One byte, eight devices: monitoring and control

A greenhouse controller has to switch a heater, a fan, a pump, a lamp, an alarm and a door lock. That is six devices, each either on or off.

You could run six separate wires and six separate yes/no values. Or you could use one byte, where each bit is one device:

One byte carries all six states. The processor can read them all at once, but it is usually interested in exactly one — and that is where masks come back.

Two different jobs use the same trick:

• Monitoring — reading a bit to find out what is happening (is it too hot? is the door open?).
• Control — writing a bit to make something happen (switch the fan on).

A sensor's status byte arrives in ACC (from its address with LDD, or from an input port). Each bit of that byte is a flag set by the hardware.

Suppose bit 2 of the status byte means "too hot". To find out:

100  LDD 400        ; read the status byte from address 400
101  AND B0000 0100 ; keep only bit 2
102  CMP #0         ; is it zero?
103  JPE cool       ; equal to zero -> not too hot
104  ; too hot: switch the fan on (see below)

AND throws away the seven other flags, so the result is either 0000 0100 (bit set) or 0000 0000 (bit clear). Comparing with 0 then turns that into True or False, which JPE/JPN can act on.

One warning: test a bit with AND, never with OR. OR leaves the other bits in place, so a non-zero answer would tell you nothing about bit 2 specifically.