[PLUG] Segmented memor architecture -- Why is it bad?

[Greg Long]

Programmers of course had to do the most work, but there were extensive
support needs generated by Joe User who didn't understand why his (then
high end) 32mb system didn't have enough [conventional] memory to run an
old DOS app that installed from a single floppy disk.

I'm glad I didn't deal with segment:offset any more than was recently
necessary for the x86 Assembly Language Course at OIT.

Greg

-----Original Message-----
From: plug-admin at lists.pdxlinux.org
[mailto:plug-admin at lists.pdxlinux.org] On Behalf Of Craighead, Scot D
Sent: Thursday, May 30, 2002 2:27 PM
To: 'plug at lists.pdxlinux.org'
Subject: RE: [PLUG] Segmented memor architecture -- Why is it bad?


>Please elucidate this subject.  Why did they choose a segmented memory 
>architecture, and why is it bad?

I'm surprised you don't know what this refurs to.  Maybe it's because
the 32-bit operating systems have pretty much solved the problem.  Back
in the DOS days, if you wrote assembly language programs, you cursed
this daily. Memory locations were made of 2 16-bit numbers, the segment
and the offset. This meant that there were were 6 "memory models" for a
program.  In C, you just had to tell your compiler which to use, but it
was a big hassle if you wanted to link in assembly modules to your C
code.  You had the tiny memory model, which used 1 16-bit addess for all
code and all data, for example. This means your entire program had to
fit into 1 64K segment as well as your variables.  Executable files the
have the extension .com are this way.  Then you had Small, one segment
for code and another for data.  You can extrapolate the rest from here.
To make things even worse, there eas the 640K barrier.  All the memory
locations above this point were resurved by the operating system.  So
that's all the memory you get for your applications programs.  Deal with
it.  Then along came extended and expanded RAM.  Two different ways to
do the same thing, have access to more memory. The only problem was that
it was a major hassle to work with.  It is amazing that PC's became so
huge when they were designed so poorly.  Fortunate, only programmers had
to deal with this and Joe user had no idea.

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