There is an unused code for the HP-67 calculator that displays as “h PI”. (It comes up as keycode “35 73”.) It is one of the six codes that you can’t normally enter into the calculator. What does it do? Does it key in PI, like the real one? Does it do something more interesting?
Unfortunately, like the “f LBL (i)” unused code, the other “h PI” does absolutely nothing. It doesn’t produce PI. It doesn’t do anything useful at all.
Here’s the actual microcode for a 0x4f hex code (other h PI) instruction:
do_h_pi_rsvd: ; do nothing ; go straight to tidy up after a function 07117 if n/c go to 07313 07313 delayed rom 00 07314 if n/c go to 0074
It is enough to lead me to suspect that the original programmer (Dave Cochran?) tidied up all the loose ends when he wrote the microcode. That’s pretty impressive when you realize they had to get the entire calculator to run in 5K of ROM. There wouldn’t have been much left much over for the finer programming points.
You can see the process of SST-ing a 0x4f (fake h PI) instruction in 67-h-pi-4f.txt.
You can find details about the missing codes and how to enter them in HP67 Unused Program Codes.
