HP11C ++

[brouhaha]

(01-30-2022 02:04 PM)Mike T. Wrote:  Did anyone ever get to the bottom of this ...?

https://www.hpmuseum.org/cgi-sys/cgiwrap...read=11083

By now I imagine that you, and everyone who is interested, knows the answer, but for the record...

The extra chip in the 15C is the second R2D2, with the display signals not bonded out. Some, but not all, Voyager calculators of other models had the footprint for the second R2D2, in order that a common PCB could be used for all models.

None of the Voyagers have microcode to dynamically recognize more memory.. Puting the 15C second R2D2 into any other model will not (by itself) accomplish anything.

The first R2D2 in any Voyager holds ROM addresses 0x0000..0x17ff, RAM addresses 0x00..0x07, and the display registers at 0x09..0x0a.

The second R2D2 in the 15C holds ROM addresses 0x2000..0x37ff, RAM addresses 0x10..0x17, and (unused) display registers 0x19...0x1a. The 15C uses the second R2D2 display registers as scratch space, although the low six bits of those registers are believed to be unimplmented.