This one became a three-part project:

1. disassemble the Game Unit
2. desolder the BIOS ROM
3. dump the contents

The BIOS chip is labelled MPR-15768-T. It is a mask ROM rather than an erasable 27C1024 EPROM, but I was able to read it using a 27C1024-compatible setup. A 27C1024-class device stores 1 megabit as 64K x 16 bits, which works out to 128 KiB. That is also the size of the BIOS image I dumped.

At the time, I believed this was the first public dump of the Aiwa BIOS. MAME now catalogues mpr-15768-t.bin for the Aiwa with a size of 128 KiB and CRC32 8052c7a0. That confirms the catalogue entry, but not who released the first public dump.

That makes this little exercise rather more interesting than simply practising with the programmer.

I later came back to this same area of the Game Unit to install a BIOS socket, repair damaged traces and burn replacement EPROMs.

For now, though, the important bit is done:

BIOS preserved.

More details from the original investigation can be found in the archived ASSEMblergames thread.

Related posts

• Aiwa Mega-CD Game Unit BIOS Socket Installation and Trace Repair
• Aiwa Mega-CD CSD-GM1 Game Unit EPROM Programming and Repair Test

Sources

• Atmel - AT27C1024 Datasheet - documents the 1 Mbit, 64K x 16 organisation; this does not itself establish the Aiwa mask ROM’s compatibility.
• MAME - Aiwa Mega-CD ROM definition in mdconsole.cpp - lists mpr-15768-t.bin, a load size of 0x020000 bytes (128 KiB) and CRC32 8052c7a0.