Path: utzoo!attcan!uunet!lll-winken!lll-tis!helios.ee.lbl.gov!nosc!ucsd!ucbvax!decwrl!pyramid!leadsv!laic!darin
From: darin@laic.UUCP (Darin Johnson)
Newsgroups: comp.arch
Subject: Re: Self-modifying code
Message-ID: <302@laic.UUCP>
Date: 20 Jul 88 02:20:11 GMT
References: <5254@june.cs.washington.edu> <76700032@p.cs.uiuc.edu> <1988Jul18.231158.19500@utzoo.uucp>
Organization: Lockheed AI Center, Menlo Park
Lines: 31

In article <1988Jul18.231158.19500@utzoo.uucp>, henry@utzoo.uucp (Henry Spencer) writes:
> 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.

However, a hardware BitBlt using DMA can be operating while the CPU is
servicing other processes, with no slowdown in CPU speed.  So even if
you can squeeze your BitBlt routine down to the bare minimum and it
takes x CPU cycles to do the blit, that is x cycles that are wasted.
Granted, if you are using a single-tasking machine, you won't get
much speedup using hardware blits.  Since the Amiga is multi-tasking,
its blitter chip does provide speedup.  Also, the Amiga blitter can do
logical operations (on three inputs) just as fast as a normal memory
copy - It'd be pretty hard to hack some 680x0 code to XOR three inputs
and write to three outputs as fast as it can move memory.  Also, the
overhead is very low compared to the overhead involved in dynamic compilation.

If dynamic-compilation is that much faster, then you would have expected
someone to have written a program that does uses this technique on the Amiga,
rather than using the blitter.  So far, I haven't seen any.  If one shows up
that is comparable in speed to the hardware, I would bet that the overhead
and memory involved are high enough that the programs usefulness is diminished.

-- 
Darin Johnson (...pyramid.arpa!leadsv!laic!darin)
              (...ucbvax!sun!sunncal!leadsv!laic!darin)
	"All aboard the DOOMED express!"