Subroutines and the Stack

T32 has just enough stack machinery to make structured programs pleasant.

Calling and returning

Use:

• JSR addr to call a subroutine
• RET to return

JSR saves the current program position on the stack before jumping to the target address. RET restores that position.

Saving values across calls

Because T32 has only one general-purpose register, PSH and POP are very important.

printnl:
    PSH
    LDI $0A
    PRT
    POP
    RET

This pattern saves A, does some work, and restores A before returning.

That is a very T32 way to write helper routines: preserve the caller’s value when it matters, and be explicit about what your routine clobbers.

A practical rule of thumb

When writing a subroutine, decide three things:

• what input it expects
• which state it may change
• whether it should preserve A

Writing those expectations in comments makes larger T32 programs much easier to read.