The Apple Macintosh Plus was one of the most long-lived Apple computers and saw three revisions of its 128 kB-sized ROMs during its life time, at least officially. There’s a fourth ROM, sized 256 kB, that merges the Western ROMs with Japanese fonts. This would save a user of a Western MacIntosh Plus precious start-up time & RAM when starting software using these fonts. Unfortunately, this particular ROM existed mostly as a kind of myth, until [Pierre Dandumont] uncovered one (machine-translated, French original).
Since this particular ROM was rumored to exist somewhere in the Japanese market, [Pierre] went hunting for Japanese Macintosh Plus mainboards, hoping to find a board with this ROM. After finally getting lucky, the next task was to dump the two 128 kB EPROMs. An interesting sidenote here is that the MacIntosh Plus’ two ROM sockets use the typical programming voltage pin (Vpp) as an extra address line, enabling 256 kB of capacity across the two sockets.
This detail probably is why this special ROM wasn’t verified before, as people tried to dump them without using that extra address line, i.e. as a typical 27C512 64 kB EPROM instead of this proprietary pinout, which would have resulted in the same 64 kB dump as from a standard ROM. Thanks to [Doc TB]’s help and his UCA device it was possible to dump the whole image, with the images available for download.
Using this ROM image was the next interesting part, as [Pierre] initially didn’t have a system to test it with, and emulators assume the 128 kB ROM format. Fortunately these are all problems that can be solved, allowing the ROM images to be validated on real hardware as well as a modified MAME build. We were informed by [Pierre] that MAME releases will soon be getting support for this ROM as well.
Looks like they figured out the extra address line that nobody else twigged. That’s a hack in my book.
Obscure mac hardware history blog posts 👍👍
You can get a refund for your hackaday subscription at the front desk.
The screenshot has Japanese but the chips are made in Korea?
Is your OS in English despite running on Chinese chips?
對，是
Did the long-availabke tools to dump the ROMs from within the OS not work either?
The first 128 KB of the 256 KB dump is exactly identical to the already-well-known Mac Plus v2 ROM. Looking at the source code to CopyRoms, it uses the ROM checksum (stored in the first 4 bytes of ROM) to identify the ROM and decide how much to dump. So if anybody had used CopyRoms to do an in-system dump, it wouldn’t have detected the extra 128 KB of data after the normal ROM, and they would have thought they just had a normal 128 KB v2 ROM instead of a special 256 KB Kanji ROM.
I think the first half being identical is why this Kanji ROM went undumped for so long. On the hardware ROM dumping side, there’s another issue: I think there is somewhat of a hole in ROM programmer support for mask ROMs that use all the address pins. For example, when I search through the XGecu software, I don’t find any 1024-kilobit devices in DIP28 form factor. There are lots of devices with 512 in their name, but nothing with 1024. So if I had these chips, I couldn’t have completely dumped them with my T56, at least I don’t think so. It’s like there needs to be an extra “dump only” mode for certain ROM pinouts that don’t have any kind of VPP/!WE pin but have an extra address line in their place instead. Maybe other ROM programmer devices already have this issue solved — I’m not sure.
28-pin 128KiB (1 mebibit) mask ROMs are all over the place in NES=Famicom games … and they have an explicit JEDEC spec … but otherwise they’re pretty rare.
Unlike the previous time (24-pin 8KiB mask ROM) where there was a ‘PROM that put /OE on the same pin as Vpp, no one ever seems to have made a 28-pin 128KiB PROM with the same trick. Instead, we got things like the 27C301, where the bottom 28 pins matched the 23C1024 pinout, but there were an extra 4 pins at the top just for Vpp and /WE.
Thanks for the tip about how they’re common in NES games, and the pinout similarity to the 27C301. Seems like that would be a potential workaround for dumping the full contents of such a chip using a programmer like mine that doesn’t seem to support the 28-pin 128 KiB DIP mask ROM pinout. Looks like VCC would have to be routed to pin 32, but that’s doable.
Yes, I was looking at the JEDEC pinout myself (figure 3.2.1-2) as I wrote my comment. I’m guessing the reason they didn’t do the same trick on the 28-pin pinout is because /OE was already gone from that pinout (A16 replaces it). 17 address pins, 8 data pins, /CE, VCC, and GND use up all 28 pins.
I realize I wasn’t quite explicit enough: the 23C64 mask ROM has 24 pins: one 5v, one ground, eight data lines, 13 address lines, and a single output+chip enable. Motorola made two UVEPROMs (MCM68764, MCM68766) that exactly matched that pinout where the /CE pin is now also Vhh=PGM.
The curious question is why did that not happen again for 28-pin UVEPROMs.
“this mythical ROM existed mostly as a kind of myth”. Yes, most myths are mythical.
The photo caption mentions EPROMs, but those are masked ROMs, not Erasable or Programmable. The program content was fixed at the time of manufacture. Once the code is finalized, it is much cheaper to manufacture thousands of copies as a ROM.
All it took to find this mythical chip was a bit of search on Japanese ebay? Was he that lucky or no one really looked? I am surprised that Japanese hackers were not blogging about having better Apple equipment than Americans.
I was hoping for a weeks long deep dive of Akihabara or something.