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Path: utzoo!utgpu!water!watmath!clyde!rutgers!cbmvax!hedley
From: hedley@cbmvax.UUCP
Newsgroups: comp.sys.amiga
Subject: Re: Monitor mishmash
Message-ID: <2671@cbmvax.UUCP>
Date: Fri, 30-Oct-87 20:16:42 EST
Article-I.D.: cbmvax.2671
Posted: Fri Oct 30 20:16:42 1987
Date-Received: Sun, 1-Nov-87 09:51:07 EST
References: <3113@ccicpg.UUCP> <4410014@hpcvcd.HP>
Reply-To: hedley@cbmvax.UUCP (Hedley Davis)
Organization: Commodore Technology, West Chester, PA
Lines: 43

In article <4410014@hpcvcd.HP> charles@hpcvcd.HP (Charles Brown) writes:
>> > 	I would dicker with the not cheap comment. Ever heard of interleave?
>> > I also made this comment, until I realized that if you delay ram timings
>> > through interleaving, You don't need fast rams.
>> >	 UUCP: uunet!ccicpg!harald
>
>If you fetch 32bits at a time and deliver 4 bits per pixel, you only
>have to read your RAM at 895KHz, even after doubling the scan rate.
>With half of the time for CPU access, this leaves approx 500ns per
>access.  Can you say cheap RAM?
>	Charles Brown		hplabs!hp-pcd!charles

Video comes out of the amiga in bursts during a line time, and
goes away for retrace. I hope you weren't planning to save
the data somewhere to run the rams full tilt during retrace also.

The bandwidth during the video time is 14.32 Mbits per bit per pixel.
	( four pixels per 3.58 mhz cycle in highres mode )

For 4 bits this is 57.28 Mbits/sec.

Video to the monitor is twice this, or 114.56 Mbits/sec.

Total bandwidth needed is 57.28+114.56 = 172.84 Mbits per sec.

Assuming 32 bit wide memory, the memory needs to run at 172/32 = 
5.375 Mwords/sec => 186 ns cycle time.

How'd you get this 500ns number ?

Also, what kind of drams you going to use ? Look at the memory layout.

Consider further the amount of shifting you need. ( 32 bits wide x 4
bits in + 32 bits wide x 4 bits out => 256 bits of shift register
storage ).

Also, you need to time align all this stuff. You're gonna need
even more bandwidth to achieve that.

How many chips did you say this would take ? The goal here is
inexpensive overall. Not inexpensive in RAM only.

Hedley