The ROM-BIOS supports up to three parallel printer ports which are
assigned as LPT1-LPT3.  During POST the BIOS tests these ports in this
order:

1) 3bcH   Port found on Monochrome Display / Printer Adapter
2) 378H   printer port adapter #1
3) 278H   printer port adapter #2

It assigns the ports as LPT1-LPT3, as they are discovered and stores the
port addresses that respond starting at 0040:08.  See BIOS Data Area.

BIOS service INT 17H provides full printer support for up to 4 parallel
printers.  To add a forth printer, you would need to place its base
address into the BIOS data word at 0040:0010.

You can trick the system into thinking that one printer is another.  It is
a simple matter of swapping base port addresses in low memory:

;        this example swaps LPT1 for LPT2
mov      ax,0
mov      ds,ax
mov      si,0408H  ;point to start of printer table at 0:0408
mov      ax,[si]   ;get base port for LPT1
xchg     ax,[si+2] ;swap with port for LPT2
mov      [si],ax   ;and put old LPT2 into LPT1 spot

  Bidirectional I/O  
PS/2s and many later parallel-port adapters support high-speed
bidirectional I/O.  In this "extended mode", you can set bit 5 of p+2
(e.g., 37aH) and then read data via IN p+0 (e.g., 378H).

The BIOS does not provide support for bidirectional I/O.

Port  Description
                                                                            
378H-
37fH  Parallel Printer Adapter #1
3bcH is base of MDPA
278H is base of printer adapter #2
                                                                            
378H  Data Latch
Write: send byte to printer
Read:  (PC/XT/old:    Fetch last byte sent
Bidirectional: Read byte from connected device (when bit 5 of
port p+2 (37aH) has been set to 1).
                                                                            
379H  Read-only: Printer Status
 7 6 5 4 3 2 1 0 
           0 0 0 
                  bit
                  3: -ERROR (pin 15) 0=printer signals an error
                  4: +SLCT (pin 13)  1=printer is selected
                  5: +PE (pin 12)    1=out of paper
                  6: -ACK (pin 10)   0=ready for next character
                  7: -BUSY (pin 11)  0=busy or off-line or error
                                                                            
37aH  Read/Write: Printer Controls
 7 6 5 4 3 2 1 0 
 0 0             
                  bit
              0: +Strobe (pin 1)         1 when sending byte
              1: +AUTO LineFeed (pin 14) 1 causes LF after CR
              2: -INIT (pin 16)          0 resets the printer
              3: +SLCT IN (pin 17)       1 selects the printer
              4: +IRQ Enable (hdwr INT when -ACK goes false)
                              LPT1  IRQ 7 (INT 0fH)
                              LPT2  IRQ 5 (INT 0dH)
              5: direction (PS/2 and later ATs)
0=write data to printer or device
1=read data from device
                                                                            
See Also: INT 17H (printer support)
- -

Parallel Printer Adapter Ports