When the HP-65 calculator is sitting idle, waiting for you to get it to do something, what is it thinking? What’s going through its head?

Whilst seemingly idle, the HP-65 and any HP65 microcode emulator is busily spinning through a “wait loop” checking to see if you need it to do something. It looks like this:

waitloop1:
00726 if s9 = 0          ; S= 0...........
00727 goto 00622

waitloop2:
00622 12 -> p            ; P= 12
00623 if s0 = 0
00624 goto 00452
00452 0 -> s10
00453 if s5 = 0
00454 goto 00750
00750 if s8 = 0
00751 goto 00441
00441 0 -> s11
00442 0 -> f0
00443 if s11 = 0
00444 goto 00620
00620 if s8 = 0
00621 goto 00531
00531 0 -> s3
00532 1 -> f2
00533 1 -> p            ; P= 1
00534 c <-> m           ; C= 00000000000221 M =04820000000001
00535 0 -> s11
00536 0 -> f3
00537 if s3 = 0
00540 goto 00552
00552 if s11 = 0
00553 goto 00764

waitloop3:
00764 if c[p] = 0
00765 goto 00776
00766 c - 1 -> c[p]     ; C= 00000000000211
00767 if c[p] = 0
00770 goto 00717
00771 c - 1 -> c[p]     ; C= 00000000000201
00772 if c[p] >= 1
00773 goto 00611

waitloop4:
00774 c + 1 -> c[p]     ; C= 00000000000211
00775 c + 1 -> c[p]     ; C= 00000000000221
00776 c <-> m           ; C= 04820000000001 M =00000000000221
00777 goto 00726

If you press a key, S0 gets set to 1 so the sequence

00623 if s0 = 0
00624 goto 00452

proceeds to 00625.

There’s more here which I’ll explore and cover shortly.

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