Jump tables

Yesterday's tank filler has three states and twelve lines of transitions. Every tick works through all transitions, including those of states the machine is not even in. With five or six states this becomes a long chain in which nobody can see any more which line belongs to which state.

Better: the chip jumps straight into the block of the current state and only calculates there.

The comparison chain#

beq state READY s_ready
beq state FILLING s_filling
beq state FULL s_full
beq state FAULT s_fault
move next FAULT         # catch: unknown state
j output

Every line compares the state with a number and jumps to the label if they are equal. beq is right here: the state is a whole number that only your code sets – not a reading where an exact comparison could fail.

One block per state#

Each block only deals with its own transitions and then jumps to the shared output:

s_filling:
sge tmp pressure TARGET
select next tmp FULL next
j output

The output at the end writes pump and display for all states in one place. That way every rule is in the code exactly once, and a new state only needs one line in the chain and one block.

Check the state instead of guessing#

The second-to-last line of the chain is the catch branch. It catches every number that has no block – for example because you deleted a state while rebuilding or a register gets overwritten by accident. Without a catch the chip simply runs into the first block below the chain and carries on there as if everything were fine. With a catch it ends up in the fault state: pump off, display 3, and you see straight away that something is wrong.

Variant: state as a subroutine#

With beqal every block becomes a subroutine that returns with j ra instead of j output:

beqal state READY s_ready
beqal state FILLING s_filling
beqal state FULL s_full

This only works because the blocks set next while the chain checks state. If a block changed state directly, the next beqal line would hit the next block within the same tick. You need the catch separately here: after the chain, check that the state lies in the allowed range.