Conditional calls: beqal, bltal & co.

The gas tank should vent on overpressure and refill on underpressure. Both jobs sit neatly in subroutines of their own. Instead of comparing first and then writing jal, it works in one line: call conditionally.

The family#

The al instructions (and link) compare like the branches you know. If the comparison holds, they write the next line into ra like jal and jump:

bgtal press MAXPRESS toohigh    # only on overpressure
bltal press MINPRESS toolow     # only on underpressure

The subroutine ends with j ra as usual and returns behind the call.

Calling only on a condition#

If it is not called, the chip simply continues on the next line. The subroutine then does nothing, not even reset anything. That is why a default state belongs before the calls:

move vent 0            # default state
move pump 0
bgtal press MAXPRESS toohigh
bltal press MINPRESS toolow
sbn VALVE HASH("Ablass") On vent

Every subroutine only sets what it is responsible for to 1. Writing happens after all calls, exactly once.

Setting ra correctly#

ra only gets a new value when the call really happens. If nothing is called, ra still holds the value from last time. As long as every subroutine is only entered through its own call, that does no harm. It becomes dangerous when the chip runs into a subroutine: then its j ra jumps to an old place.