And, or, either-or

An alarm should go off when the pressure is too low or the temperature too high. With jumps that would be two branches and four labels. With logic instructions it is one line.

and and or#

l r0 d3 Pressure
l r1 d3 Temperature
slt r2 r0 50          # 1 = pressure below 50 kPa
sgt r3 r1 313.15      # 1 = warmer than 40 C
or r4 r2 r3           # 1 = one of the two
s d4 On r4            # warning light

xor: the two-way switch#

A long hallway, a lever at each end. Whichever you flip, the light should toggle. That is exactly what xor does: if one lever stands differently from the other, the light is on. Flip either one and the result changes.

l r0 d0 Open
l r1 d1 Open
xor r2 r0 r1          # two-way switch
s d2 On r2

nor is bitwise#

All four instructions work bitwise on whole numbers. With and, or and xor you do not notice as long as only 0 and 1 go in. With nor you do: it flips all bits. nor r0 0 0 therefore does not give 1 but −1, and with a 1 involved the result is −2.

For a clean “neither of them” you better use two instructions:

or r3 r0 r1
seqz r3 r3            # 1 only if both are 0

Or you keep nor and cut off the last bit: and r3 r3 1 turns −1 into 1 and −2 into 0.

Deciding without jumps#

With set instructions, logic and select you build whole decisions without a single label. The program runs from top to bottom every tick and always writes all devices. There is no branch in which an s could be forgotten.