I’d installed 64 MB of RAM in my Macintosh IIcx as I wanted to install NetBSD on it, and UNIX needs all the memory it can get. However these computers perform a memory check on startup that can take up precious seconds of your time, especially when a large amount of RAM is fitted. The instructions for this memory check are stored in the Macintosh ROM. I want to change that ROM and disable it… The logic boards of these ‘030 Macs have a socket that can accept a ROM in the form of a 64-pin SIMM module. There are already newly manufactured flash based ROM SIMMs available to purchase, like BMOW’s ROM-inator II, but I figured I might as well save some pocket change by assembling one myself. I already own a ROM programmer.
I’d seen that someone on the VCFED forums had already designed an open hardware ROM SIMM, but I found that some aspects of the PCB design could be improved so I asked for the KiCad files and received them. I used a cleaner SIMM-64 footprint by dougg3, corrected the copper zone fills, and routed the WE# signal so it may be possible to program my SIMM in a ROM SIMM programmer (I have not yet tested this as I don’t own one).
Here are photos of what the blank PCBs look like. I got them manufactured by JLCPCB.
This SIMM uses 4 PLCC packaged EEPROMs that can be up to 4 megabits in size, which gives you 2 MB of ROM. Through-hole PLCC sockets are used as they are easy to solder by hand. Individual EEPROMs were burned using a Relatively Universal ROM Programmer.
I used SRecord to split a single, contiguous Macintosh IIsi ROM into four smaller binaries to then flash onto the EEPROMs.
The following excerpt comes from documentation for MACSIMM by GGLABS:
The 4 flash ROM can be programmed individually using any EPROM programmer that can program the relevant PLCC-32 flash ROM. The ROM file must be split in 4 sections according to: U4 is byte 0,4,8... U3 is byte 1,5,9... U2 is byte 2,6,10... U1 is byte 3,7,11... If the srecord package is available the following commands will generate the 4 files: srec_cat rom.bin -binary -split 4 0 -o U4.bin -binary srec_cat rom.bin -binary -split 4 1 -o U3.bin -binary srec_cat rom.bin -binary -split 4 2 -o U2.bin -binary srec_cat rom.bin -binary -split 4 3 -o U1.bin -binary
The Macintosh IIsi ROM is a universal ROM that also works on several ‘030 Macintosh II models (and SE/30). frontein1 of 68kMLA forums was kind enough to drop me a IIsi ROM modified to disable the memory check on startup. It is also a 32-bit clean ROM, so all RAM above 8 MB can be addressed without the need for the MODE32 patch.
Installing the ROM SIMM in my IIcx is as simple as slotting it in and moving a jumper.
With a SIMM like this one, hacks like modifying the happy Mac icon or changing the startup sound are possible. I will be looking into this.
I’ve published the gerbers and related design files on GitHub.
At the time I write this I still have two ROM SIMM PCBs left over, so if you’re interested in building your own and are in the United States, feel free to drop me a line…