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From: hcj@lzaz.ATT.COM (HC Johnson)
Newsgroups: comp.os.minix
Subject: Re: PC Minix/64KB limit
Summary: OK, It takes lots of software to make up for hardware
Message-ID: <417@lzaz.ATT.COM>
Date: 15 Feb 89 20:15:33 GMT
References: <415@lzaz.ATT.COM> <2485@sun.soe.clarkson.edu>
Organization: AT&T ISL Lincroft NJ USA
Lines: 39

In article <2485@sun.soe.clarkson.edu>, jk0@sun.soe!clutx.clarkson.edu (Jason Coughlin,221 Rey,,) writes:
> 	I'm no engineer, but can't the PC address 1 meg?  Would it be
> difficult to change Minix to use this much space?  Since it isn't Messy-Dos,
> could it address 1 meg without those funny little LIM boards?

Ok, for the non engineers, heres the low down.

1. the 8088, 8086,80186, 80286 totally lack the ability to address beyond 64KB.
   There simply is no such machine operation.  Microsoft added the ability
   to there C compiler output by adding 'far' pointers that basically pull
   2 16 bit addresses from memory (segment:offset) and access one byte/word
   with the effective address.  Slow, Clumsey, but the only way to do it.

2. the .exe format allows for text, data, or both to exceed 64K, by finishing
   the relocation to physical address when the program is loaded.  This is
   the same idea as the ATARI ST uses when it loads.

3. Extened (or is it expanded) memory uses a window in the address space of
   the machine to access more memory.  This is the only scheme on the
   8088/8086. While useful for data bases, and other dedicated programs it
   a real mess for loading programs into.

4. the 80286 supports expanded(or is is extended) memory that addressable
   when running in protected mode.  This paging mode was invented by someone
   who hated programmers.  Its use in MINIX will take much patience!

The bottom line is that:
A. only the base 640K is readily useable.  But it is available.
B. A program can exceed 64 K only by using the Microsoft extensions.
   A MINIX would need to create Microsoft C 5.1 to have a CC command.
C. Beyound that, the port from the Atari ST base should be straight forward.

Maybe someone with a PC and ATARI ST will buy both MINIX versions and do
the port before Easter as Penance.  Good Luck.

Howard C. Johnson
ATT Bell Labs
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