Newsgroups: comp.arch
Path: utzoo!henry
From: henry@utzoo.uucp (Henry Spencer)
Subject: Re: Self-modifying code
Message-ID: <1988Jul28.171444.7068@utzoo.uucp>
Organization: U of Toronto Zoology
References: <1988Jul22.164129.5495@utzoo.uucp> <4912@killer.DALLAS.TX.US> <4324@cbmvax.UUCP>
Date: Thu, 28 Jul 88 17:14:44 GMT

In article <4324@cbmvax.UUCP> jesup@cbmvax.UUCP (Randell Jesup) writes:
>	Don't forget, the 68000 doesn't have the BFEXTU instruction, so
>the inner loop of a BitBlit isn't as tight as on a 68020.

The inner loops of normal large-area BitBlts do not usually do bitfield 
extractions.  Look at how the operation is actually used.

>>> ... The techniques are
>>> not that well known (especially in the micro community, which is notorious
>>> for its ignorance of the state of the art -- the people at Commodore most
>>> likely had no idea it was even possible to do a fast software BitBlt).
>
>	Excuse me, I have a paper in front of me published in EUUG last fall
>by a researcher at AT&T on optimized software BitBlits in C on an 68020.  If
>you want to insult people please do it to the XYZZY-clone makers and the people
>who write software for them.

I don't consider a 68020 a micro, myself.  The 68020 machine I am writing
this on is much bigger and faster than the minicomputer I was using a
month ago.  When I say "micro", I mean something that competes with PCs.

As for insulting the wrong people, if you're reading papers like that, I
would consider you a (laudable) rare exception to the (deplorable) rule.

>	Fast software implementations of BitBlit tend to be 2 sources, 1
>destination blits (therefor 16 operations).  If you expand them to 3 sources,
>1 destination (256 operations), they tend to take up a LOT of code/ROM
>space, or they slow down a lot, or both...

This is where dynamic compilation, the original subject of this conversation
(remember back then? :-)) wins big:  you don't need to duplicate all that
code N times, just know how to generate it.
-- 
MSDOS is not dead, it just     |     Henry Spencer at U of Toronto Zoology
smells that way.               | uunet!mnetor!utzoo!henry henry@zoo.toronto.edu