BIOS call INT 17h has three subfunctions, selected with AH set to 0-2. Subfunctions assume DX = printer number (0-2); they return with AH = printer status byte (see below). The subfunctions are:
AH = 0: Print character specified in AL. If BUSY does not go low within about 16 seconds, a "timeout" is declared and BIOS returns with bit 0 of the statues byte set (and the character in AL is lost).
AH = 1: Initialize (set ~SELECT low, pulse ~INITIALIZE low, set ~AUTO FEED high, and disable IRQ7 interrupts from ~ACKNOWLEDGE). IBM- or EPSON-compatible printers respond to the INITIALIZE pulse by performing a carriage return and establishing the current line as top-of-page.
AH = 2: Get printer status byte into AH. The meaning (some of the signals have been inverted on the way from the connector) is:
DOS function call INT 21h with AH = 5 prints the contents of DL interpreted as an ASCII character.