Path: utzoo!utgpu!news-server.csri.toronto.edu!clyde.concordia.ca!mcgill-vision!snorkelwacker!tut.cis.ohio-state.edu!purdue!haven!mimsy!jds
From: jds@mimsy.umd.edu (James da Silva)
Newsgroups: comp.lang.scheme
Subject: Re: (none)
Message-ID: <26034@mimsy.umd.edu>
Date: 16 Aug 90 05:47:17 GMT
References: <9008151749.AA23832@mailhost.samsung.com>
Reply-To: jds@cs.umd.edu (James da Silva)
Organization: University of Maryland, Department of Computer Science
Lines: 33

In article <9008151749.AA23832@mailhost.samsung.com> gjc@mitech.com writes:
>>From: Alfred Kayser
>>I've once tryed to port it to MSDOS or OS2 (compiler and cpu are the same)
>>but it crashed enormously. The code was too unreadable to be debugged.
>
>Under the microsoft C compiler, small code and data model at least the 1.3
>version of SIOD compiled and ran the first time I tried it.

I would guess that SIOD wants sizeof(pointer) == sizeof(int).  This holds
for 8086 small model but not for large model, where sizeof(pointer) == 4.

>Anybody who would want to utilize the large data model 80286 mode would
>have to be an expert MSDOS/C programmer, I am told by someone who did
>a port. The details were something I had a hard time grasping. Very strange
>stuff along the lines of PADDING the "struct obj" structure so it would
>be 16 bytes long instead of 10 bytes long.

This is an old 8086 programmer's trick.  On the 86 addresses are divided
into two 16-bit parts; the segment, and the offset.  To get the physical
address, the hardware calculates `segment << 4 + offset'.  Thus segment
numbers have 16 byte granularity.  If all your objects are 16-byte aligned,
you can use just the segment number as the address (the offset is always 0),
thus you are back to sizeof(address) == sizeof(int).

Rather than using this 86-specific hack, it would be better to just make the
SIOD code portable to architectures where ints, pointers, and longs are
not interchangeable.  I think making SIOD pass lint would be enough.  Any
volunteers?

Jaime
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: domain: jds@cs.umd.edu				     James da Silva
: path:   uunet!mimsy!jds	 	    Systems Design & Analysis Group