Reprinted from A.C.E.C. BBS (614)-471-8559
THE ATARI 800 PLUS 256K
by David G. Byrd
1513 Commanche Drive
Las Vegas, NV 89109
Provided free for the personal use of ATARI enthusiasts everywhere, but not
Copyright (c) 1985, D. G. Byrd
Atari 800 is a trademark of Atari, Inc.
This article may be reprinted if printed in whole and proper credit is given to
the original authors and this publication. For their repetitive failure to
provide credit to the original author, this permission is expressly DENIED to
Jay Torres and The Windhover Project.
First published in the July 1985 S.N.A.C.C Newsletter.
(Southern Nevada Atari Computer Club)
Revision A - Oct 1985
THE ATARI 800-PLUS 256k
288k Total Memory for the 800
by David G. Byrd, Las Vegas, Nv.
I am one of those incorrigible hardware hackers who is never satisfied with
simply operating an appliance computer with purchased software. I am
fascinated by taking a commercial product and trying to extend its capabilities
beyond those the original supplier provided.
After reading the "48/64K UPGRADE FOR THE ATARI 400" by Claus Buchholz, I was
inspired to design a similar upgrade for the ATARI 800. Full credit for the
installation of 5v only chips in the 16k board goes to Claus and the "MACE
This article will describe how to modify, test and use a 256k memory board in
the middle slot of the 800. This will cost you under $50, a few hours of your
time and will give you a total of 288k of RAM.
All logic additions and track cutting is limited to the memory boards, so you
may wish to pick up a "spare" to hack on. This will allow you to return to a
stock 800 if you should desire. One jumper is installed in the Personality
Module (ROM), and three backplane jumpers are necessary to provide the
Remove the middle memory board (or use the spare) and Look at the memory board.
The eight chips along the top are the RAM chips. The other four chips are the
addressing circuitry. The edge pin connectors at the bottom are labeled as in
The first step is to eliminate the 12V and -5V sources on the board and to move
the 5V source to where the 12V used to be. As shown in Figure 3, cut the trace
going from pin "X" of the boards's edge connector to the capacitor C521. Also
cut the trace going from edge pin "Y" to C523. Cut the traces cleanly and
completely. Be careful not to slip and damage adjacent traces.
Now remove the capacitors C521 and C523. The trace coming from pin "W" carries
5V. Using a short piece of wire, make a solder bridge between this trace and
the old 12V and -5V traces, at the point where C523 and C521 used to be (see
Fig ure 3). Next, remove the eight capacitors C503, C505, C507, C509, C511,
C513, C515, and C517, which are usually in a row along the top of the board.
We now have 5V going to pins 1, 8 and 9 of the RAM sockets. Remove the eight
16k RAM chips and insert the new 256k RAMS in their place, properly orienting
their notched ends. With an ohmmeter, make sure there is NO connection between
edge pin "Y" and pin 8 of the chips, nor should there be any connection between
any two of the edge pins "W", "X", and "Y".
If all has gone well the board should function like a 16K memory board, since
the addressing circuitry has not been altered.
Put the modified memory board in the middle memory slot of your 800, and check
to see if it functions normally. The blue screen should appear quickly. If
not, recheck all modifications made so far.
Now take the 5V supply off pins 1 and 9 of the RAM chips. To do this, cut the
two rightmost wide traces on the chip side of the board (see Figure 4).
Pick up a new 74LS158 chip, which is the same as the chips Z503 and Z504 on the
memory board. With needlenose pliers, carefully bend up all pins except 1, 8,
15, and 16 (see Figure 5). Remove chip Z503, place the new chip on top so that
the four pins listed above touch the same four pins on the lower chip.
Carefully, solder each of the four pairs together, being careful not to get too
much solder on the end of each pin. Now insert the pair in socket Z503.
The second new 74LS158 chip is to be mounted in the same fashion on top of Z504
except that pin 1 is also elevated and not soldered. Now insert this pair in
Next we will prepare the additional chips for mounting. These chips will be
mounted "Dead Bug" style, so it is a good idea to put a stick-on label on the
underside of each chip indicating its type and the location of pin 1.
Install the four new chips in the vacant area below the memory chips. I used
"Super Glue" to attach them. Make sure you get them in the right place the
Now wire the new chips per the diagram in Figure 6. Connect 5V and ground pins
first to each dead-bug chip. Convenient connection points for 5V and ground are
the left and right ends of each capacitor located directly below each RAM chip.
Connect each chip individually with short wires.
Connect a temporary jumper from pin V to W.
LETS CHECK OUR PROGRESS
Return the memory board to the middle slot for testing as a 16k board.
If the blue screen doesn't come up quickly, turn it off immediately and check
If it checks OK, remove it and remove the temporary jumper connecting pins V
and W, and cut the track connecting pins S and T.
The 256k board is complete so put it aside for now.
STANDARD MEMORY BOARD MOD
Remove the two remaining 16k memory boards from the computer, and remove the
case. On the reverse side of the board, find the tracks connecting S to T. Cut
this track on both 16k boards.
If you wish, you may reinstall these boards in their case.
PERSONALITY BOARD (ROM) MOD
Remove the ROM board, and take it out of the case. Find Z401 and connect a
jumper between Z401 pin 7 and pin 20 (the only unused pin) of the card edge. Do
not allow solder to flow down the card edge pin.
Reinstall the ROM board in its case.
NOW THE BACKPLANE JUMPERS
Now it will be necessary to get to the 800 backplane. This requires removal of
the case. First remove all cables from the computer.
Turn the 800 over, with the game-ports facing you, and remove the five recessed
phillips screws holding the bottom cover. It may now be removed by lifting at
the rear and then sliding it forward to clear the game-port connectors. This
will expose the bottom RF shield.
Remove the nine phillips screws holding the bottom RF shield and mother board
in the RF cage. Remove the phillips screws on the sides of the RF cage. Gently
lift the front edge of the mother board and power supply boards and disconnect
the keyboard ribbon cable, the power supply cable (note its orientation), and
speaker cable. Separate the power supply board and mother board. The mother
board and lower RF shield may now be lifted up and out of the RF cage. Now
remove the CPU card.
The bottom shield must be removed. It is attached with four plastic expansion
pins. Use a screwdriver to push the center pin out until it can be removed
from the bottom. Now remove the outer pins.
The bottom shield will lift off and expose the entire backplane area under the
Using Figure 7 as a guide, install the three jumpers using a fine, solid
conductor, insulated wire.
Recheck your work with particular attention to correct pins, solder bridges and
Reinstall the bottom shield and replace the plastic pins.
Reinstall the CPU board, place the mother board in the RF cage, and re-insert
the power supply connector (remember proper orientation). Reconnect the
keyboard cable and speaker cable. Replace the screws in the RF cage and then
the nine screws holding the bottom shield. Reinstall the bottom cover and
replace the five phillips screws. The 800-PLUS 256k modification is now
NOW FOR THE LAST TEST
Turn the 800 upright and reinstall all cards making sure the 256k memory board
is in the middle slot. Replace the top cover and close the cartridge lid.
Again, power up the computer and watch for the blue screen to appear. As
before, if it does not appear, turn the machine off and check your work.
If all is normal, congratulations, you now have an ATARI 800 with 48k of normal
user memory and 240k of extended memory.
EXTENDED MEMORY DISK EMULATOR (EMDE/OS)
The most effective use of extended memory is as a disk emulator (virtual disk).
I used GENEMDE, (by H. V. Stacey), to extend Atari DOS II, version 2.0S. This
produces EMDE/OS which was written specifically for the 800 PLUS mod. It
effectively provides another "very fast" disk drive. DUP.SYS and MEM.SAV may
be made resident on the virtual disk and switching from application programs to
DOS and back again will occur almost instantaneously. With 288K total memory
installed the virtual disk may be configured as a full 720 sector single or
double density ASIC (slow) Extended Memory Diagnostic Programs.
The possibilities for use of this extended memory are many and varied. Very
fast disk reads and writes can speed up Data Base searches.
Use of the "virtual disk" when running AMODEM will significantly reduce the
upload and download times (and your long distance phone bill).
Disk duping becomes a breeze when you can use J. (Duplicate disk) or C. (Copy
files *.*) to move data to and from virtual disk.
Programs that leave your drive spinning for extended periods can often be
modified to perform much faster and with no wear on the drive.
I would be very interested in the uses you find. That is part of the fun and
challenge. Just send me a copy.
800 PLUS 256k-PARTS LIST
Desig Qty Type Description
ZX1 1 74LS02 Quad NOR Gate
ZX2 1 74LS175 Quad Latch
ZX4-5 2 74LS158 Quad Multiplexer
ZX6 1 74LS112 Dual Flip Flop
ZX7-14 8 41256 DRAM (150ns)
ZX3 0 Deleted in Rev. A