;-------------------------comoff routine begins--------------------------+
; from BLUEBOOK OF ASSEMBLY ROUTINES FOR IBM PC & XT.
;         page : 38
;
; NAME COMOFF
; ROUTINE FOR COMMUNICATIONS LINE OFF
;
; FUNCTION: This routine turns off the handshaking signal DTR (line 20)
; on the specified communications line.  RTS (line 4) is kept as it was.
; 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 OFF INPUT FROM A COMMUNICATIONS LINE
;
comoff	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,2		; clear DTR (line 20)
	out	dx,al		; send out control byte
;
	pop	si		; restore registers
	pop	dx
	pop	ds
	ret			; return
;
comoff	endp
;-------------------------comoff routine ends---------------------------+