Comparing against zero: sltz, sgtz & co.

With a heater you rarely care about the temperature itself. What matters is how far it is from the target and in which direction. There is a group of set instructions for exactly that: they compare a value with zero.

The six z instructions#

Otherwise the result is 0. The z stands for zero. Compared with slt r1 r0 0 you save the zero and a few characters – with 4096 characters per chip that adds up eventually.

Deviation first, comparison second#

l r0 d0 Temperature
sub r0 r0 293.15   # deviation from 20 C
sltz r1 r0         # 1 = too cold
sgtz r2 r0         # 1 = too warm

r0 is now negative when it is too cold, positive when it is too warm, and exactly zero only in the rare case of exactly 20 °C. You can put the deviation straight onto a display: a value of −2.5 says more than 290.65.

Adding a small band#

If the heater reacted to every tiny deviation it would switch all the time. Simply shift the limit with an addition:

add r1 r0 1        # only negative below -1
sltz r1 r1         # heater on below 19 C
sub r2 r0 1        # only positive above +1
sgtz r2 r2         # cooler on above 21 C

Between 19 and 21 °C both stay off. That is not a real hysteresis with memory yet – that comes in week 7 – but for many rooms it is enough.

seqz as “not”#

For values that are only 0 or 1, seqz flips the value: 1 becomes 0, 0 becomes 1. snez does the opposite job and turns any non-zero number into a clean 1.

seqz r3 r1         # 1 if the heater is NOT running