How does the HP-65 display a program step? If you run the HP-65 emulator with logging turned on and flick the switch to W/PRGM you’ll see. Here’s the details:
The HP-65 wait loop goes from 00726 – 00777. That’s the main starting point for everything other than “power up”. The wait loop jumps around a lot in the process of getting to 00777 and, in the course of that, it goes through:
; S= ............ P= 1 C= 00000000000221 M =00000000000000 00537 if s3 = 0 ; if RUN 00540 goto 00552 ; NOTE: W/PRGM overrides s3. s3 appears 0 but goto doesn't happen ; ; display current prgm step ; 00541 if s11 = 0 00542 goto 00721 00721 if c[p] >= 1 00722 goto 00742 00742 c - 1 -> c[p] ; C= 00000000000211 00743 c - 1 -> c[p] ; C= 00000000000201 00744 if c[p] >= 1 ; see 00746 if c[p] gets set to 3, skip all this 00745 goto 00774 ; ; work out what to display ; 00746 load 3 ; C= 00000000000231 P= 0 (skip this next time) 00747 goto 00724 00724 0 -> c[m] 00725 jsb 00543 .00543 c <-> m ; C= 00000000000000 M =00000000000231 .00544 delayed select group 1 ; 1*04000 .00545 rom 3 ; 3*00400 .05546 dispoff ; 05546= 04000+01400+0146 .05547 0 -> f2 ; S= ...........b .05550 c -> a[w] ; A= 00000000000000 .05551 a <-> b[w] ; A= 02009999999999 B= 00000000000000 .05552 1 -> s5 ; S= .....5.....b (s5=mem full) .05553 memory full -> a ; A= 02009999999990 .05554 0 -> p .05555 if a[p] >= 1 ; if prog[100] != nop .05556 goto 05560 .05557 0 -> s5 ; S= ...........b (wasn't full) .05560 0 -> a[w] ; A= 00000000000000 .05561 a - 1 -> a[w] ; A= 99999999999999 .05562 shiftl a[m] ; A= 99999999990999 .05563 shiftl a[m] ; A= 99999999900999 .05564 0 -> c[w] .05565 4 -> p ; P= 4 .05566 0 -> s4 .05567 buffer -> rom address ; setup for computed goto (in this case with 077) .05570 goto 05400 ; to 077 = start marker, show "00 00" ; .05477 1 -> s4 ; S= ....4......b .05500 jsb 05744 ; but only 1 level jsb supported .05744 if s4 = 0 .05745 goto 05711 .05746 0 -> s4 ; S= ...........b .05747 a <-> c[w] ; A= 00000000000000 C= 99999999900999 .05750 shiftl a[m] .05751 shiftl a[m] .05752 shiftl a[m] .05753 7 -> p ; P= 7 .05754 load 0 ; C= 99999909900999 P= 6 .05755 load 0 ; C= 99999900900999 P= 5 .05756 a <-> c[w] ; A= 99999900900999 C= 00000000000000 .05757 4 -> p ; P= 4 .05760 return 05501 goto 05744 05744 if s4 = 0 05745 goto 05711 ; 05711 if s5 = 0 ; if not mem full, skip next bit 05712 goto 05723 05723 a <-> c[w] ; A= 00000000000000 C= 99999900900999 05724 b <-> c[w] ; B= 99999900900999 C= 00000000000000 05725 clearstatus ; S= ............ 05726 delayed select group 0 ; 0*04000 05727 rom 1 ; 1*00400 00730 0 -> s9 ; 00730=00000+00400+0330 00731 1 -> s10 ; S= ..........a. 00732 disptoggle ; =" 00 00 " 00733 goto 00622 ; 00622 ... ; (back into the wait loop) 00745 goto 00774 ; ; if already done, skips to here ; 00774 c + 1 -> c[p] ; C= 00000000000221 00775 c + 1 -> c[p] ; C= 00000000000231 00776 c <-> m ; C= 00000000000000 M =00000000000231 00777 goto 00726
If the current program step is other than 077 (top of program marker) then “05570 goto 05400” will goto a microcode step appropriate for the program step and the display will show something other than “00 00”.
See also:
HP-65 Display X in fixed mode
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