HP50g small tips and trick

[DavidM]

(06-17-2018 07:59 AM)grsbanks Wrote:  For those interested, the SysRPL listing of this is:

Code:

::
  NULL::
  ' xORDER
  >HCOMP
  >R
;

"NULL::" creates an empty program on the stack (":: ;", a bit like the UserRPL "<< >>")

"' xORDER" places the ID of the UserRPL command "ORDER" on the stack, then:

">HCOMP" adds it to the empty program. leaving us with ":: xORDER ;" on the bottom of the stack.

">R" pushes that to the run stream so that ":: xORDER ;" is run immediately after the SysRPL program has finished and before returning control to the user.

Ingenious

Very nice description! Thanks for adding the explanation.

I'd add one more thing that is important but subtle: the ":: xORDER ;" program is created in such a way that it is guaranteed to be in TEMPOB (temporary memory). This is important to make sure that the address pushed onto the return stack isn't part of the running program.