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From: russ@m-net.ann-arbor.mi.us (Russ Cage)
Newsgroups: alt.msdos.programmer,comp.sys.ibm.pc.programmer
Subject: Re: Why won't DOS allow more than 20 open files?
Summary: Hard table size limit in the PSP.
Message-ID: <1990Jun16.161612.2509@m-net.ann-arbor.mi.us>
Date: 16 Jun 90 16:16:12 GMT
References: <90160.191651JXS118@psuvm.psu.edu>
Organization: M-net -- Public Access UNIX & Conferencing: (313)994-6333
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In article <90160.191651JXS118@psuvm.psu.edu> JXS118@psuvm.psu.edu (Jeff Siegel, Op from Atherton Hall) writes:
>Even though I have the statement FILES = 40 in my config.sys, DOS refuses
>to open more than 20 files at once and returns error code 4 (no handles
>available) if I try to open more. I've tested this on 3 different version
>of DOS (all of which allegedly support the FILES statement) Any clues why this
>is?

Down in the PSP of your process, there is a table which maps
handle numbers for your process to DOS internal file reference
numbers.  (Examine the table and you will see entries of 01
for stdin, stdout, and stderr on an interactive process.  I
think 02 is stdprn unless redirected, dunno what 0 is.  The
unused entries are marked with 0xff.)  This table is only
20 bytes long; it goes from PSP:0018 to PSP:002b.

However, this table can be re-located.  In DOS 3.x, there is
a FAR pointer to the handle table at PSP:0034, and the length
is at PSP:0032.  So, by moving the table to an area of memory
with more space, filling the remainder with 0xff, and changing
the pointer to (and length of) the table in your PSP, your
process can have more than 20 files open at once.  I have done
this, it works quite well.  Condor 3 v. 2.20 uses it to good
effect.

My knowledge of this ends with DOS 3.x, there may be cleaner
methods of doing this with more recent versions of DOS.
-- 
  I am paid to write all of RSI's opinions.  Want me to write some for you?
(313) 662-4147        Forewarned is half an octopus.
Russ Cage, Robust Software Inc.            russ@m-net.ann-arbor.mi.us