Everyone is moving toward 3.3V and lower. After a ton of searching, I settled on the Micron M29F160FB5AN6E2, a 5V 16-megabit chip that can be accessed in parallel using either a 16-bit data bus or an 8-bit data bus. It turns out that 5V chips are becoming harder and harder to find, especially in larger sizes. You can probably see where I’m going with this–I designed a bigger ROM SIMM!įirst of all, I had to find NOR flash chips that ran on 5V and were big enough. After taking into account the 512 KB ROM size, that would leave 7.5 MB of usable space for a ROM disk. Each address points to 4 bytes of data, which provides for a total maximum ROM SIMM size of 8 MB. However, A0 and A1 are unused due to the fact that the data bus is four bytes wide, so there are really only 21 usable address pins. The 64-pin ROM SIMM slot has 23 address pins labeled A0 through A22. It boots up extremely quickly!Īfter he came up with that brilliant idea, I figured I should maximize the potential size of the disk image. It works perfectly and allows you to create a 1.5 MB bootable ROM disk to append to the 512 KB IIsi ROM.
However, bbraun came up with an even better idea - a ROM disk driver! He has patched the IIsi ROM (which should be compatible with the SE/30, IIx, IIcx, IIci, IIfx, and of course the IIsi itself) to enable booting from a ROM disk that uses up the remaining space on the SIMM and also has some other tweaks like disabling the slow RAM test that the previously-listed machines perform on every cold boot.
I always figured that the 2 MB size of the SIMM was plenty - I never really envisioned using the custom ROM SIMM for anything other than customizing the startup chime. The combination is pretty nifty and enables people to do all kinds of crazy things that previously weren’t possible. Everything from the hardware design to the firmware is completely open source. These SIMMs (and programmers) are available for purchase here.Īs you may have seen from my previous posts, I have developed a programmable ROM SIMM for older Macintosh computers and a programmer to go with it.
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