;-------------------------comon routine begins--------------------------+
; from BLUEBOOK OF ASSEMBLY ROUTINES FOR IBM PC & XT.
;         page : 38
;
; NAME COMON
; ROUTINE FOR COMMUNICATIONS LINE ON
;
; FUNCTION: This routine turns on the handshaking signal DTR (line 20)
; and RTS (line 4) on the specified communications line.
; INPUT: Upon entry DX contains the unit number (0 for comm1: and 1 for
; comm2:).
; OUTPUT: Just to the communications line.
; REGISTERS USED:  No registers modified.  DX is used for input.
; SEGMENTS REFERENCED:  During the routine the system data segment is
; referenced.
; ROUTINES CALLED:  None
; SPECIAL NOTES: None
;
; ROUTINE TO TURN ON INPUT FROM A COMMUNICATIONS LINE
;
comon	proc	far
;
	push	ds		; save registers
	push	dx
	push	si
;
	mov	si,dx		; look up address of comm line
	add 	si,si		; double to index into table
	mov	dx,40h		; segment of system I/O table
	mov	ds,dx		; set data segment to this table
	mov	dx,[si]		; look up data
	add	dx,4		; modem control register
	mov	al,3		; set DTR and RTS
	out	dx,al		; send out control byte
;
	pop	si		; restore registers
	pop	dx
	pop	ds
	ret			; return
;
comon	endp
;-------------------------comon routine ends---------------------------+