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From: grr@cbmvax.UUCP (George Robbins)
Newsgroups: comp.sys.amiga
Subject: Re: Amiga memory and serial questions
Message-ID: <2907@cbmvax.UUCP>
Date: Sat, 5-Dec-87 23:29:03 EST
Article-I.D.: cbmvax.2907
Posted: Sat Dec  5 23:29:03 1987
Date-Received: Fri, 11-Dec-87 04:43:36 EST
References: <4558@pyr.gatech.EDU>
Reply-To: grr@cbmvax.UUCP (George Robbins)
Distribution: na
Organization: Commodore Technology, West Chester, PA
Lines: 54
Keywords: slow ram, fast ram, MIDI

In article <4558@pyr.gatech.EDU> gt4785b@pyr.UUCP (David Carter) writes:
> Is there a magazine article or technical reference manual that gives a good,
> clear explanation of the differences between slow ram, fast ram, and slow-fast
> (half-fast?) ram?

Chip memory is memory that can be accessed by the Amiga custom chips for
various purposes including video refresh.  In the higher resolution* video
modes this video refresh uses up some or all of the cycles normally usable
for processor access resulting in contention, which is to say that the
processor effectively runs slower when accessing chip memory.

Fast memory is memory that is not on the Amiga chip's memory bus, and thus
avoids contention, allowing the processor to run at full speed without regard
to the video mode. 

The internal expansion memory in the A500 and A2000 is on the chip memory
bus but is not accessible by the Amiga custom chips due to limitations in
the addressing capabilities of the Agnus chip.  Since it is on the chip
memory bus it suffers from the same contention constraints as the other
chip memory.

Originally, the C00000-D7FFFF area where this memory is position was
documented as being for "fast" internal expansion for future machines,
however when we designed the A500, we found that putting the internal
expansion memory on the chip memory bus, but having it respond to these
addresses would allow us to include provision for up to 1MByte of internal
expansion memory without the additional expense** and complications of the
circuitry required to implement "fast" memory and would still be compatible
with the existing 1.2 software.

The confusion of having "chip" or "slow" memory residing in this putativly
"fast" memory allocation has led to the rather perverse "slow/fast" or
"half/fast"*** memory designations.

Notes:

* It should be noted that the Amiga hi-res modes are not present in the Atari
  ST family and that no contention occurs when using the lower resolutions
  such as those implemented in the ST.  This means that the only performance
  difference results from the Amiga's 7.16 Mhz CPU clock rate vs the ST's
  8 Mhz clock.  However it is this relation of the Amiga's clock rate to the
  various NTSC video timings that allows simple implementation of the Amiga's
  video related features and accessories.

** rule of the computer biz: if it costs us money to put in a feature, then
   the list price goes up - it had better be worth it to the majority of
   your customers.

*** this is supposed to be funny, I think - time will tell...

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