Now that the HP-65 Delayed Select Group instruction is behaving just like the real thing, it becomes possible to see what the real microcode does for the [R/S] key and as a program step. Here’s the details:

When pressing the [R/S] key to start a program it does this:

01101 0 -> c[m]
01102 delayed select group 1
01103 c <-> m        ; C= 03000000000000 M= 10000000000221
05104 delayed select rom 1
05105 goto 04604
04604 if s8 = 0
04605 goto 04405
04405 if s9 = 0
04406 goto 04434
04434 rom 2
05035 goto 05376
05376 delayed select group 0
05377 goto 05223
01223 clearstatus
01224 1 -> s8        ; S= ........8...
01225 1 -> s10       ; S= ........8.a.
01226 goto 01217
01217 rom 1
00620 if s8 = 0
00621 goto 00531
00622 ...

When encountering an R/S instruction in a program it does this:

01101 0 -> c[m]
01102 delayed select group 1
01103 c <-> m        ; C= 04000000000000 M= 10000000000221
05104 delayed select rom 1
05105 goto 04604
04604 if s8 = 0
04605 goto 04405
04606 goto 04502
04502 shiftr a[w]    ; A= 04000000000000
04503 c -> a[s]
04504 delayed select group 0
04505 rom 2
01106 goto 01354
01354 if s1 = 0      ; if data entry not in progress
01355 goto 01213
01213 clearstatus    ; S= ............
01214 goto 01266
01266 1 -> f3        ; f= .123.5..
01267 goto 01240
01240 jsb 01167
01167 if s8 = 0
01170 goto 01123
01123 clearstatus
01124 return
01241 goto 01334
01334 if c[m] >= 1
01335 goto 01337
01337 if p # 14
01340 goto 01217
01217 rom 1
00620 if s8 = 0
00621 goto 00531
display:
00531 ...

The two could be summarized as:

01101 0 -> c[m]      ; clear prefix flags (f f-1 g STO ...)
01103 c <-> m        ; X to C, state to M
04604 if s8 = 1      ; if running
04502   shiftr a[w]  ; undo 00755 shiftl
04503   c -> a[s]    ; and get sign back (C has it)
01354   if s1 = 1    ; if data entry in progress
01356     0 -> f1    ; clear auto-enter flag
01357   endif
01213   clearstatus  ; clear the running flag (&others)
01266   1 -> f3      ; set update display flag
01334   if c[m] = 0  ; if mantissa = 0
01336     0 -> c[w]  ; set exponent = 0 too
01337   endif
04405 else
01223   clearstatus
01224   1 -> s8      ; set running
01225   1 -> s10     ; set don't interrupt
00620 endif

The full trace is in 65-rs.txt

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