The printer control register provides necessary control over the output of the characters to the printer. Each bit in this register has a specific function. For example:

Bit 0 - start bit. Setting this bit (to 1) tells the printer to print the character in the data port. This bit must be set for a certain length of time (0.5 microsec) before it is cleared again (to 0).

Bit 4 - interrupt enable bit; when set the printer will send an interrupt signal to the processor when ready; otherwise, the processor must check repeatedly check the status port until the status is ready.