Sherry Haibara said:

I wonder if it would still work with the full modification in place and only 128k of RAM or if it would require a chip swap?

I've got a friend who should have an extra pair of right ROM chips, so that modification could be easily reverted; the floppy, though, would likely remain a 800k one, unless I'm able to find a 400k drive in working order. I'm wondering if a 800k would work with the original ROM set and just be "castrated" to read 400k floppy, or if would just stop working.

Click to expand...

Do you mean leaving all that wiring in place but just have it magically work with the 64k chips in there? No. There's no identifier in these DIP RAM chips to tell the computer what they are, it's hardwired. The modification to expand a 128k to a 512k cuts free a normally grounded ("nc") line on the motherboard and uses it as an additional multiplexed address line. (IE, the 64k chips have A0-A7 which are time-multiplexed to translate a 16 bit address into two 8-bit row/column values; the 256 chips use the not-used-on-a-4164 pin1 as A8 to make the range of possible values 18 bits, IE, 256k. RAM is 16 bit wide, IE, two banks, which is why that translates to 128 and 512k RAM sizes respectively.) To make it work with the smaller chips you *have* to rip out the extra circuitry which does the address multiplexing and expands the memory map.

(And to make it even *more* intimidating, that memory mod, which undoubtedly involved piggy-backing a second set of 256k RAMs onto the first set, almost certainly means one of the piggybacked chips that's left behind was tasked in part with controlling which set of chips got the chip-select signal... which also implies a cut trace somewhere.)

Again, I'm sure it's totally un-doable, but the board's going to be a cut-up mess. Uniserver can probably help you identify exactly what rev board you have and provide you some pictures/schematics of what it's *supposed* to look like, but... I don't know, I think if I had that sitting on my workbench I'd probably start googling like mad to see if I could find the DIY article about upgrading a 128k/512k to a meg of RAM (it's gotta be out there somewhere) and try to put it back the way it was.

Per the disk drive, it depends on the mechanism. The first couple revs of the 800k mechanism are incompatible with the 64k ROM unless the HD-20 init is loaded from a compatible drive, which makes them completely incompatible with a 128k. (HD-20 init won't load on a 128k.) Later 800ks will work as a 400k drive, if you google around you can find the magic number to look for.