{"id":1423,"date":"2015-12-12T22:21:19","date_gmt":"2015-12-12T22:21:19","guid":{"rendered":"http:\/\/www.sydneysmith.com\/wordpress\/?p=1423"},"modified":"2017-05-14T00:43:26","modified_gmt":"2017-05-14T00:43:26","slug":"hp-65-rtn","status":"publish","type":"post","link":"https:\/\/www.sydneysmith.com\/wordpress\/1423\/hp-65-rtn\/","title":{"rendered":"HP-65 RTN"},"content":{"rendered":"

The RTN key does a return from a subroutine. It also stops a main program and, if nothing’s running at all, it will take you to the “top of memory marker” (“00 00” in w\/prgm mode).<\/p>\n

Here’s how it does all that:<\/p>\n

00752 – 01365. Get a step and switch(prog_code)
\n01052 – 02351. switch again but with access to rom 4 now
\n02052 – 02223. Figure out it is a RTN and need to run it
\n02240 – 02247. Which case is it?
\n02334 – 02336. In a main program.<\/p>\n

\r\n00752 pointer advance\r\n00753 disptoggle\r\n00754 c <-> m            ; C= 10000000000221 M =00000000000000\r\n00755 shiftl a[w]        ; A= 00000009990000\r\n00756 0 -> s11           ; S= ........8...\r\n00757 0 -> f5\r\n00760 if s11 = 0\r\n00761 goto 00757\r\n00757 0 -> f5            ; S= ........8..b\r\n00760 if s11 = 0\r\n00761 goto 00757\r\n00762 0 -> f5\r\n00763 rom 2\r\n01364 buffer -> rom address\r\n01365 goto 01000\r\n\r\n01052 goto 01347         ; [RTN]\r\n01347 rom 4\r\n02350 buffer -> rom address\r\n02351 goto 02000\r\n\r\n02052 goto 02164\r\n02164 p - 1 -> p         ; P= 11\r\n02165 p - 1 -> p         ; P= 10\r\n02166 p - 1 -> p         ; P= 9\r\n02167 if p # 8\r\n02170 goto 02305\r\n02305 if c[m] = 0\r\n02306 goto 02220\r\n02220 if p # 9\r\n02221 goto 02120\r\n02222 if s3 = 0          ; if not just keying into prgm mem\r\n02223 goto 02240         ; go run it\r\n;\r\n; RTN0:\r\n; Do a RTN step\r\n02240 if s8 = 0          ; if not running\r\n02241 goto 02332         ; go position to \"00 00\" instead\r\n02242 if c[s] = 0        ; got a subroutine RTN\r\n02243 goto 02210\r\n02244 c - 1 -> c[s]      ; C= 00000000000221\r\n02245 if c[s] = 0\r\n02246 goto 02334         ; RTN in main prog (tidy up and stop)\r\n; else c[s] was > 1. go back to main prog (goto 02254)\r\n;\r\n; RTN3:\r\n; RTN from main program\r\n02334 0 -> c[ms]         ; clear prefixes AND subroutine level\r\n02335 c <-> m            ; C= 00000000000000 M =00000000000221\r\n02336 rom 1              ; tidy up INC clear the running flag\r\n\r\n; tidy up\r\n00737 1 -> f3\r\n00740 clearstatus        ; S= ............\r\n00741 goto 00620\r\n00620 if s8 = 0\r\n00621 goto 00531\r\n; display:\r\n00531 ...\r\n00777 goto 00726         ; wait loop\r\n<\/pre>\n
if s8 = 0 we’re not running and we should end up at “00 00”.<\/p>\n
\r\n; RTN1:\r\n; RTN when not running (from the keyboard)\r\n02332 if s9 = 0\r\n02333 goto 02250\r\n; else continues into RTN from main program\r\n02250 search for label\r\n02251 jsb 02127 (=MCIRC0 = wait until done)\r\n02252 1 -> f7\r\n02253 jsb 02047\r\n02047 return\r\n02254 search for label\r\n02255 jsb 02127 (=MCIRC0 = wait until done)\r\n02256 goto 02334\r\n; 02334 is RTN from main program\r\n<\/pre>\n
It looks like two searches take us to “00 00”.
\nThe seeming NOP of “jsb 02047 : 02047 return” has a side effect if f7 is set. It copies the bottom 6 bits of the jsb address into “the buffer”. That means the second search looks for program code 047. That is a reserved instruction. It shouldn’t be present in memory. <\/p>\n
s9 = 1 skips the go “00 00” process. It seems to get set when doing a subroutine but it needs to be explored further.<\/p>\n
The other end of the RTN is the GSB. On a HP-65, GSB is inferred – you just use [A] or [B] etc in a program to mean GoSuB A or B etc. (Interestingly on a HP-67, if you press [A] or [B] etc in W\/PRGM mode, the one key gets entered as “31 22 11” (f GSB A) or “31 22 12” (f GSB B) etc; so the principle carried forward.) You can see an [A] pressed from the keyboard and then a [B] run from within a program, in 65-rtn-rtn.txt<\/a><\/p>\n
[A] from the keyboard gives:<\/p>\n
\r\n01316 key -> rom\r\n01036 goto 01351 <\/pre>\n
[B] in the program gives:<\/p>\n
\r\n01364 buffer -> rom address\r\n01365 goto 01000\r\n01034 goto 01351 <\/pre>\n
Clearly the scan code for [A] is 036 and for [B], 034.
\nThe common bit afterwards is:<\/p>\n
\r\n01351 rom 4\r\n02352 0 -> f5                            ; S= ...........b\r\n02353 if c[m] = 0\r\n02354 goto 02371\r\n02371 0 -> c[ms]\r\n02372 if s8 = 0                          ; if not running\r\n02373 goto 02102 <\/pre>\n
It checks that there aren’t any prefixes (f f-1 STO RCL LBL DSP g) in the “if c[m] = 0” step. Then it clears the sign “bit” in C and clears any prefixes (perhaps something branches here with some set).<\/p>\n
The thing that determines what comes next isn’t “was it a key press?” nor “are we already in a subroutine?”. It is simply “are we running?” (if s8 = 0).<\/p>\n
If we aren’t already running, it goes to:<\/p>\n
\r\n02102 if s9 = 0\r\n02103 goto 02302\r\n\r\n02302 search for label\r\n02303 goto 02106\r\n02106 c + 1 -> c[s]      ; C= 10000000000221\r\n02107 clearstatus        ; S= ............\r\n02110 1 -> s10           ; S= ..........a.\r\n02111 jsb 02376 <\/pre>\n
That causes a “search for label” and sets c[s] to 1 (as shown).<\/p>\n
The alternate, if we were already running, is:<\/p>\n
\r\n02374 mark and search\r\n02375 clearstatus           ; S= ............ <\/pre>\n
That uses the other “search” microinstruction. This one leaves the current pointer in place and adds a secondary pointer. After this, it is the secondary pointer that moves – not the normal one. Note that c[s] remains zero if we are in a subroutine (one level down) instead of the main routine (top level).<\/p>\n
The HP-65 only supports two levels of subroutine. The doco refers to the top level as the “main routine” and the lower level (called from inside the main routine) as the “subroutine”. You can’t call another level below that. The manual shows a call to another layer (eg LBL B, …, C, …, RTN) as equivalent to a “GTO C”. There isn’t a tertiary pointer to move around and c[s] is only ever 1 (in the main routine) or 0 (in the subroutine).<\/p>\n
Regardless of whether it was [A] from the keyboard or [B] within a program, the process continues on from microinstruction 02376 which sets the “running” flag (s8 to 1). Don’t get confused by the “jsb 02376”. It never returns to the instruction after that as, like user program memory, the microcode only supports one return address. It is effectively a goto 02376 back into common code.<\/p>\n","protected":false},"excerpt":{"rendered":"
The RTN key does a return from a subroutine. It also stops a main program and, if nothing’s running at all, it will take you to the “top of memory marker” (“00 00” in w\/prgm mode). Here’s how it does all that:<\/p>\n","protected":false},"author":3,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":[],"categories":[16,5,39],"tags":[37,32],"_links":{"self":[{"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/posts\/1423"}],"collection":[{"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/users\/3"}],"replies":[{"embeddable":true,"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/comments?post=1423"}],"version-history":[{"count":6,"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/posts\/1423\/revisions"}],"predecessor-version":[{"id":1478,"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/posts\/1423\/revisions\/1478"}],"wp:attachment":[{"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/media?parent=1423"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/categories?post=1423"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/www.sydneysmith.com\/wordpress\/wp-json\/wp\/v2\/tags?post=1423"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}