HP HP-67 - Page 211

Subroutines 207
The calculator can return back to the main program from subroutines
that are three deep, as shown. However, if you attempt to call up
subroutines that are four deep, the calculator will execute only three
returns:
Only three returns can be pending...
Main Program
cE6 4 Gm 4 @Ee p e
// I Y /
v, I =23
=0 / N=]c
\
I
\
@=a \
v \\ \ \ \
RTN
f____ ...80 execution will stop here.
Naturally, the calculator can execute the instruction as a stop
any number of times. Also, if you press [§ through @, (&) through
G e.0 3 through [ 6.6 0 through % 9,
or B} GEE1(E) through B &) from the keyboard, all pending
RTN instructions are forgotten by the calculator.
If you are executing a program one step at a time with the key
and encounter a or instruction, the calculator will execute
the entire subroutine before continuing to the next step. However,
only one instruction may be executed as the result of a or
instruction during single-step execution, so if a program con-
tains a subroutine within a subroutine, execution will not return to the
main program during execution.