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From: dougp@sbphy.ucsb.edu
Newsgroups: comp.sys.amiga
Subject: Re: OS/2 vs AmigaDOS
Message-ID: <1587@hub.ucsb.edu>
Date: 2 May 89 03:35:53 GMT
Sender: news@hub.ucsb.edu
Organization: UC, Santa Barbara. Physics Computer Services
Lines: 29

In article <6728@cbmvax.UUCP>, jesup@cbmvax.UUCP (Randell Jesup) writes...
>	The exact same capability exists in AmigaDos: overlays.  They aren't
        ^^^^^ Not quite.
>	Another way of getting the same results is to use loaded libraries.
                                                      ^^^^^^^^^^^^^^^^
This is closer, but still different. I unfortunately have to program
in windows at work. Windows allows segments of code to be loaded
when functions in that segment is called, in that much it is like
overlays. Unlike overlays, this does not necessarily force out another
block of code. This code, like unused libraries on the Amiga only
gets kicked out when the system is low on memory (all too often in 
windows). So a program in windows may if there is sufficient memory,
be entirely loaded in memory, or it may act like an overlaed program
if memory is tight. To do this, windows takes advantage of the nasty
segmented architechture which allows them to move code arround in 
memory while the program is running by only messing with a few
segment variables. The tradeoff is that code and data segments are
limited to 64K and the CPU is constantly wasting cycles shuffling
the memory arround.
	I believe that something similar to what windows does could
be done using LoadSegment, but this would be complicated, and I don't
see how segments could be discarded when another program was attempting
to get more memory.
	This is the kind of thing that might be good to add in version 2
of the OS, but I hope it can be done without the compromises in 
windows.
>Randell Jesup, Commodore Engineering {uunet|rutgers|allegra}!cbmvax!jesup

Douglas Peale