![]() It’s not rocket science, but it had been 20 years since I last did any real classic Macintosh programming, so it took me a while to dig up Metrowerks Codewarrior and remember how to create a “hello world” type program. To make this happen, I needed to write a flash updater program that could run on a vintage Mac. From this mode, it’s possible to query the chip parameters, erase blocks of flash memory, and write new values to the flash. That’s a good thing – you wouldn’t want a software bug to go modifying your ROM! But if you write a series of magic “unlock” bytes to a series of magic addresses in the ROM, it will enter a special command mode. Writing to a ROM is a strange concept, and with the 29F040B, it will ignore standard attempts at writing. So I knew in-system modification of the flash ROM data was possible. By coincidence, I had just finished my 68 Katy breadboard computer, which also used a 29F040B flash ROM and a 68000-family CPU. In a discussion on the forum, I proposed connecting the CPU’s R/W pin to the flash ROMs, to make in-system reprogramming of the disk image possible. ![]() But the disk image is fixed in ROM – to change it, you need to open up the Mac and put the flash ROM chips in an external EPROM programmer. It even has a custom “pirate Mac” icon at the boot screen, to remind you that you’ve got a modified ROM installed. Originally designed for custom ROMs in Mac II-series machines, Rob updated the ROM disk driver to work with the modified Mac Plus ROM as well. Rob and Doug Brown studied how the Classic’s ROM disk driver works, and developed a stand-alone ROM disk driver for vintage Macintosh computers. This can be done using IC test clips, or by soldering wires directly to pins on the 68000 CPU.Īpple provided the Macintosh Classic with a built-in ROM disk: hold Command-Option-X-O during startup, and the Classic will boot from the built-in disk image. In order to take advantage of the entire 1 MB, two extra wires must be connected to address pins on the CPU. Rob Braun designed an adapter board that fits in the stock ROM sockets of a Mac Plus, and accepts modern (well, less ancient) 29F040B flash ROM chips. Swapping the stock ROMs for larger 1 MB flash ROMs requires a physical adapter, because the flash ROMs have more pins and have some signal pins in different locations than the stock ROM chips. This project is a combination of three smaller projects, two of which were developed by other people. The same thing should be possible for the Mac 128K and Mac 512K too. You can fill the ROM disk with system software and a couple of games, or whatever you want to show off without needing a disk. When necessary, the contents of the ROM disk image can be rewritten from within the running system, using a custom-made flash updater program. ![]() When powered on, the Mac can be booted from the built-in ROM disk with a single key press. The flash ROM contains a modified copy of the original Apple ROM data plus a special disk driver, and a disk image occupies the remainder of the 1 MB of flash. Using a small adapter board, the original ROM chips on the Mac’s logic board are replaced with 1 MB of flash ROM. How about a bootable, rewritable ROM disk for a Macintosh Plus, using modern flash memory? Dream no more, the reality is here. Stuffing new technology into old hardware is fun.
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