Path: utzoo!utgpu!attcan!uunet!seismo!sundc!pitstop!sun!imagen!atari!apratt
From: apratt@atari.UUCP (Allan Pratt)
Newsgroups: comp.sys.atari.st
Subject: Re: bios function 0x7f
Message-ID: <1261@atari.UUCP>
Date: 14 Dec 88 19:39:32 GMT
References: <12076@hall.cray.com>
Reply-To: apratt@atari.UUCP (Allan Pratt)
Organization: Atari (US) Corporation, Sunnyvale, California
Lines: 25

In article <12076@hall.cray.com> rosenkra@hall.UUCP (Bill Rosenkranz) writes:
> [...] it executes bios function 0x7F [...]

There is no such function.  The only way it could usefully do anything
with that BIOS call is if it installed a handler itself, or if it
is probing for another installed handler which it knows about.
(That is, if some other product installs a handler for BIOS call 0x7f,
and this program knows how to use that other product.)

It is a documented guarantee that if you make a BIOS or XBIOS call and
there is no handler for that function number, the function number itself
will be returned in D0.W.  If there *is* a handler there, it is free to
return whatever it wants, of course, but every attempt should be made
*not* to return the function number in D0.W so you can tell that it's a
handler, not an invalid function number.  (If you never know what
you'll be returning, there should be a special sub-function of your
handler which is the "probe" call -- it should return a known value
which is not the function number, just to prove it exists.)

This guarantee holds true for all versions of ROM TOS: things turned
out this way, as it's so useful we decided to document the fact.

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