The “f LBL (i)” instruction is one of the missing codes that you can’t normally key into a HP-67 calculator. But if you do, what happens?

The original advice was “it doesn’t do anything”. That is both right and wrong at the same time. You see, it does exactly the same as every other LBL instruction and runs the exact same code as every other LBL instruction. However, LBL instructions don’t do anything. The calculator just skips over them and goes to the next step.

Here’s the actual microcode for a LBL instruction:

do_f_lbl_any:
; do nothing
; go straight to tidy up after a function
06017 delayed rom 00
06020 if n/c go to 0074

You can see the process of SST-ing a f LBL (i) instruction in 67-f-lbl-(i).txt.

You can find details about the missing codes and how to enter them in HP67 Unused Program Codes.

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