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From: russ@crlt.UUCP
Newsgroups: comp.os.minix
Subject: Re: Minix and compiler models
Message-ID: <631@crlt.UUCP>
Date: Wed, 11-Feb-87 12:58:27 EST
Article-I.D.: crlt.631
Posted: Wed Feb 11 12:58:27 1987
Date-Received: Fri, 13-Feb-87 05:36:32 EST
References: <966@ulowell.cs.ulowell.edu> <1565@cit-vax.Caltech.Edu> <500@bobkat.UUCP>
Organization: CRLT , Ann Arbor, MI
Lines: 52
Summary: Correction on 8086/88 memory models.

In article <500@bobkat.UUCP>, m5d@bobkat.UUCP writes:
>I have BIG BIG problems believing that Minix allows separate code,
>data, and stack segments.  This is NOT small model, at least not as
>defined by Intel.  In fact, it's no model at all:
>
>	Small:	one code seg, one data seg (incl. data, stack, and constants)
>	Compact: code, data, stack, and "memory"
>	Medium: one code seg. per module, only one each data, stack, "memory"
>	Large: one code seg & one data seg per module, one stack and 
>        one "memory"

Memory models supported by Lattice C:

	S (small):	64K code, 64K data/stack.  16 bit pointers.
	P (Prog):	To 1 MB code, 64K data/stack.  16 bit data
			pointers.  Function pointers by lookup table.
	D (data):	64K code, to 1 MB data, 64K stack.  32 bit
			pointers.
	L (large):	To 1 MB code, 1 MB data, 64K stack.  32 bit
			pointers.

>[...] It could be that PC-DOS calls >this 8085 model "small".

Nope, the combined I&D (64K total) model is called the 8080 model.

>In all the models except "small" (or 8085 model, which is almost the
>same for the purposes of this discussion), pointers MUST be 32 bits.
>Why?  Because otherwise the location referenced by the pointer cannot
>be determined.

The 8080, small and P models can use 16-bit pointers, because they have
only 64K of data/stack space.  Separate I & D segments do not affect the
required pointer size unless you want to do something silly like write
over your code.  The P model (with more than 64K of code) can still use
16 bit pointers to data; its pointers to functions are indirect through
a table of function addresses (32 bits/entry, of course).

So far as relocatability is concerned, any 8080 or S model program can
be shoved to an arbitrary segment address dynamically and all pointers
will still be valid.  A P model program could survive relocation of its
data segment; if the code was re-entrant and re-used, then a P model
program could also be fork()ed without difficulty.  D and L model programs,
forget it.  It looks like Minix supports the 8080 and S models only,
which is not suprising.
-- 
The above opinions and figures are mine; my employer endorses them implicitly.

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