Making do with 16 registers

The base grows. Three rooms with their own heaters, plus pumps, lights and displays – and at some point your register plan shows: all 16 taken. sp and ra do not count, you need them for the stack and subroutines. A second chip would be one way out. Usually it is enough to be more careful with registers.

What really needs a register#

You only need a register for values that change at run time and that you need again later. Everything else is free:

An alias is just a name, it reserves nothing. Two aliases on the same register are therefore two names for one storage: if alias press r3 ends up below alias temp r3 by accident, every pressure value silently overwrites the temperature.

Reusing registers#

A helper register like tmp may mean something different on every line, as long as you no longer need the old content. A subroutine that uses the same registers for several rooms saves the most:

move name HASH("Raum1")   # input: which room
jal room
move name HASH("Raum2")
jal room                  # same registers, other room

Instead of three sets of temperature, state and helper value you need one.

Avoiding intermediate values#

Many calculations work without a result register of their own. Keep calculating in the same register and write the result straight to the device:

lbn t SENSOR HASH("Lager") Temperature Average
sub t t CELSIUSOFFSET           # in degrees C, same register
slt tmp t FROST                 # too cold?
sbn LIGHT HASH("Frost") On tmp  # out right away, do not keep it

The stack as storage#

Whatever has to survive several ticks but is not needed all the time goes into the stack. With get and put at fixed addresses sp stays untouched:

define ADR_ROOM1 0      # stack slot for state of Raum1
get heat db ADR_ROOM1   # fetch state
put db ADR_ROOM1 heat   # put state back

One stack slot thus replaces a register that would otherwise be taken for the whole run time. In return every access costs one line.