Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!uunet!mcsun!ukc!tcdcs!maths.tcd.ie!ecarroll
From: ecarroll@maths.tcd.ie (Eddy Carroll)
Newsgroups: comp.sys.amiga.advocacy
Subject: Re: (Video) Hardware Idiots ?
Summary: CpuBlit cheats a bit
Message-ID: <1991Jun11.124627.3450@maths.tcd.ie>
Date: 11 Jun 91 12:46:27 GMT
References: <1991Jun8.191231 <1991Jun10.040317.25396@ncsu.edu> <1991Jun10.065129.4135@mintaka.lcs.mit.edu>
Organization: Dept. of Maths, Trinity College, Dublin, Ireland.
Lines: 34

In article <1991Jun10.065129.4135@mintaka.lcs.mit.edu>
rjc@wookumz.gnu.ai.mit.edu (Ray Cromwell) writes:

>                                  The core of the gfxlib routines
>are portable (all the geometric primitives like MOve/Draw(),
>Read/WritePixel, DrawImage/Border(), Text(), BltBitMap (proven by
>CPUBlit)

BltBitMap is certainly portable, but don't let CpuBlit fool you into
thinking it is easy. CpuBlit handles exactly one special case, which is
where the blit operation is REPLACE (i.e. no special processing) and
the source and destination have the same longword alignment. It gets its
speed because it can essentially use the full bandwidth of the CHIP bus.
Once you start doing any sort of processing at all, you introduce delays
between each read and write, and it isn't long before the blitter takes
the lead again.

Even so, CpuBlit only gives a substantial speedup on the A3000 (because of
the 32-bit path to CHIP memory vs the blitter's 16-bit path). On other
accelerated Amigas, the main benefit is really the removal of flicker when
scrolling multiple bitplanes (the blitter can do this in exactly the same way
as CpuBlit, it's just not supported too well under the current OS).

Finally, as food for thought, the source code for the three operand BitBlt in
the Windows VGA driver is about 10,000 lines of assembly (this is with no
hardware assistance of course). It is definitely _not_ a trivial task to
implement a general purpose BitBlt in software! (Not that this slows down the
introduction of DIG, just the implementation of drivers that support it :-)

Eddy
-- 
Eddy Carroll           ----* Genuine MUD Wizard  | "You haven't lived until
ADSPnet:  cbmuk!cbmuka!quartz!ecarroll           |    you've died in MUD!"
Internet: ecarroll@maths.tcd.ie                  |   -- Richard Bartle