The simplest (and most common) use for semaphores is simply to have your program remember some on/off or yes/no item of information for later action. If you are accustomed to relay logic, this is equivalent to using an internal relay to hold onto a bit of information or an intermediate result.

Semaphores use RAM memory, and memory must be set aside for them. The easiest way to do this is with the defSEM directive.

Example 1:

sCooling        defSEM
sFillLow        defSEM
sFillHigh       defSEM
sOverTemp       defSEM

The following subroutine CheckLevel would return the state of a low, medium or high level probe depending on two of the above semaphores:

CheckLevel:     GoIfST          sFillLow,CheckLow    ;Go to CheckLow if low semaphore set
                GoIfST          sFillHigh,CheckHigh  ;Go to Checkhigh if high semaphore set
CheckMedium:    Input           iMediumProbe         ;By default must medium
                Return
CheckLow:       Input           iLowProbe
                Return
CheckHigh:      Input           iHighProbe
                Return

Example 2:

Semaphores can be set and cleared in several ways. They are directly set and cleared using SetS and ClrS instructions. For example, to set or clear the sFillHigh semaphore depending on whether the button on input iHighFill has been pressed or not:

                SetS            sHighFill          ;Assume switch pressed
                GoIfInK         iHighFill,HighFill ;Test the switch, skip next inst if pressed
                ClrS            sHighFill          ;Switch was not pressed ... reset semaphore again
HighFill:

Example 3:

You can also change a semaphore by storing the outcome of a previous operation (in the X register) to it. In that case any non-zero result will set the semaphore, and a zero result will clear it. Using the StoreS instruction, the previous example can be simplified:

                InputK          HighFill          ;Read the switch
                StoreS          sHighFill         ;Save result

If you are using MultiTrack, SPLat’s easy to use multitasking, you should also look at semaphores for task synchronization.