Casio Pocket Calculator CPU Question

[David Hayden]

The pages on this site for individual calculators all have links labelled "Technology" that take you to a page describing the hardware. Lots of details are known - enough to create high fidelity emulators of all(?) older HP models.

HP used it's own processor which evolved over time. The biggest difference from a general-purpose CPU is that the HP processors generally had 56-bit registers that could store a floating pointer number with mantissa and exponent BCD-encoded. Hardware instructions could do BCD arithmetic.

The address space is generally small, growing from 8 bits in the classics to 20 bits in Saturn CPU (the pioneers and the 48/49 series). Each address accesses a 10-bit value in the classics and in the NUT (41C) also. In the Saturn processor, each address accesses a 4-bit nybble.

Another thing I haven't seen in general purpose CPUs: the CPU registers could be accessed by "fields" - predefined groups of bits within the register. This let you read/write the parts of a number (sign, mantissa, exponent, exponent's sign), or individual nybbles within the number.

Starting in the 49(?), the Saturn CPU is emulated by an ARM processor (a general purpose processor that's very common in small devices). The 39gii and Prime use the ARM processor directly rather than via an emulated Saturn.

I'm sure that some of what I've written is wrong and that others will correct me.