Path: utzoo!utgpu!water!watmath!clyde!bellcore!tness7!killer!elg
From: elg@killer.DALLAS.TX.US (Eric Green)
Newsgroups: comp.arch
Subject: Re: Self-modifying code
Message-ID: <4893@killer.DALLAS.TX.US>
Date: 23 Jul 88 05:19:25 GMT
References: <1988Jul18.231158.19500@utzoo.uucp>
Organization: The Unix(R) Connection, Dallas, Texas
Lines: 37

in article <1988Jul18.231158.19500@utzoo.uucp>, henry@utzoo.uucp (Henry Spencer) says:
> In article <1090@ficc.UUCP> peter@ficc.UUCP (Peter da Silva) writes:
>>You can save an even greater factor by building the BitBlt into the
>>hardware....
> No.  You can save a large factor over *poor* software implementations of
> BitBlt.  Not over good ones.  The dynamic-compilation implementations on
> the Blit et al run the memory at nearly full speed for the usual cases
> of BitBlt; it is fundamentally impossible to do better.  Commodore et al
> find hardware implementations attractive because they don't know how to
> write good software.

I won't comment on Commodore's software guys (I'm typing this on an
Amiga, so, depending on when I last found a bug in the OS, I'm either
in complete agreement or violent disagreement), but there's one big
advantage of a bitblt chip over even a tight assembly-language
memory-move loop (using DBcc, no less): instruction fetch.

On newer processors such as the 68020 and 68030, instruction fetch
during a loop is no problem, since such a small tight loop is almost
certain to be in the cache. But on older processors such as the 68000
or 68010, you would burn up 3/4ths of your bus bandwidth just fetching
instructions! (no cache).

Also note that even fetching instructions out of cache, it still takes
at least the 68020 some time to execute them (presumably, the 68030's
pipelining will keep instruction fetch & execution overlapped). The
hardware blitter, of course, will never have instruction fetches...

Given the technology of 1983-1984, a hardware blit chip made a lot of
sense, then. Now.... well, the 68030 costs a bundle, so why pay more
for a blit chip that can't go any faster anyhow?

--
Eric Lee Green    ..!{ames,decwrl,mit-eddie,osu-cis}!killer!elg
          Snail Mail P.O. Box 92191 Lafayette, LA 70509              
   PC Pursuit, n., a service specializing in mangling your postings,
     by eating characters so you can't see what you're typing.