into GEMDOS for the Fgetdta call.

Making a "real" trap 1 call can be done two ways: either you replace
the old trap 1 vector, make your call, then write yours in again,
or your "fake" the trap 1 by creating a stack frame and jumping to
the old vector.

Any time you catch GEMDOS or BIOS traps, or create a trap or interrupt
stack frame, you should be sure to check the system variable "longframe"
at $59e (word).  If it's nonzero, you are on a machine with "long 
frames" -- an extra word on the stack after the SR and PC.  Only  the
68000 has short frames; every other 680x0 has long frames.

I am answering this in such detail because I want to forestall somebody
else pointing out that you can look in the current process' basepage for
its current DTA pointer.  This is not a published fact, and therefore the
location and meaning of that variable is subject to change.

============================================
Opinions expressed above do not necessarily	-- Allan Pratt, Atari Corp.
reflect those of Atari Corp. or anyone else.	  ...ames!atari!apratt