Category : Assembly Language Source Code
Archive   : ASM-SUBR.ZIP
Filename : COMOFF

;-------------------------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---------------------------+