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From: gnu@hoptoad.uucp (John Gilmore)
Newsgroups: net.micro.amiga
Subject: Re: interlace mode
Message-ID: <437@hoptoad.uucp>
Date: Sat, 25-Jan-86 15:12:48 EST
Article-I.D.: hoptoad.437
Posted: Sat Jan 25 15:12:48 1986
Date-Received: Sun, 26-Jan-86 17:18:50 EST
References: <570@amiga.amiga.UUCP> <877@h-sc1.UUCP> <511@well.UUCP>
Organization: Nebula Consultants in San Francisco
Lines: 39
Summary: Video memory bandwidth problems are solvable.

In article <511@well.UUCP>, farren@well.UUCP (Mike Farren) writes:
> In article <877@h-sc1.UUCP>, breuel@h-sc1.UUCP (thomas breuel) writes:
> > WHAT I WOULD LIKE TO KNOW IS: is Commodore/AMIGA planning on
> > releasing a version of the AMIGA in which this problem has
> > been fixed (e.g. in which the monitor can be run at either
> > 60Hz or 70Hz)? Is there a hardware patch that can be applied
> > to current machines to solve the problem?
> 
>   The necessary fix would be fairly expensive. The biggest reason for the
> 640 X 200 limitation on each field is to reduce the bandwidth requirements...

There is a technical fix for video memory bandwidth problems, called
"video rams".  These are ordinary dynamic RAM chips which also have a
second access port which scans out a row of 512 bits serially based on
an input clock.  While this scanning goes on, there is NO interference
with the normal memory access.  Once per 512 bits, you must do a special
memory cycle to load a different row of bits into the shifter.  This gives
the CPU something like 99.8% access and the video 100% access.

For example, each row might be a line of a 512-wide screen.  If you need
N bit planes, you use N chips.  (These are 256Kx1 RAMs anyway, so you will
have at least 16 of them in a 68K-based system).  This does place some
constraints on how video memory is allocated but they can be lived with.

While video rams are more expensive than normal DRAMS, in a system with
the price of an Amiga, the added features (large # of bitplanes in the
display with no performance degradation) easily outweigh the cost
difference.

Of course, these can not be retrofitted into an existing design like
the Amiga; they need to be designed in from scratch.   A lot of
designers are skeptical about video rams, as I was, until I saw them
used to halve the access time of a frame buffer and reduce its cost
too.

-- 
# I resisted cluttering my mail with signatures for years, but the mail relay
# situation has gotten to where people can't reach me without it.  Dammit!
# John Gilmore  {sun,ptsfa,lll-crg,nsc}!hoptoad!gnu    jgilmore@lll-crg.arpa