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motosega

Macintosh Classic (XO) Custom Rom Disk?

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Has anybody ever made a Custom boot rom for the Classic?
the XO classic boots really quickly with the rom disk and the rom slot can hold a bigger ROM, it'd be nice to have my own custom Rom.

i have a Classic and an eprom programmer. but my knowledge of the mac rom and floppy format are quite limited, i'd like to do some RTFM, but i'm not sure where to start, any hints? 

i hope that the ROM contains a floppy image that i can exchange for my own image, but i wouldn't be suprised if it turns out a bit more complex.
how could i identify the floppy image in the rom? 

p.s. i'm new here, i've been lurking occasionally over the years, loads of great projects happening here!

 

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Hey, welcome in!

 

As far as I know, the built-in/default ROM isn't modifyable, but there have been projects around building custom ROM simms for certain machines, including building XO-like ROM boot features for other machines. The SE/30 and members of the II family are popular recipients of such ROMs, for a few different reasons.

 

This is one of the more popular ones: http://www.bigmessowires.com/mac-rom-inator-ii/

 

Using a custom ROM SIMM would involve, if one hasn't been built yet, ripping the existing ROM, writing it to a new SIMM with a flashable chip, and then re-integrating whatever you want on your boot image (hypothetical example: system 7.1 with some networking updates to make connecting to a file server more convenient) into the ROM file, and then rewriting it to the actual ROM.

 

 

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The XO classic has it's rom on a 32 pin Dip mask rom, which looks at first glance to be compatible with a 27c020 eprom

i suppose my first step should be to dump the rom, copy it to a compatible eprom and see if it works.

 

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On 10/1/2019 at 10:41 AM, motosega said:

The XO classic has it's rom on a 32 pin Dip mask rom, which looks at first glance to be compatible with a 27c020 eprom



 

IIRC, the Classic had a 40-pin rom chip, which is less common nowadays. Am I wrong ?

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i haven't had the chance to open it up again, and i found a rom image online,
it's 512k, so thats going to fit into a 27c040.
the socket actually has extra unused pins for a bigger chip.

according to Designing Cards and Drivers for the Macintosh Family 3rd Edition 1992.pdf the classic has a 512k rom which is upgradable to 2mb.

Edited by motosega

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A 27c040 won’t work. The original ROM is 16bits wide and a 27c040 has only 8 data bits.

You would have to build some sort of adapter that plugs into the ROM socket and has sockets for two 27c chips.

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i think i have some 27c400 and 27c800 16 bit eproms too (used for amiga kickstart) hopefully the pinout is the same.

i've made a few adapter pcbs like that recently as i've been reprogramming the sound Roms on old drum machines, sometimes they have mask roms with non standard pinouts. i made an adapter with a zif socket with a load of jumpers so i can test adapter wiring before making pcbs, maybe that will come in handy.

 

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