DM42 and labels

[Didier Lachieze]

(05-24-2020 08:52 PM)Mike (Austria) Wrote:  When above program is entered into the 65, behavior depends on how you start the program. Hit any of the A-D keys, and execution stops at RTN as expected (e.g. A -> 1). However, when execution is started with R/S, the first RTN is ignored, and the program stops only at the next RTN (e.g.: GTO A ; R/S -> 2). I did not find mention of this anomaly in the 65 manual, which on page 62 states,

"The RTN at the end of the program stops calculator execution, halts the stepping of the program pointer, and returns control to the keyboard"

Yet another example for the pleasure of playing with old calculator models. Thanks Bob for the suggestion. Maybe someone with a physical 65 wants to verify this?

Confirmed on my 1603S HP-65. This seems to be linked to the peculiar way the HP 65 manages the program memory through a circulating shift register, and main/secondary pointers. The 65 User’s manual from July 74 explicitly forbids to start with R/S a program ending by RTN (page 93/94):

"The rule for using R/S is simple: Pair a R/S with R/S. In other words, if you plan to initiate execution of your program with R/S, a R/S must be used as a program step to halt the program. Similarly, if your program begins with a label (A thru E), and ends with a RTN you must start it with the program control key identified by the label."

This is also described in the HP-65 User News V1N1 in the Q&As page 4. (EDIT: SlideRule beat me to it)