Free42 LSTO REGS

[Thomas Okken]

To elaborate a bit: every local variable is associated with a level of the RTN stack. LSTO creates variables on the current level, and when that level is popped, the associated variables are deleted. And, any action that clears the entire RTN stack, like GTO, XEQ, or RTN in RUN mode, clears all local variables.

There is one quirk to this: LSTO can also create variables when the RTN stack is already empty. It has to be able to do this, in order to work in a subroutine that is invoked from the keyboard. Say you do XEQ "FOO" from the keyboard with this in program memory:

Code:

01 LBL "FOO"
02 LSTO "X"
03 R↓
04 LSTO "Y"
[...]

When the LSTO instructions are performed, there is nothing on the RTN stack, unlike when the XEQ "FOO" is done from a program, but of course you'd want LSTO to work even in this scenario.

When a program performs RTN while the RTN stack is empty, this causes the empty-stack locals (referred to as "level 0" in the source code, the RTN stack "level" being the number of pending returns) to be deleted, so when the program "FOO" above ends with a RTN, its locals disappear, even though they were never associated with an actual RTN. And this same mechanism can be used in RUN mode.