changes a pin's data direction to output.
In assembly language, converting a pin to output requires writing a 0 to the corresponding bit of the port's TRIS register. For example, to make pin 7 of port RB an output (and the rest of RB's pins inputs), you could use the instructions mov !rb, #01111111b. (In assembly, a 1 in TRIS means makes a pin an input; a 0 makes it an output. This is the opposite of the BASIC Stamp.)
When the PIC first resets, all I/O pins are set up as inputs by default. Pins that are to be used as outputs must be set up by writing 0s to the appropriate TRIS registers.
According to the PIC's manufacturer, Microchip Inc., the TRIS registers can be corrupted by static discharges on the I/O pins. They recommend periodically rewriting the TRIS registers, just in case.
See also: