steve@tic.UUCP (Steve Drake) (02/19/88)
I am posting this for a friend please send all replies to: ..!ihnp4!alberta!edson!tic!tac!amunds Thank you... I understand that the atari 800XL can be upgraded to 256K by replacing the 64K chips and doing some appropriate address decoding with the ACIA. I was wondering if anybody could direct me to an appropriate source to find the nescessary information or tell me what is required. I am a 4th year computer engineering student so technical descriptions would be just fine. Thank you Dave Amundson
jhs@MITRE-BEDFORD.ARPA (02/23/88)
You can indeed upgrade your Atari 800XL to 256K quite easily, i.e. by replacing the 64K chips with 256K ones. The 64K chips have 8 address pins which are multiplexed 2:1 to give effectively 16 address bits, which figures out to 64K of address space. The 256K chips need effectively 18 address bits, or 9 pins if multiplexing is done in the same way as for the 64K chips. The bottom line is, the 256K chips have one extra pin that must be controlled. If you just change the chips to 256K chips, that extra pin will be held at a logic "1" by a 3K pullup resistor near the corner of the board. So the machine will still work normally but will only utilize 1/4 of the 256K chip address space. To get at the rest of it, you need to build a simple logic circuit that figures out when to yank down on that line and select one of the other 64K regions. The logic needed is fairly tricky because there are two clock phases, corresponding to the multiplexing of the address lines; there is also the issue of whether the CPU or ANTIC chip is requesting access; and finally, the logic has to take into account the program's commands for which section of memory to access. In the 130XE, and therefore in most 800XL upgrades, this information is represented by controlling previously-unused bits in the "Port B" PIA register at $D301 or decimal 54017. Even though there are 6 or 7 logic conditions to consider in calculating the correct bit value for "Address Bit 9", by using multiplexors to implement combinatorial logic functions it is possible to do the job in about 7 or 8 chips. The details of memory bank selection in the 130XE are described in the most recent version of "Mapping The Atari", and you should familiarize yourself with this scheme (basically the bit definitions at $D301) before attempting to design a compatible board. Several commercial companies have built such boards. I have been using the board from Newell Industries, the so-called 256KXL "Newell Board". I was disappointed to find that this board departed from 130XE compatibility more than I thought was necessary, so I reconstructed the schematic and made a design modification so that it would do a more nearly perfect job of emulating the 130XE. I can supply you with the original and revised schematics if you wish. Being an engineering student, you should probably build your own board, but it might help to have the Newell schematic and the design information I used to modify it. Or, you could buy a Newell board and consider making the patch to it that I worked out. Also, there have been a few magazine articles on how to design such a board in recent years. If you want to order from Newell, his address/phone number are: Newell Industries 602 E. Highway 78 Wylie, TX 75098 (214) 442-6612. Usually Wes Newell himself is available for technical questions. By the way, he is also handling sales of the CDY "OMNIVIEW" 80-column upgrade chip, which I strongly recommend getting if you plan to do a lot of word processing on the 800XL. (An 80-column word processor program comes with it.) You will also need software to make the expanded memory do you any good. One of the most useful things to do with it is to run a RAMdisk. If you buy or build a standard Newell board, you may be interested to know that OMNIVIEW256 includes a RAMdisk program which is ROM-resident, i.e. available by a couple of keystrokes. The word processor that comes with OMNIVIEW also lets you take advantage of the extra memory to set up a really BIG text buffer. For example, I routinely run an 80K text buffer simultaneously with a full-sized SS/SD RAMdisk. This lets me write almost 25 pages of text before I have to save to disk! Also, saving to RAMdisk is very fast. If you'd like a copy of the design info for the modified Newell board, send me a mailing address and I'll mail you a copy. -John Sangster, jhs@mitre-bedford.arpa