Date   : Mon, 02 Sep 1996 10:47:01 +0100
From   : Ian Stephenson <ians@...>
Subject: Re: Point Blank

>.,/\ 6. Windows as an emulation platform. /\,.

>Emulators are one of the few programs that rely on raw
>CPU power, rather than RAM and lots of harddisk space.

Whoever wrote this (unfortunatly) does not know what they are talking
about :-(

This may be true for simple (static) emulation of older systems. However
more  
advanced emulation can be very heavy on both RAM and diskspace. While
a realise  
that these issues may not be relevant to BBC emulation, the original
article  
was concerning generic emulation, and hence needs to be critiqued on those  
terms.

diskspace is the simplest issue - you need to hold an entire filesystem
image.  
Easy for BBC's, but of that filesystem is Win95 (as in the latest SoftWindows)
 
then you need about 200Meg free. Thats a LOT on a Mac, when fileMaker Pro  
takes up 1Meg of disk space.

RAM is more complex - start with the same idea: You need more than the
system  
you're emulating (not strictly true if you've got VM, but if you start
paging,  
then you're really in trouble). However for high speed emulation, you need  
dynamic compilation. This relies on converting emulated instructions onto  
native host instructions ON THE ASSUMPTION THEY"RE GOING TO GET EXECUTED
AGAIN.  
When you come back round a loop for the second (over simplification!) time,  
then you've already done the conversion, so it runs at near native speed.

This only sucseed's if you get sufficient cashe hits on converted code.
This  
means you need as large a set as possible of converted routines, in the
hope  
they might be executed again. Bigger cashe = better performance.

In dynamic compilation amount of RAM can relate DIRECTLY to performance.


I do aggree with the author, that Windows is a nasty hack on top of a nasty  
system. I'm also tempted to agree that Windows has little to offer the  
emulation writer, but these are generic problems with MS, not specific to  
emulation.

Get a real system*, and stop worrying which heap of MS junk is bigger.

$an

* it is left as an exercise to the reader to determine what constitutes
a REAL  
system.