WIP: 16C firmware hack for more memory

[brouhaha]

Indirect addressing was working to over 3300 in integer modes (HEX, DEC, OCT, BIN), but the code for FLOAT mode was limiting it to 99. I just got a patch working to allow up to 999, though with the current memory map, the maximum 56-bit register range is 0 to 237. Actually only to 236 at the moment due to the register address computation bug that I still need to fix.

Hopefully no one needs to store more than 237 floating-point values (or 56-bit integer values) in an HP-16C. :-)

So far I've patched 26 words in the base ROM, and added 57 words of expansion ROM.

Next on my TODO list for the 16C: track down the skipped nybble bug in the address computation. At the moment, I only know where that computation is done, but I don't at all understand how it does it.

This 16C hacking has been fairly fun. The previous attempt at 16C hacking, back in 2004 or 2005, was frustrating and nonproductive.

I did work back then on a hardware hack to do the ROM patching and RAM expansion for a real 15C. I need to dust that design off and bring it up to date, and try hacking the real machines (original Nut versions, not ARM).

But before that, I really should get back to 12C hacking, because while the 16C is of more use to me personally, the 12C improvements are probably useful to a wider audience, and I should be able to program them into a current production 12C (ARM-based).

In principle, I think I could do the memory expansion for the 11C fairly easily, but I don't think I'll bother, since the 15C is pretty much a superset of the 11C. Also, it wouldn't be hard to increase the available program steps of the 10C from 79 to 99, but I think that's even less worthwhile..