From mboxrd@z Thu Jan  1 00:00:00 1970
X-Spam-Checker-Version: SpamAssassin 3.4.4 (2020-01-24) on polar.synack.me
X-Spam-Level: 
X-Spam-Status: No, score=-0.8 required=5.0 tests=BAYES_00,INVALID_DATE
	autolearn=no autolearn_force=no version=3.4.4
X-Google-Language: ENGLISH,ASCII-7-bit
X-Google-Thread: 103376,18385551e0d37b37
X-Google-Attributes: gid103376,public
X-Google-ArrivalTime: 1994-10-31 12:25:43 PST
Path: 
 bga.com!news.sprintlink.net!howland.reston.ans.net!news.moneng.mei.com!hookup!yeshua.marcam.com!news.mathworks.com!news2.near.net!news.delphi.com!usenet
From: tmoran@bix.com
Newsgroups: comp.lang.ada
Subject: Re: Ada replacements for DOS I/O
Date: 31 Oct 1994 19:31:51 GMT
Organization: Delphi Internet Services Corporation
Message-ID: <393gn7$kmf@news.delphi.com>
NNTP-Posting-Host: bix.com
Date: 1994-10-31T19:31:51+00:00
List-Id: <comp.lang.ada>

>The most logical step is of course to write the IO functions we need
There are two problems: traditional hard disk IO in DOS is done via
CPU instructions, not DMA, so it's not convenient to interrupt in
the middle - and you couldn't get much IO/CPU overlap anyway.
The more general problem is that MSDOS is not re-entrant.
So the effect is as if there is a single (DOS) task that does
any and all DOS activity.  Any task wanting a DOS action must
wait for rendezvous with that single DOS task.
  The first problem is lessened under DOS by doing disk IO with
large block sizes, full track reading, disk caching, etc.
For the second problem, you'll have to write your own multi-tasking
DOS.  A non-trivial undertaking.  (OTOH, IBM and Microsoft have
already done that and sell the results as OS/2 and Windows NT. ;)
  Is your problem that you would like useful CPU cycles overlapped
with disk IO, or that (presumably due to interrupts) high priority
tasks can suddenly become unblocked while a low priority task is
running inside non-preemptible DOS?