Hacking old Commodore Calculators.

[orac81]

Hacking old Commodore Calculators.

This works for the Commodore Scientific Calculator Model SR4912, also the PR50, and others.
Many moons ago I discovered it is possible to confuse this calculator into showing illegal (hex) digits.

Type: 1 EE 80 STO 0 Mx 0 C/CE RCL 0 (Repeat until exponent in range 06-36)
Then type: X^2
You will now see something like "5. 93E71o 8'" displayed with illegal
(probably hex) digits mixed with the number.
Now try: x RCL 0
Type = to resolve the original number. (You can then figure what these hex digits are!)
Also try: 1 EE 1 0 - 1 = You will see 1 EE 00! Hit = to show 1 EE 10.

I think what is happening is that the BCD mode in the on-chip (probably 6502) CPU is being overflowed into illegal digits,
because of bugs in some of the internal ROM routines.
After some messing about, I believe the hex digits are: " = 10, ' = 11, o = 12, E = 13, - = 14, " " = 15.
(I have tried to make an approximation of the LED display using ASCII here)
I doubt you can go too far with this, ie: get the calc to run code, since the system only has a minimal amount of RAM, and there is no way
to execute a jump to RAM, but it is interesting.
Has anyone ever reverse engineered the processors on these calculators? I am guessing they are 6502 based.

[wimoos]

This calculator holds the MPS 7561, which is a dedicated processor for calculators...nowhere near a 6502.