Beck-Tech's modified ROM was exactly the same as the original, except that in about a dozen places, Andy had hard-coded the screen address for 512KB of RAM, and those needed to be bumped up to the 4MB limit that the hardware allowed. When the 128K (Plus) ROM came along, it fixed the memory limits. A 512Ke could take a MacMegabytes card without any ROM changes and have a meg of system RAM. We were quite relieved when Apple released the 128K ROM upgrade. Of course it made everybody else's RAM upgrades work properly too, so we gained some, we lost a little. Apple's 128K ROMs are also 100% compatible with the 64K ROMs, so a Mac Plus will run all the early pre-release demos like Pepsi Caps or the first Alice, which aren't even Finder-compatible.

I have a Beck-Tech logic board poster around here someplace. I guess I should photograph it and post it. Too big for my scanner. I also have my original MacDraw schematics which the poster was done from as well as Phat's timing diagrams and PAL analysis. I'll dig them out and put them on dotmac. You'll need a classic Mac with MacDraw to open the files. I have the IM Underground stuff filed away someplace, and a 1986 Hackintosh I should probably take pictures of for history buffs. Phat and I weren't the only ones reverse-engineering the Mac.

Thanks for the pointers to the review and the schematic, JDW. I had forgotten those. My God, is there anything about the early Mac you don't know? 8-o