The Int 17h service routine supports one serial printer. This is different from the standard PC-XT BIOS in which Int 17h drives a set of up to three parallel printers. However, it is similar to the situation when the DOS command
MODE LPT1:=COM1:
is used to redirect LPT1 output of a PC to a serial printer. In this case, MS-DOS redirects the Int 17h interrupt vector to point to an interrupt service routine that supports a serial printer.