Path: utzoo!utgpu!cunews!bnrgate!bwdls58!bwdlh417!dbscoop2
From: dbscoop2@bwdlh417.BNR.CA (Alun Fryer)
Newsgroups: comp.sys.amiga.misc
Subject: Re: Agnus
Message-ID: <6516@bwdls58.bnr.ca>
Date: 19 Apr 91 16:56:36 GMT
References: <1991Apr17.082037.27978@cs.uow.edu.au>
Sender: news@bwdls58.bnr.ca
Reply-To: dbscoop2@bwdlh417.BNR.CA (Alun Fryer)
Distribution: world
Organization: Bell-Northern Research Ltd.
Lines: 39

There are at least 5 different Agnuses (Agni?) that I know of.  The
first is the "normal" agnus, which is a standard DIP chip and only came
in the old A1000's.  I'm not sure of the part no of it.  Then came the
"FAT" agnus, which has a part number "8370" (I beleive).  There are two
of these.  One is for NTSC (8370) and on is PAL (not sure of part no.). 
Then came the (SUPER | ONE-MEG | OBESE | FATTER | REALLY BIG |
OH-WOW-I-NOW-HAVE-A-WHOLE-1-MEG-OF-CHIP-RAM ) agnus (pt. no. 8372). 
This is that same chip for both NTSC and PAL.  Finally comes the
SUPER_GODLY 2 meg agnus which is only avaiable in the A3000/3000T. (pt.
No. - I don't know).

I'm not 100% sure of the chip numbers but I do know the old Fat agnus
ends in a "0" and the 1 meg ends in a "2".

Now as to the question about "why 2 types of RAM?"...

The Amiga has 2 types of RAM: CHIP ram and FAST RAM.  CHIP RAM is so
called since it is the RAM which is accessable by the CUSTOM chips
(Agnus, Denise, Paula).  The limiting factor is the AGNUS chip.  The old
FAT agnus would only allow 512k to be "CHIP" ram, where the Newer Super
Agnus allows 1 Meg.  The A3000 agnus allows 2 Meg.  

FAST RAM is all RAM which is not CHIP RAM.  It is called FAST RAM
because the CPU has faster access to it.  Well, really what I mean is
that since the custom chips don't have access, the CPU doesn't have to
wait for them to gain access to the RAM.

There is also so-called "SLOW-FAST" RAM which is the second 512k in a
machine with a 512k Agnus in it, but I wouldn't worry about it.

Basically, CHIP RAM is the only RAM which you can directly use for
graphics and sound.  If you want more than that (e.g. a 2MB sound
sample), your software has to swap the data into CHIP RAM when it is needed.

I hope this clears things up a bit.

			- Alun Fryer