Path: utzoo!utgpu!water!watmath!clyde!att!ucbvax!hplabs!pyramid!cbmvax!jesup
From: jesup@cbmvax.UUCP (Randell Jesup)
Newsgroups: comp.arch
Subject: Re: Sw vs. Hw BitBlit.
Keywords: BitBlit.
Message-ID: <4350@cbmvax.UUCP>
Date: 27 Jul 88 19:52:14 GMT
References: <399@ma.diab.se>
Reply-To: jesup@cbmvax.UUCP (Randell Jesup)
Organization: Commodore Technology, West Chester, PA
Lines: 40

In article <399@ma.diab.se> pf@ma.UUCP (Per Fogelstr|m) writes:
>Someone pointed out that placing characters is the main work for the BitBlit.
>Yes, that is correct in some systems and this is a problem in many cases.
>Placing a character can take from 20micro seconds and up, and the cpu has to
>wait for the blitter to be ready before placing the next character. Then 
>20 microseconds is to litle for a context switch and the cpu is wasitng time
>waiting. But 20-30 microseconds is at least faster than the cpu can place
>the caracter anyway. And if the graphic engine is smart enough it can fetch
>characters from the buffer or main memory itself and offloding the micro
>until it needs some help with a special character (scrolling etc.)

	The only trouble with doing characters with a blitter is setup
time is usually longer than the actual blit (at least for 8N wide fixed-
width fonts that happen to be aligned on "nice" boundaries.)  For this
reason, 1.3 of the amiga OS has something that intercepts Text() calls and
renders them via CPU if the font is 8 pixels wide and aligned on a byte
boundary.  This makes things like emacs really blazingly fast, no discernable
rendering time.  The blitter and normal software handles the more general
cases, and scrolling.

>And talking about scrolling ! Everyone who has seen a Sun scroll must agre
>that the screen must be scrolled in less than one frame (1/70 of a second)
>to make a pleasant impression. I would like to see a 68k do that with
>a 1k x 1k x 8 plane display in one frame time.

	I get real annoyed at sun-2 (no blitter) scrolling speeds.  And it's
only dealing with 1 bitplane!

>An other argument i have heard is "But look at the Mac, its cpu driven and its
>fast !". Well, i just have one answer to that, what is the display resolution ?

	Not compared to an amiga with blitter.  Try dragging a color window
on a Mac-II (remember, it has an '020 at twice the speed of the amiga's
'000.)

	Blitter's also can do VERY fast line draws, with a few extra gates/ 
registers, as well as area fills, etc.

-- 
Randell Jesup, Commodore Engineering {uunet|rutgers|allegra}!cbmvax!jesup