Xref: utzoo comp.os.minix:7795 comp.sys.ibm.pc:37448 comp.unix.xenix:8369
Path: utzoo!yunexus!maccs!cs4g6ag
From: cs4g6ag@maccs.dcss.mcmaster.ca (Stephen M. Dunn)
Newsgroups: comp.os.minix,comp.sys.ibm.pc,comp.unix.xenix
Subject: Re: IBM and Apple Operating Systems (Re: dosread.c again)
Message-ID: <2550689E.13397@maccs.dcss.mcmaster.ca>
Date: 2 Nov 89 16:17:33 GMT
Article-I.D.: maccs.2550689E.13397
References: <254532d1@ralf>
Reply-To: cs4g6ag@maccs.dcss.mcmaster.ca (Stephen M. Dunn)
Organization: McMaster University, Hamilton, Ontario
Lines: 24

In article <254532d1@ralf> Ralf.Brown@B.GP.CS.CMU.EDU writes:
$What few people seem to remember is that IBM originally offered a choice
$of THREE operating systems: MSDOS, CPM-86, and UCSD p-System.  Of these,
$MSDOS was not only the cheapest, but also quite compatible with CPM-80,
$allowing mechanical translation of 8080 programs (in fact, MSDOS still
$supports the CP/M entry point and calling convention).

   CP/M-86 and p-System were indeed offered as alternative operating systems,
but they were slow to appear at dealers, cost more, and lacked software
to run under them.  Also, IBM released all of its own software for the PC
to run under MS-DOS.  Essentially, it's "You can use one of these three
operating systems ... but if you want to run our software (whic, of course,
you will, because we're IBM :-) you'd better buy this one."

   If you look at the structure of the PSP (Program Segment Prefix) under
DOS, you will indeed find lots of things that don't need to be there but
were left in to allow for the mechanical translation of CP/M programs.
Also, the first seven (and 23 of the first 27) DOS calls are similar or
identical to CP/M's BDOS calls.
-- 
Stephen M. Dunn                               cs4g6ag@maccs.dcss.mcmaster.ca
          <std_disclaimer.h> = "\nI'm only an undergraduate!!!\n";
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