Otherware/Otherware_1_SB_Development.iso/amiga/info/memhack.zoo/1meg.txt

Text File | 1990-02-09 | 19.3 KB | 425 lines

NOTE: This is the text of an article that appeared in Amazing Computing. It
      refers to some pictures that were in that article. It can be done
      without the pictures. Also reference is made to 'make512' to get
      around the problem with WB 1.2 & following. See the accompanying
      articles that show better ways around that problem.
----------------------------------------------------------------------------

            A Megabyte without Megabucks

                by Cris Erving
             Copyright 1986, All Rights Reserved

   If you have tried to do any serious development work with
C, or if you have worked with memory-intensive programs like
Aegis Draw, you have undoubtedly become acquainted with the
Amiga's biggest limitation...  its lack of memory.  Although
512K seems like a lot at first, it is very easy to use it
all up, especially if you do a lot of graphics.  The
conventional solution is to go out and buy an external
memory expansion board.  Although these boards are nice,
they cost about $350 and up, which puts them out of the
reach of many of us impoverished Amiga users.  Fortunately,
there is a solution.  For about $70 and an evening of
hacking, you can internally expand your 512K Amiga to a full
megabyte.  A word of warning first:  this hack is not
supported by Commodore, and voids your warranty.  If you
aren't comfortable playing around inside your computer, you
would probably be better off saving up for an external
board, or finding somebody who can do the upgrade for you...
ask around at your local users' group.  That being said,
let's start hacking!

   First, you will need to get some memory chips from your
favorite chip supplier.  IC Express, Microprocessors
Unlimited, and JDR Microdevices are good sources of memory
chips...  look them up in 'Byte' or 'Computer Shopper'.  You
need sixteen 4464-type 256K RAM chips, rated 150 ns or
faster.  These are NOT the same 256K RAM chips that the IBM
PC uses...  those are 41256-type RAM chips, and will NOT
work in the Amiga!  You will also need some wire (28-gauge
wire-wrap wire is nice), eight 18-pin solder-tail sockets
(NOT 16-pin!), and eight 33 ohm, 1/4 watt resistors.  You
will also need a low-power soldering iron with a very fine
tip; I recommend that you use one rated under 20 watts.
Radio Shack sells a decent 15-watt iron with a grounded tip
for about $6...  ask for stock number 64-2051.

   OK, disconnect everything from your Amiga, and lay it
upside-down on a table.  Unscrew the phillips-head screws
that hold the top on; they are inside deep recesses, and
there are 5 of them.  Also remove the two smaller screws
near the front of the computer, which hold the bezel on.
(See Photo 1).  After you have done this, put a piece of
tape over the holes to keep the screws from falling out, and
turn the computer rightside-up.  With a flat-blade
screwdriver, carefully pry the lid off the top of the
computer; you will hear a pop as each of the four catches
comes off.  Lift the lid off, and you will see a large metal
shield.  (See Photo 2).  Remove it; you will have to twist a
few metal tabs to get it completely off.  There are about 15
screws holding this shield on, so if it won't come off you
probably missed a screw.  Lift off the bezel; you will have
to pull two small plastic tabs back to remove the two LED's



from the bezel.

   You will now see the main board, with a smaller
"daughter" board piggybacked upside-down on top of the main
board.  (See Photo 3).  This is the Kickstart RAM board.
Remove the 3 screws holding the daughter board, and
carefully pry the daughter board out of its sockets by
gently rocking it back and forth.  Use care, as the header
pins that connect the daughter board bend easily if you
apply too much force.

   If you look at the front and the left side of the Amiga's
mother board, you will see some letters and numbers
silkscreened on the board.  Chips on the Amiga's PC board
are located at the intersection of a row number and a column
letter.  For example, a chip located at 1C is in the first
row, column C.  Near the expansion RAM port you will see
three IC's - two 74F138's and a 74F399.  From left to right,
these IC's are located at position 1H, 1I, and 1J.  Also,
you will see the eight RAM chips on the Amiga's mother
board, located at 1B-1E and 2B-2E.  These RAM chips will be
marked "4464", "41464" or "50464".  (See Photo 4).

   Take the eight 18-pin sockets, and clip pin 16 off of
each one, as close to the socket as possible.  Now, solder a
socket on top of each of the RAM chips on the Amiga's PC
board, making sure that the socket matches the RAM chips'
orientation.  Don't use any more solder than it takes to get
a good connection, and let the RAM chips cool down a few
seconds between each solder joint.  A solder bridge here is
extremely difficult to remove, so be careful and take your
time.  If you did this correctly, you will have soldered all
18 pins except for pin 16, which you clipped off of each
socket.  (See Photo 5)

   With a pair of fine cutting pliers, CAREFULLY clip pin 3
of the two 74F138's (at 1H and 1I) as close to the PC board
as possible, and bend the pins up away from the PC board.
IC pins are numbered counterclockwise from the notch (or
dot) at one end of the chip, so pin 3 will the the third pin
from the front on the right side.  Do the same with pins 11
and 12 of the 74F399 at 1J.  Connect a wire between pin 3 of
both 74F138's and pin 10 of the 74F399.  Connect another
wire between pin 12 of the 74F399 and pin 8 of the 74F399
(which should still be soldered to the PC board).  Cut a
wire about 8" long, and solder it to pin 11 of the 74F399;
leave the other end loose for now.  Label the loose end of
this wire "A19" with a small piece of tape; this will make
life easier later on.  (See Photo 6).  Check your work
before you go any further.

   Take the sixteen 4464-type RAM chips you bought, and
CAREFULLY cut the narrow part of pin 16 off of each chip.
Do NOT cut the pin off entirely...  only cut off the narrow
part!  Now comes the tricky part.  Solder one of these chips



on top of another one, being very careful that you have them
pointed the same direction; you can tell by the notch on one
side.  You will be soldering all pins EXCEPT for pin 16,
which you clipped short.  These chips are static and heat
sensitive, so it's a good idea to use a grounded soldering
iron and as little heat and solder as is necessary to get a
good electrical connection.  Be extremely careful not to
accidentally create any solder bridges between the RAM chip
pins.  It wouldn't hurt to solder them together over a piece
of aluminum foil to reduce static, either.  Repeat the
process for the other chips.  What you will end up with is
eight "piggybacked" RAM chip pairs, that is, eight sets of
two RAM chips stacked on top of one another.

   Make eight wire jumpers about 1 1/2" long.  Take two of
the piggybacked RAM chip "stacks", and solder a jumper from
pin 16 of the top RAM chip on one of the stacks to pin 16 of
the top RAM chip on the other stack.  Solder a 6" wire to
pin 16 on one of the chips; the other end should be left
free.  Do the same for the bottom RAM chips.  You should now
have two stacks with two short wires connecting them, and
two longer wires attached to them.  Repeat this process for
the other RAM chip stacks.  When you are done, you should
have four sets of two chip stacks, with each set connected
by two jumpers and each set having two longer free wires.

   Insert a set of RAM chip stacks into the RAM sockets on
the mother board at 1B and 1C, making sure that the
orientation is correct.  The notches on the RAM chips should
be facing towards the front of the computer.  Do the same
for the other three RAM chip stacks, placing them in the
sockets at 1D and 1E, 2B and 2C, and 2D and 2E.  Now, label
the eight free wires coming from the RAM chip stacks as
follows:  (See Photo 7)

From the Bottom RAM at 2E/2D  : LC4
From the Top RAM at 2E/2D     : LC6
From the Bottom RAM at 1E/1D  : LC5
From the Top RAM at 1E/1D     : LC7
From the Bottom RAM at 2C/2B  : UC4
From the Top RAM at 2C/2B     : UC6
From the Bottom RAM at 1C/1B  : UC5
From the Top RAM at 1C/1B     : UC7

   Take the eight 33 ohm resistors, and trim the leads of
each one to about 1/4".  Solder one resistor to pin 7 of the
74F138 at 1H.  Do the same at pins 9, 10, and 11.  Repeat
the process with the other four resistors and the 74F138 at
1I.

   Solder the eight wires coming from the RAM chip stacks to
the free end of the eight resistors as follows:  (See Photo
8)

UC7 to the resistor at 1H, pin 7



UC6 to the resistor at 1H, pin 9
UC5 to the resistor at 1H, pin 10
UC4 to the resistor at 1H, pin 11
LC7 to the resistor at 1I, pin 7
LC6 to the resistor at 1I, pin 9
LC5 to the resistor at 1I, pin 10
LC4 to the resistor at 1I, pin 11

   Solder the wire marked "A19" to pin 3 of the IC on the
daughter board marked "DPALCAS".  It's located at the rear
end of the daughter board at location 6K, and the IC has the
marking "PAL16L8" on it.  (Photo 8) Put the daughter board
back, making sure that all of the header pins fit into the
header sockets on the daughter board.  Finally, replace the
256K expansion ram cartridge in its slot.

   You are now ready for the moment of truth.  With the
cover off (in case you need to debug this mess!), hook the
keyboard, monitor, and power to your Amiga.  Turn it on.  If
the computer won't boot (i.e.  you don't see the Kickstart
or Workbench screens), you have wired something incorrectly
or you have a solder bridge somewhere...  turn the Amiga off
and retrace your work.  Pay particular attention to the
order of the wires; this is quite critical.  If you got to
Workbench, you will need to use the AddMem utility to tell
your Amiga to make the extra memory available.  This utility
came with the developer's package, and is also on Amicus
Disk  4 (along with the C source!).  To add all 512K to free
memory, type 'AddMem 80000 FFFFF'.  You should now have a
total of 1 megabyte to play with; if something seems wrong,
turn the computer off and check your wiring.  There is
another utility called Avail on the utility disks, which
will tell you how much memory you have.  To use it, simply
type 'Avail'; it will tell you how much Chip and Fast RAM
you have.  (The 'Maximum' column should read 1048568).  You
can also use the Gfxmem utility to show you how much memory
you have; this program appears on several of the Amicus and
Fish disks.  A hint:  if AddMem seems to work, but you get
an AmigaDOS error when you do a DIR, you might have reversed
the UC and LC wires...  go back and check your work.  When
you are satisfied of the memory's operation, replace the RF
shield, the bezel, and the covers, and you are in business.

   OK, time for the technical discussion.  The reason this
works is that Amiga most generously did not fully decode the
lower 2 megabyte RAM space that the internal 256K and 256K
expansion RAM fit into.  They DID decode 1 megabyte, but
they only used half of the available memory select lines.
This makes it fairly easy to add the other 512K, because we
don't even need to add any decoding IC's, just some jumpers.
Basically, we take the memory Column Address Strobe (CAS)
signals that are unused, and connect them to our piggybacked
memory.  The only problem with doing this is that we have to
restrict the 3 custom chips to the lower 512K.  Fortunately
again, there is already a multiplexer on board which selects



between Chip and 68000 addresses (the 74F399, which is a
latched quad 2/1 multiplexer), and it has some unused
sections.  So, we connect input I0C at pin 11 of this IC to
A19, which is the source for the extra address line from the
CPU, then we connect input I1C at pin 12 to ground, which
assures us that the custom chips will always "see" the lower
512K.  The output of the multiplexer, QC at pin 10, drives
the upper address bit of the two 74F138 3/8 demultiplexers,
which are gated with UDS and LDS to produce the extra upper
and lower CAS signals that the new RAM chips need.  The 33
ohm resistors help reduce ringing on the CAS lines; this is
especially important with long lengths of fine wire, such as
we are using.  Each pair of 74F138 outputs represents 128K
of memory; since there are four selection pairs available,
we can select up to 512K.  See the schematic below for all
the gory details.

   What you end up with is your normal 512K of Chip RAM,
mapped from $000000 to $07FFFF, and another 512K mapped from
$080000 to $0FFFFF.  This 1 megabyte "shadows" over the next
1 megabyte, (from $100000 to $1FFFFF) because the next
highest address bit (A20) is not used for decoding.  This 1
megabyte was "reserved" by Commodore for internal memory
expansion, and you could theoretically add another megabyte
of RAM chips (along with a few more decoder and glue chips
to decode this address space), giving you 2 megabytes of
RAM.  I say theoretically, because you will run out of power
long before you have reached 2 megabytes.  One megabyte,
however, does not tax the Amiga's power supply, and leaves
you with enough left over for a few goodies like A-Time and
Digi-View.

   A few other topics before I sign off.  First, you may be
wondering whether this is "Fast" memory or "Chip" memory.
The answer is that it is neither.  Fast memory resides
directly on the processor buss, with no possibility of
contention by the custom chips.  Chip memory is memory which
the custom chips can access.  What we have here is Chip
memory which the custom chips cannot access.  (Of course,
AmigaDOS thinks it's Fast RAM...).  A better term for this
memory might be "Synchronous" memory, since it is
synchronized to the custom chips because it is on their
memory buss, but it is not available to them.  This means
that it will slow down when heavy DMA activity is going on;
in practice, however, this seldom happens.  Also, because
this memory is not on the 68000's expansion buss, it cannot
auto-configure; this is why you need to use AddMem.  Of
course, this also means that it doesn't use up any of your
auto-config address space, too.

   You may also be concerned about compatability with future
Amiga hardware, namely Sidecar and Zorro expansion boxes.
Since the lower 2 megabytes of address space "shadows" if it
is not fully decoded internally, it is not possible to use
this address space from the expansion port, because both the



expansion buss and the internal memory would try to be on
the processor's buss at the same time.  Because of this,
there is no way an external expansion box could conflict
with your synchronous RAM.  Since this RAM does not map into
the auto-config space, you don't have to worry about
conflicts with any Zorro boxes you may add later, since
Zorro boxes only map into auto-config space.

   Finally, there is one last question...  if it's so easy
to add the memory, why didn't Amiga do it themselves?  I
don't know why; maybe they thought 512K was plenty of
memory.  Considering the software they had at introduction
(i.e.  Textcraft), that may have been understandable.  It
would have been so easy for Amiga to allow us to expand to 1
or even 2 megabytes by simply plugging in a RAM cartridge
that any excuse on their part is unforgivable.  Not only are
the CAS lines already on the board, but the are a whole
bunch of lines on the expansion RAM cartridge which are
arbitrarily tied to +5 volts or ground, so there's already
plenty of room on the connector for the extra RAM select
lines.  The extra power required for 2 megabytes would only
have been a few amps, so it wouldn't have cost them very
much to upgrade the power supply.

   One final note is in order.  If you try to use this
memory with Workbench 1.2, you will find that Workbench 1.2
tries to auto-configure the extra memory as Chip RAM.  This
is probably due to the fact that the new "Fat Agnus"
graphics chip will be able to address 2 megabytes, rather
than the 512K that the current Agnus chip supports.  If
there was no fix, this would cause serious problems, because
graphics and other chip data could be loaded into a location
which the chips could not access, resulting in an almost
certain visit from the Guru.  Fortunately, there is an easy
way around this.  The program listed below, which is a
Lattice C program called make512k, will automatically turn
your computer back into a 512K machine.  Then you can use
AddMem to make the extra memory available, just as you would
with Workbench 1.1.  Make sure that make512k and AddMem are
called at the very beginning of your s/startup-sequence
file, and you will have no problems.  I have been running
with late beta versions of 1.2 for over 2 months without any
problems.  ( see accompanying articles for better ways to fix
this problem).

   All in all, it's a nice little hack, and I hope a lot of
you take advantage of this cheap way to double your memory.
It's amazing how many uses for more memory you can find once
you have it.  You can copy your C:  directory to RAM:, do an
ASSIGN C:  RAM:, and your CLI commands will execute almost
instantaneously.  You can use RAM:  for all your workfiles,
then copy them to the disk when your're all through...  a
large Aegis Draw drawing takes much less time to save this
way.  Lattice C compiles speed up enormously when you copy
the include and library files to RAM:.  You can finally take
full advantage of the Amiga's multi-tasking without worrying



about the Guru...  I like to play Reversi while I'm waiting
for Aegis Draw to redraw the screen.  It sure makes using
the Amiga a lot more fun.  After all, that's what we bought
an Amiga for!

About the Author
Cris Erving is the Systems Manager of a Digital Equipment
VAX 8600 installation at a large California heavy equipment
dealership.  He is very active in the AmigaTech Users Group,
which is based in the Los Angeles area.  His interests are C
programming, CAD/CAE, hardware hacking, and "taking the
Amiga apart".



Photo Captions
==============

Photo 1:
The underside of the Amiga.  The arrows point to the 5
screws holding the case together, and the 2 screws holding
the bezel on.

Photo 2:
The Amiga with the top cover removed.  The RF shield is the
large shiny object in the middle.  This particular shot is
with the screws already removed.

Photo 3:
The Amiga with the RF shield removed.  The Kickstart
daughter board is the S-shaped board sitting on top of the
main board.  The 3 arrows point to the screws holding the
daughter board on.

Photo 4:
The Amiga's RAM chips and the two 74F138's and the 74F399
that we are concerned with.  If you look hard you can see
the notch and the dots on the chips that point to pin 1.

Photo 5:
The RAM chips with the sockets soldered to them.  I can't
stress enough that you have to be careful when soldering in
these close quarters, in order to prevent solder bridges.
The arrows point to the pins on the two 74F138's and the
74F399 which are clipped...  the extra arrow at pin 7 of the
74F399 should be ignored.

Photo 6:
Closeup of the 3 logic chips and the added jumpers.  Note
that pin 3 of both 74F138's have been cut, as well as pins
10 and 11 on the 74F399.  The wire marked "A19" is about 8"
long.

Photo 7:
The piggybacked RAM chips with their pin 16 wires.  Although
you don't have to mark them, it makes life much easier.

Photo 8:
The completed upgrade.  Although it looks like a plate of
spaghetti, it's really not THAT bad.  You could route the
wires nicely around the IC's, if you are so inclined.
Notice the resistors soldered to pins 7,9,10, and 11 of the
74F138's, and the "A19" wire soldered to the PAL16L8 at 6K
of the daughter board.

Photo 9:
The moment of truth.  Does it work?....  you bet!