Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!husc6!rutgers!cbmvax!grr
From: grr@cbmvax.UUCP (George Robbins)
Newsgroups: comp.sys.amiga
Subject: Re: Chip RAM
Message-ID: <2537@cbmvax.UUCP>
Date: Sun, 18-Oct-87 21:15:10 EDT
Article-I.D.: cbmvax.2537
Posted: Sun Oct 18 21:15:10 1987
Date-Received: Mon, 19-Oct-87 02:50:36 EDT
References: <1821@gryphon.CTS.COM> <2719@xanth.UUCP>
Reply-To: grr@cbmvax.UUCP (George Robbins)
Organization: Commodore Technology, West Chester, PA
Lines: 31
Keywords: 1/2 M, 1M, 4M, PAL's

In article <2719@xanth.UUCP> kent@xanth.UUCP (Kent Paul Dolan) writes:
> 
> A while back, we were being told that, because the address pins for
> chip ram addressers were time multiplexed, adding one wire (pin?) took
> us from 0.5 meg to 2 meg of chip ram, and that there "wasn't" an
> intermediate 1 meg position.  Two meg is a comfortable amount of chip
> ram for the current screen resolution and depth; 1 meg is still pretty
> skimpy, especially considering that it is shared with sound info and
> disk decoding buffer space.
> 
> Now we're being told that 1 meg is the answer, and that two meg can't
> be attained for technical reasons.  What gives?
> 
> Did someone finally find a way to split a pin in half?  ;-)

The Fat Agnus chip used in the A500 and now the A2000 can only internally
address 512K of chip memory, however the processor side access supports
this 512K plus an additional 512K for external expansion.  The actual
outputs are 9 multiplexed address pins and one RAS line for each of the
two 256K*16bit banks of DRAM's.  To provide addressing for 2MB of "chip"
memory would require yet another address input and either external
decoding of the RAS signals (timing problems) or the use of 1MB chips
(big cost problems) or more pins for RAS lines (packaging problem).
Anyhow, the motherboards in the A500 and A2000 are wired up to support
Fat Agnus and a total of 1MB internal RAM plus whatever you want in the
way of external RAM.

-- 
George Robbins - now working for,	uucp: {ihnp4|rutgers|allegra}!cbmvax!grr
but no way officially representing	arpa: out to lunch...
Commodore, Engineering Department	fone: 215-431-9255 (only by moonlite)