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From: louie@umd5.UUCP (Louis Mamakos)
Newsgroups: net.micro.amiga
Subject: Memory/clock expansion for the Amiga (6/9 - mem.doc)
Message-ID: <1020@umd5.UUCP>
Date: Wed, 11-Jun-86 16:10:07 EDT
Article-I.D.: umd5.1020
Posted: Wed Jun 11 16:10:07 1986
Date-Received: Sat, 14-Jun-86 05:01:45 EDT
Reply-To: louie@trantor.umd.edu (Louis Mamakos)
Organization: U of Md, CSC, College Park, Md
Lines: 94

This document describes the operation and construction of a one megabyte
memory board with optional battery backed up clock for the Amiga. You can
also build a 512k memory by only populating one bank. I do not mention
elsewhere that the software needs to be modified to match the new limits if
you do this. 

The memory system consists of two banks of 256k words each. The banks are
selected by the row address strobe (RAS) which is qualified by an address
bit (A19) to select the bank. The upper and lower data strobes are used to
generate seperate column address strobes (CAS) for the upper and lower
bytes. Refresh is accomplished by activating the row address strobes for
both banks while supplying a refresh address. Refresh cycles occur, if
possible, when the memory is not being accessed (because some other resource
is being addressed) or when necessary by holding off the processor to force
time for the refresh cycle. All timing is derived from the 68000 clocks.
This allows a fully synchronous design so that there are no glitches in
timing possible. This also means that the processor must wait one clock
cycle when it accesses this memory. The processor clock is simply too slow
to go through the necessary states without the wait. If one eliminated the
wait the system would be 20% faster. This would require a delay line or an
asynchronous design. The cost of these alternatives is more than I was
willing to pay. Also, the design would be harder to duplicate because of the
more delicate timing.
The clock is a Motorola MC146818. It keeps track of the date and time
including daylight savings and leapyear. A nicad battery is charged by the
Amiga when it is powered up. This battery runs the clock when the Amiga is
off. As long as the Amiga is on 1% of the time, the clock will remain
accurate.

This memory board does not autoconfigure. This means that the Amiga doesn't
know about it until it is told. A program called AddMem is distributed on
the assembler or C disks. This program is needed to tell the Amiga that the
memory exists. I have not written a clone of this program. I have written a
memory test program to test the board. It is included in this release.

At this time, a number of programs, including Deluxe Paint, do not work
properly with external memory. I hope that as more Amigas get added memory
the suppliers of these programs will fix the problems. Mostly, graphics
objects cannot reside in external memory. If a graphics object is a program
constant, the program must be chip memory resident. There is no way to do
this within a language.  When a program allocates memory, it must specially
ask for chip memory if needed because the operating system will allocate
>from external memory if it can. If you want to use one of the programs that
does not work with external memory, simply do not use the AddMem command.
There is no need to remove the card. Amiga has released the ATOM program to
developers which is used to tell the loader that some hunks must be loaded
into chip memory. If you can get this program, it will fix the above
problems.

The hardware clock isn't used by the Amiga during normal operation since it
keeps its own internal clock. I have written a program to read the hardware
clock and set the system clock (this is more trouble than I wanted). This
program is also included in this release.

The easiest way to use these new resources is to modify your
s/startup-sequence file to include:

AddMem 400000 500000
Clock

 as added commands at the beginning. I also usually copy the entire system
disk to ram: and reassign all the logicals to the copy in ram:. The system
is very fast when running entirely out of ram.

I wirewrapped the memory since I expected to make changes. It took three
tries before the control circuit worked well. There were a number of pins
that were essentially undocumented and I had little choice but to try things
until they worked. Because this design has no support from Amiga, I cannot
guarantee that it works with your machine. On the other hand, I design
memories (and other things) for a living. I know what I'm doing. The design
is very forgiving of small timing changes (another reason to keep it slow)
and I do not expect anyone to have problems. I have no intention of
supplying a PC board. I don't mind if someone else takes on that project as
long as it is non-commercial. (If you are interested in a commercial design
let me know, I would do many things differently if it were a high quantity
commercial product. This design is bent in the direction of ease of
duplication with no special parts.)

There are four additional files I will mention here. The mechanical
description is in mem.mechanical. This includes the connector and board info
and the chip socket locations. Some comments on construction are also
included. The parts list is in mem.parts. The parts given will work.
Substitutions are possible, but at your own risk. The schematic is in
mem.schematic and clock.schematic (this section can be built at a later
time.) This is not the most beautiful method of drawing schematics,
but it allows this project to be electronically documented so it can reach a
lot of people without the 10th generation xerox problem. The schematic files
look best if printed at eight lines per inch.


-- 
Louis A. Mamakos WA3YMH   University of Maryland, Computer Science Center
 Internet: louie@trantor.umd.edu
 UUCP: {seismo!umcp-cs, ihnp4!rlgvax}!cvl!umd5!louie