Downtown Doug Brown

These SIMMs (and programmers) are available for purchase here.

As 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. Everything from the hardware design to the firmware is completely open source. The combination is pretty nifty and enables people to do all kinds of crazy things that previously weren’t possible.

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. 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. It works perfectly and allows you to create a 1.5 MB bootable ROM disk to append to the 512 KB IIsi ROM. It boots up extremely quickly!

After he came up with that brilliant idea, I figured I should maximize the potential size of the disk image. The 64-pin ROM SIMM slot has 23 address pins labeled A0 through A22. 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. That provides for a total of 2²¹ = 2,097,152 = 2*1024*1024 usable addresses. Each address points to 4 bytes of data, which provides for a total maximum ROM SIMM size of 8 MB. After taking into account the 512 KB ROM size, that would leave 7.5 MB of usable space for a ROM disk.

You can probably see where I’m going with this–I designed a bigger ROM SIMM!

First of all, I had to find NOR flash chips that ran on 5V and were big enough. It turns out that 5V chips are becoming harder and harder to find, especially in larger sizes. 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’s designed with automotive applications in mind, which probably explains why it still exists as a 5V chip. 16 megabits (Mb) is equal to 2 megabytes (MB). In order to reach the full 8 MB size, I need four of them per SIMM. I use them in 8-bit mode so each of the four chips provides one of the four bytes per address, just like how I designed the 2 MB version of the SIMM.

The chips are available in a fine-pitch TSOP package, which pretty much eliminates the possibility of socketing them. These chips really need to be soldered down, so this new SIMM requires my programmer board in order to put a disk image onto it. Here is the final product:

Quickly after I finished the new SIMM, bbraun updated his driver to work with a 7.5 MB ROM disk. So now it is possible to create a 7.5 MB ROM disk and boot from it. If you’re interested in buying one of the SIMMs and programmers (believe me, I have plenty to go around), you can purchase them here. As always, the 8 MB ROM SIMM is completely open source so you can make one yourself if you’d rather do it that way.

Some newer machines like Quadras have pads on their motherboards for these SIMMs–you just have to solder on your own socket. Unfortunately, it’s looking like they are hardwired to only address 1 MB of ROM space. Unless someone discovers a way to get around that, those machines will probably not be able to take advantage of the ROM disk. So for now, this ROM disk is restricted to the SE/30, IIx, IIcx, IIci, IIfx, and IIsi.