Downtown Doug Brown

TL;DR: The pinout of modern programmable Mac ROM SIMMs needs to be changed for correct operation in Quadras. If you’re interested in how I reached that conclusion, or at least want to see some cool pictures, read on below.

If you’ve been paying attention to the Classic Mac scene these days, you’re probably familiar with custom Mac ROM SIMMs such as the ROM-inator II. If not, here’s some intro material about them. JDW also made an awesome YouTube video explaining them.

One thing that you usually discover when searching for info about these programmable ROM modules is that they’re compatible with the earliest 68030-based desktop Macs: the SE/30, IIx, IIcx, IIci, IIfx, and IIsi. The most common use of them is to set up a special custom bootable ROM disk using Rob Braun’s driver, Big Mess o’ Wires’ driver based on Rob’s, or Garrett’s Workshop’s driver. In general, the compatible ROM images out there are all using the IIsi ROM which is capable of booting any of the aforementioned Macs.

What you may not know is that most of the later 68k Macs also have provisions for a ROM SIMM socket, but aside from the very first Quadras (700/900/950) which always have the socket installed, it’s not usually populated. Some early production or prototype units have it, but most just have empty through-holes filled with solder where the socket would go.

Several months ago, Will from CayMac Vintage reached out to me looking to resurrect my old Mac ROM SIMM programmer project. As a quick summary of that project, it provides a convenient way to program custom 64-pin ROM SIMM modules for vintage Macs from the late ’80s to early ’90s. There are several reasons you might want to do this, including: replacing an original ROM module that has gone bad, disabling the startup RAM test to decrease boot time in systems with a lot of RAM, bbraun’s amazing bootable ROM disk hack, or my startup chime hack. JDW recently made a cool YouTube video explaining custom ROM SIMMs if you’re curious about them. He even included some footage from 2003 of me playing basketball!

I used to make programmer boards and programmable ROM SIMMs and sell them to hobbyists, but it burnt me out. In particular, assembling the boards and the logistics of shipping were not fun to deal with. Thankfully, in 2016, Steve from Big Mess o’ Wires stepped in to take over. He made his own customizations to the programmer and made some really neat improvements to the bootable ROM disk driver. He still sells the Mac ROM-inator II SIMM to this day, but he stopped selling the programmer board. In the meantime, many other players have entered the market with custom ROM SIMMs, but nobody has been making the programmer available to the community, likely due to my non-commercial license on the PCB design.

Will was looking to fill that void. I helped him get going, but we discovered that the AT90USB646 microcontroller that I originally used was hard to find due to the chip shortage. At the time, it was easier to find the AT90USB1286 instead, which is essentially just the exact same chip, but with 128 KB of flash instead of 64 KB of flash.

I recently ran into an interesting warning on newer versions of ARM GCC, including the latest (as of this writing) Arm GNU Toolchain 12.3.Rel1. In particular I’m dealing with arm-none-linux-gnueabihf-g++. Here’s a very simple example program that demonstrates the warning:

#include <vector>
#include <iostream>

int main(int argc, char *argv[])
{
	std::vector<double> test;
	test.push_back(50.0);

	std::cout << test[0] << std::endl;

	return 0;
}