Around 1983, HP published the HP 45423A Financial Calculator software package for their HP-150 Touchscreen computer. It was in fact a microcode-level simulation of the HP-12C calculator. Internally HP had the 11C, 15C, and 16C running on the 150 as well, but didn’t release those.

The ROM file format was a bit strange:

• 48 byte header
• 12288 words of ROM code
• one word of checksum (I think)
• 206 zero bytes, apparently to pad the file out to a multiple of 256 bytes

They packed the 10-bit ROM words into two bytes, which is what I’d expect, but they put the two LSBs of the ROM word into the LSBs of the first byte, and the remaining eight bits in the second byte. And they added a pseudo-random 6-bit sequence to the otherwise unused 6 bits of the first bytes. It’s just a sequence that repeats every 64 words, and is the same for all the models. I can’t imagine any reason to do that other than a crude attempt at obfuscation.

The dates on the files are 1983. The data is bit-for-bit identical with the 11C, 12C, 15C, and 16C calculators I’ve dumped so far, which confirms that my romsucker works reliably.