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[AndyFreestone]

As it does with any other register - by the registers name, which on the 12F509 is either PortB or GPIO for the actual port and TRISB or TRISGPIO for the "direction".
You know, assumption is the mother of all f**k-ups so never assume anything - a simple look at the datasheet for the device would have told you that there is no PortA.

/Henrik.

Again way over my current knowledge. How do I address THE PORT in PBP3 ?

[HenrikOlsson]

Like I said, on the 12F509 THE PORT is named PortB or GPIO not PortA.
Otherwise you had it right - HIGH PortB.0 or HIGH GPIO.0 instead of HIGH PortA.0

EDIT: I'm sorry, make that GPIO.0 only, PortB was for the 12F605 only which share the same datasheet.