HP-71B to sacrifice in the name of SCIENCE!

[J-F Garnier]

(03-10-2024 12:28 AM)brouhaha Wrote:  I expect that the undocumented behavior of BCD arithmetic on non-BCD values is probably identical.

I guess so, because some parts of the HP-71B OS depend on this behavior !

Quote:It will be interesting to determine what length the 1LF2 thinks the level 1 opcodes are, and what (if any) affect they have on registers.

I already checked that the PC=(A) opcode is correctly fetched on the 1LF2 and doesn't break the opcode flow.
I used it to identify the CPU type by software on the 71B (here).

The opcode for PC=(A) is 808C and is part of the 808x group, in the 1LF2 only the 8080 INTON and 808F INTOFF are defined.
So it depends on how the opcode decoding is done in the 1LF2, but I would expect the 808C opcode to be executed as INTOFF.

On the other hand, I always wondered why the RSI opcode is 80810 and not just 8081.
Due to this 5th nibble, an attempt to execute RSI on a 1LF2 will probably put the CPU out of sync with the next opcodes.

J-F