Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Posting-Version: version B 2.10.3 alpha 4/3/85; site ukma.UUCP
Path: utzoo!watmath!clyde!burl!ulysses!mhuxr!mhuxt!houxm!ihnp4!cbosgd!ukma!sambo
From: sambo@ukma.UUCP (Inventor of micro-S)
Newsgroups: net.arch,net.lang,net.micro,net.micro.pc
Subject: Integer division on the Intel iAPX8[6,8]
Message-ID: <2018@ukma.UUCP>
Date: Mon, 5-Aug-85 01:07:58 EDT
Article-I.D.: ukma.2018
Posted: Mon Aug  5 01:07:58 1985
Date-Received: Tue, 6-Aug-85 20:05:51 EDT
Organization: U of Kentucky, Mathematical Sciences, Lexington KY
Lines: 31
Xref: watmath net.arch:1648 net.lang:1666 net.micro:11321 net.micro.pc:4833


     I am currently working on a compiler for a language I have designed, and
whose target machine is the IBM PC.  I happened to look at the description of
the IDIV instruction for the Intel 8086 and 8088.  It says that the minimum
negative quotient for a byte division is -127, and the minimum negative quo-
tient for a word division is -32767.  I tested this, and lo and behold, both
-128 / 1 and -32768 / 1 do indeed generate a division-by-zero interrupt.  I
am wondering three things:
     1) Why does this happen - i.e. what hardware algorithm does this proces-
sor use?
     2) Are there any compilers for a machine using the 8086 or 8088 that can
handle both -128 / 1 and -32768 / 1 correctly?  (IBM Pascal Version 1.00 does
not.)
     3) Are there any other processors that behave the way the 8086 and 8088
do in this respect?  (I tested this on both our VAX 11/750 and our 3B2/300 by
writing a C program and modifying the assembly language output, and both of
them work correctly.)
     By the way, the algorithm on page 2-73 of the iAPX88 Book dated July 1981
is wrong - it implies that the maximum negative quotient is -128 and -32768
for byte and word division respectively, which is clearly contradicted in the
prose description for the instruction.
     P.S.: I know some of you may be tempted to express your great admiration
for the totality of Intel's product line, but this is not what I am looking for.
-----------------------------------------
Samuel A. Figueroa, Dept. of CS, Univ. of KY, Lexington, KY  40506-0027
ARPA: ukma!sambo<@ANL-MCS>, or sambo%ukma.uucp@anl-mcs.arpa,
      or even anlams!ukma!sambo@ucbvax.arpa
UUCP: {ucbvax,unmvax,boulder,oddjob}!anlams!ukma!sambo,
      or cbosgd!ukma!sambo

	"Micro-S is great, if only people would start using it."