Nonpareil status

[teenix]

(10-06-2022 11:31 PM)brouhaha Wrote:  Speaking of this being a research project, one of the things I'm looking into is that it appears that the HP-67 and -97 have three data storage registers in the hardware that are UNUSED! I haven't yet convinced myself 100%, but I can't find anything in the ROM that accesses registers at hardware addresses 0x3a through 0x3c. It seems that they could have made them available as user storage registers, allowing for 29 total instead of 26. Of course, they'd only be indirectly addressable.

I noticed this also, but didn't look too deeply into it.

Maybe it was easier to use the SAME ROM/RAM type for manufacturing even though that RAM was not going to be used.

The code for RAM transfer is a simple loop with...

$0EA0 1E2 c + 1 -> c[p]
$0EA1 23F if no carry go to $E8F

as the final test, and when C[0] > 9, the loop stops.

If the Binary mode was enabled and the test terminated when C[0] = $D, the extra registers could have been transferred. A modification to the loop may have required too many extra instructions and wouldn't fit in any remaining unused ROM.

cheers

Tony