¢¢¢¢ Atari XL OS RAM Upgrade by Russ Babylon¢ ¢ Modified for 1200XL by Kyle Dain¢ ¢ The following article shows how to modify an 800XL or 1200XL computer¢ to allow any RAM based operating system to remain resident under any¢ situation short of turning off the power to the computer. Not only¢ will the RAM operating remain active it will also be uncorruptible,¢ that is it can not be changed once it is enabled. Sound interesting?¢ ¢ As with any project involving internal modification to your computer¢ do not attempt this unless you are familiar with electronic circuits¢ and soldering.¢ ¢ You will need one 74LS00 IC, a SPST subminature switch, a 5K resistor¢ and a couple of feet of 30 gauge hook up wire. At most a cost of two¢ or three dollars.¢ ¢ To start the project first disassemble your XL and remove the metal¢ shields from the circuit board. Next locate the PIA chip, it is the¢ 40 pin chip marked U23 on both 800XL's and 1200XL's. The PIA should¢ have CO14795 or 6520 somewhere in the jumble of numbers on the top.¢ ¢ Most, if not all 1200XL's will have socketed chips. If your chips are¢ socketed, carefully remove the PIA from its socket and locate pin 10. ¢ Pin 10 corresponds to bit 0 of memory location $D301. CAREFULLY bend¢ up pin 10 slightly so that it will not be in the socket and then¢ reinsert the PIA into the socket. If you have soldered chips then the¢ easiest way to do the mod would be to carefully cut pin 10 from the¢ circuit board with side cutters so no electrical connection existed¢ between pin 10 of the PIA and the foil on the board.¢ ¢ Make SURE you leave enough of pin 10 to solder to. I do not like the¢ idea of cutting traces on the circuit board but if you do and can¢ trace them carefully enough more power to you.¢ ¢ Now solder about a twelve inch piece of hookup wire to pin 10 of the¢ PIA.¢ ¢ On the 800XL, flip the board over and find where pin 10 used to be¢ connected and solder another twelve inch length of hookup wire to this¢ point.¢ ¢ On the 1200XL, solder about 8-10 inches of hookup wire to the solder¢ pad just above the color adjust pot. This pad will be at the lower¢ right of the CPU chip (CO14806) just below pin 19. If you can't find¢ it, use the top pad of R48 instead.¢¢¢¢¢¢¢¢¢¢¢¢¢¢¢¢¢¢¢¢ Now for the next step; with the board right side up, locate the¢ 74LS375 chip. It's U28 on the 800XL, and U11 on the 1200XL. It is¢ used to latch the R/W signal from the 6502 CPU. You need to locate pin¢ 14 of this chip and then flip the board over and solder about eight¢ inches of wire to the back of the board where pin 14 connects.¢ ¢ On the 1200XL, you will also have to put a jumper from the R/W line to¢ pin 15 of the 74LS375. The best place to get the R/W signal is at the¢ middle solder pad below U17 (74LS08). Make this connection on the¢ bottom of the board and make SURE you use the MIDDLE solder pad. Take¢ this jumper to pin 15 of the 74LS375 chip (U11).¢ ¢ You will notice that pin 14 of the 74LS375 has no foil connections to¢ it since it is unused in a standard XL but we are going to make good¢ use of it.¢ ¢ Carefully bend up all pins EXCEPT 7 and 14 on the new 74LS00 chip. ¢ Stack this chip on top of another 14 pin 74LS chip (such as 74LS08)¢ and solder pins 7 and 14 to the chip. Don't over heat either chip!¢ ¢ OK, you have power to the chip and now need to hook everything¢ together. Take the wire you have connected to pin 10 of the PIA and¢ solder it to pin 1 of the 74LS00 chip.¢ ¢ Take the wire that was connected to where pin 10 of the PIA used to be¢ and connect it to pin 8 of the 74LS00 chip.¢ ¢ Take the wire that you hooked to pin 14 of the 74LS375 chip and¢ connect it to pin 13 of the 74LS00 chip. On the 74LS00 chip itself¢ connect pin 2 to pin 4, pin 4 to pin 5, pin 6 to pin 12, pin 3 to pin¢ 9 and pin 10 to pin 11.¢ ¢ You are almost home! Now take the 5K resistor and trim it down and¢ connect one side to pin 2, 4, or 5 of the 74LS00 chip and the other¢ end to a +5v source, or to pin 14 of the 74LS00¢ ¢ Solder a small piece of wire to pin 2, 4, or 5 of the 74LS00 (where¢ you put the resistor). The other end of this wire is hooked to one¢ side or the other of the SPST switch.¢ ¢ On the other switch terminal connect a wire to any convenient ground¢ point or pin 7 of the 74LS00 if none are to be found.¢ ¢ That completes the electrical work! Now you have a switch hanging in¢ mid air looking for a home.¢¢¢¢¢¢¢¢¢¢¢¢¢¢¢¢¢¢¢¢¢¢ For the 800XL, a good place is between the monitor output jack and the¢ RF output.¢ ¢ For the 1200XL, there are many good places, but I put mine right above¢ the channel select switch. That will only work if your switch is¢ small enough.¢ ¢ Drill a 1/4 inch hole in this area and mount the switch after the¢ circuit board is installed in the bottom cover.¢ ¢ Now all you have to do is put it all back together. The wires from¢ the circuit board can be passed through any circuit board and metal¢ shield openings being careful not to short or cut any wires or¢ insulation.¢ ¢ The moment of truth is at hand! With the SPST switch set so it is¢ OPEN power up your computer. It should act perfectly normally in any¢ mode or executing any program.¢ ¢ Now for some fun load in a translator program and after it is in and¢ running flip the SPST switch to close the contacts. The computer¢ should not be visibly affected. Now push RESET and you will find that¢ the translator is still there and operating. Go into BASIC with the¢ translator or other RAM operating system and try pokeing different¢ values into the RAM area where your operating system resides and you¢ find that you can't once you have closed the SPST switch.¢ ¢ This project has many possible uses. If you are proficient at machine¢ language you can modify a RAM operating system to do all sort of nice¢ things.¢ ¢ It could make an Omni-mon look sick. You can make a copy of your ROM¢ based operating system except change the RESET vector at $FFFC and¢ $FFFD. Install this system in RAM put the switch back in its normal¢ position and boot up some picky piece of software that checks for¢ custom or RAM based operating systems and wait until it loads. Then¢ close the switch and hit reset and you can go to and execute some¢ utility program. I have used this technique to get memory dumps of¢ loaded programs.¢ ¢ The things you can do with this modification are only limited by your¢ imagination. This mod gives you almost total control of your¢ computer.¢ ¢ Any questions, comments, or suggestions are gladly accepted. Russ¢ Babylon can be reached at the Ratcom BBS 437-9813 and leave a message¢ for Big Russ. Russ also can be reached from time to time on¢ CompuServe 72327,404. Kyle Dain can be reached on the XEP Net,¢ CompuServe 72347,3014, or the S.A.G.E. BBS 814-833-4073¢ ¢
