The Lost 256 KB Japanese ROM for the MacIntosh Plus Has Been Found

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 …read more

May 18, 2025 - 04:50

The Lost 256 KB Japanese ROM for the MacIntosh Plus Has Been Found

Mainboard with the two 128 kB EPROMs containing the special MacIntosh Plus ROM image. (Credit: Pierre Dandumont)

The two 128 kB EPROMs containing the special MacIntosh Plus ROM image. (Credit: Pierre Dandumont)

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 mythical 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.

Top image: Mainboard with the two 128 kB EPROMs containing the special MacIntosh Plus ROM image. (Credit: Pierre Dandumont)