pc@unisoft.UUCP (Paul Campbell) (02/07/85)
<munch>
HOW TO MAKE A 4MB MAC ...
What follows are a few thoughts on how to hack in more than 512K into
a standard Mac. Most of this is deduced from a number of sources - the DDJ
article, the Feb 84 Byte article, my recollections on how dynamic rams work
(unfortunatly all my data books are 10,000 km away) and an hours spent
peeking at various addresses in the Mac's address space. If any of what I
suggest is wrong or misleading please dont hesitate to contact me (Apple are
you listening? ... of course you are).
OK now for the rough stuff, first a little about how (256k) dynamic
rams work
- The basic ram timing is generated by two signals RAS and CAS (row and
column address strobes), I can't remember which is which (again I have
no data) but the important is that one (RAS I think) is required for
refresh and is the first one, CAS comes second and acts as a chip
select (Ie if you give a RAS and no CAS you refresh the chip but dont
read or write any data). Apple are good enough to give you a signal
(called "Sel A" in the DDJ article) that goes one way for RAS and the
other for CAS .... I don't yet know which way or what it does when
no access is being made.
- The address lines are multiplexed through 9 address pins by the
RAS/CAS signals.
- Data is read in on pin 2 and out on pin 14.
On the MAC it seems that the PALs decode the bottom 4Mb for ram, this
is done somewhat decadently - the ram turns up every 512K in this space, the
CAS signal on pin 15 of the ram chips sais when this bank is selected. Another
factor effects what happens here - the screen display ("Sel B" seems to indicate
whether a CPU access (0) or a display access (1) is in progress), this is
important as this is how your refresh is done. The top bits of the display
addresses probably never make it onto the address bus at all, instead the PALs
(the same ones do the display timing as do address decoding) just make a CAS
at the correct time. To add to these problems at reset time the memory mapping
is changed and the ram mapped to 6M (again by those same PALs).
In order to put more ram in this bottom 4Mb a number of things must
happen
- the CAS line (2 actually, one for each byte) must be interrupted
before it reaches the chips and passed thru some logic that gives
it back to the bank 0 chips if
- Sel B is asserted (display stuff going on) or
- the cpu is not addressing the ram at 512K-4M
- otherwise it is gated to the appropriate other bank of ram chips
(it must go thru fast (74S) logic as it can't be delayed long
but if you use 150ns ram there should be enough lee-way ...
however I have not done the timings yet !!)
- The address lines, RAS, R/W, are wired in // with the bank 0
chips (buffers will probably be needed so you will have to put in
multiplexers for the addresses...)
- Data lines will probably have to be buffered.
Mounting such mods in the current mac pose some problems
- heat, that box gets hot enough already - new ram would probably
have to be mounted on the side opposite the analog board, an extra
512K should be able to be accomodated without overheating but
more may require a fan.
- power, the power supply has enough extra to power an external floppy
and modem - I have neither of these so I should have no problems.
- getting all these signals from the logic board to the ram board
cleanly ... this is the really hard part, the last thing I need is
a tangled mass of wires snaking thru my Mac, it would be nice to
be able to unplug the logic board and have the mac work as normal
(after jumpering CAS).
SUMMARY
I have described my current knowledge of how I THINK the mac ram
circuitry works and how extra memory might be added, again if anyone can add
to this please don't hesitate to mail me. Hopefully we can put together a
public domain guide on how to make your mac bigger and bigger .... I intend
to try and add just such a board to my system in the next few months.
It seems to me that with the price of 256k chips at about $10 ie $160 for
512k it is cheaper to add more ram disks to your mac (and faster) than to
buy floppys!!!!
Paul Campbell ucbvax!unisoft!paul
The views expressed above are not those of Unisoft ... etcpc@unisoft.UUCP (Paul Campbell) (02/07/85)
<munch> Ps ... I forgot we have a name for a 4Mb Mac, its going to be called a "Monster Mac" Paul Campbell ..!ucbvax!unisoft!pc "Lets do the Mac ...."