torsdag 22 september 2016

MAME: Saving old computers

When writing a MAME driver for an old computer it is more to it than one can think of.

You are seldom lucky enough to get full schematics or even access to real hardware. Sometimes all you get is a rom file from someone that worked on the system ages ago and some general idea. If you are lucky there are documents about the system on the internet somewhere like www.bitsavers.org. Sometimes you find a forum thread from someone who bought something on Ebay and posted some pictures asking people what kind of machine it was. The information mining and detective work is the first phase of the adventure. The less you find the more eager you'll get to find out what it was. At least that is how I am geared. :)

The next best thing after schematics are detailed pictures of the PCB:s involved which reveals the chips and how they are interconnected. However even with this at hand the PCB has components which hides the traces on one side and there might not be a picture of the bottom.


In this picture I have both bottom and top side of the PCB. The bottom part is mirrored so I can easily follow traces from top to bottom through the via holes without getting confused. Since I don't have the actual hardware at hand I can't use a volt meter to test which pins are connected, I need to see it on the image. I used Gimp to fill in the traces from the 74LS138 chip to be able to make an address map which is needed to write a MAME driver.

MAME also has a built in debugger which can be used to see where the rom code accesses things and then guess which of the chips it is, usually setup patterns from other drivers can be seen and verify the guess together with the visual trace on the PCB image.

It is actually great fun to figure out old designs from clues like this. In the end this specific work will be documented in the driver  and on my Candela page for others to find out.

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