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Wednesday, February 9, 2011

Upgrading an IBM/XT to 640k of memory (1985)

The following recipe for upgrading IBM PCs and XTs to 640k of memory on

the motherboard was posted in 1985 by Mike Brown of Nicolet. I did this on

my PC Portable (XT motherboard) and it worked like a charm. It seems like

it's time for a repost ...

This document describes how to install 640k of RAM on the system board

in the IBM XT and the IBM portable (which uses the same mother board).

Parts list:

Quantity Description


18 256k by 1 Dynamic RAM chips (any of the following)

Manufacturer Part number


Fujitsu MB81256-15

Hitachi HM50257-15

Mitsubishi M5M4256-15

NEC uPD41256-15

OKI MSM41256-15

TI TMS4256-15

Toshiba TMM41256-15

1 74LS158 Decoder/multiplexer Integrated circuit

IBM XT Instructions:

1. Turn off the system unit, and disconnect the power cables, monitor and

keyboard cables and any cables that may be connected to expansion boards.

Remove the monitor and the keyboard. Place the system unit in a convenient

work area.

2. Take off the cover from the system unit by removing the 5 screws on the

back (4 corners and top center), slide the cover forward and tip up to

remove completely.

3. Take out any boards installed in the expansion slots by removing the hold

down screw at the rear of the chassis, and pulling the card straight up.

4. The floppy disk drives will have to be removed to gain access to portions

of the mother board. This is done by removing the screws on the left side

of the drive(s). Gently slide the drive(s) out of the unit far enough

to get at the cables plugged into the back of them. Making note of where

each cable goes, remove the data and power cables by gently pulling them

away from the drive. When the cable are off, remove the drive(s) from the

chassis and set them aside.

5. Refer to figure 1 and locate the jumper block labeled E2 on the mother

board. It is located near the edge of the board near the power supply.

6. A jumper has to be installed between pads 1 and 2 on E2. This can be

done without removing the mother board using a short piece of wire. Hold

the wire with a pair of needle nose pliers and heat up one of the pads

with a soldering iron. When the solder melts, push the wire into the pad

and remove the soldering iron. Do the same thing with the other end of

the wire and pad.

7. Refer to figure 1 and locate the IC socket labeled U84 on the mother

board. This will be an empty socket near the front of the board,

underneath where the floppies were mounted. Install the 74LS158 chip in

this socket making sure pin 1 (marked with a dot or notch) is pointing

away from the front panel.

8. Remove the 64k RAM chips in the rows labeled BANK 0 and BANK 1 (9 in each

bank) on the mother board using an IC puller. If you currently only have

128k of memory on the mother board (BANK's 2 and 3 empty) you can move

these chips to those banks. Be careful not to damage the pins when

removing them (you can sell them to a friend whose machine has amnesia).

9. Install the 256k RAM chips in the now empty sockets of BANK 0 and BANK 1

making sure they are installed correctly with pin 1 pointing away from

the front panel. You should now have 2 banks (0 and 1) of 256k RAM chips,

and 2 banks (2 and 3) of 64k RAM chips, giving you a total of 640k.

10. Refer to figure 1 and locate the switch block, SW1 on the mother board.

Make sure that switch positions 3 and 4 are in the OFF position.

11. Re-install the floppy drive(s) by sliding them into the front panel about

half way and reconnect the data and power cables in the same locations

they came off of. Push the drive(s) the rest of the way in and anchor

them with the screws removed earlier.

12. Re-install the your expansion boards (minus any memory boards that used

to be in the system) in the reverse order of when you took them out.

13. Put the cover back on, re-connect the cables and install your monitor and








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Figure 1

(XT/Portable motherboard)


Tony Movshon

Internet: movshon@nyu.edu

Usenet: movshon@cmcl2.uucp or {ihnp4

US Mail: Department of Psychology, NYU

6 Washington Place, room 1083

New York, NY 10003

Phone: (212) 998-7880

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