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Assembly Language Programming

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14
Serial Port Programming
14.1. INTRODUCTION
Serial port is a way of communication among two devices just like the
parallel port. The basic difference is that whole bytes are sent from one place
to another in case of parallel port while the bits are sent one by one on the
serial port in a specially formatted fashion. The serial port connection is a
9pin DB-9 connector with pins assigned as shown below.
1 ­ Carrier Detect
(CD)
­
6
Data  Set
2 ­ Received Data
Ready
(RD)
(DSR)
3 ­ Transmitted
7 ­ Request to
Data (TD)
Send (RTS)
4 ­ Data Terminal
8 ­ Clear to Send
Ready
(CTS)
(DTR)
9 ­ Ring Indicator
5 ­ Signal Ground
(RI)
We have made a wire that connects signal ground of the two connectors,
the TD of one to the RD of the other and the RD of one to the TD of the other.
This three wire connection is sufficient for full duplex serial communication.
The data on the serial port is sent in a standard format called RS232
communication. The data starts with a 1 bit called the start bit, then five to
eight data bits, an optional parity bit, and one to two 0 bits called stop bits.
The number of data bits, parity bits, and the number of stop bits have to be
configured at both ends. Also the duration of a bit must be precisely known
at both ends called the baud rate of the communication.
The BIOS INT 14 provides serial port services. We will use a mix of BIOS
services and direct port access for our example. A major limitation in using
BIOS is that it does not allows interrupt driven data transfer, i.e. we are
interrupted whenever a byte is ready to be read or a byte can be transferred
since the previous transmission has completed. To achieve this we have to
resort to direct port access. Important BIOS services regarding the serial port
are discussed below.
INT 14 - SERIAL - INITIALIZE PORT
AH = 00h
AL = port parameters
DX = port number (00h-03h)
Return:
AH = line status
AL = modem status
Every bit of line status conveys different information. From most
significant to least significant, the meanings are timeout, transmitter shift
register empty, transmitter holding register empty, break detect, receiver
ready, overrun, parity error, and framing error. Modem status is not used in
direct serial communication. The port parameters in AL consist of the baud
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Computer Architecture & Assembly Language Programming
Course Code: CS401
CS401@vu.edu.pk
rate, parity scheme, number of stop bits, and number of data bits. The
description of various bits is as under.
7
6
5
4
3
2
1
0
data bits
baud rate
00-5
000-110
01-6
001-150
10-7
010-300
parity
11-8
011-600
00-N
100-1200
10-N
stop bits
101-2400
01-O
0-1
110-4800
11-E
1-2
111-9600
INT 14 - SERIAL - WRITE CHARACTER TO PORT
AH = 01h
AL = character to write
DX = port number (00h-03h)
Return:
AH bit 7 = error flag
AH bits 6-0 = port status
INT 14 - SERIAL - READ CHARACTER FROM PORT
AH = 02h
DX = port number (00h-03h)
Return:
AH = line status
AL = received character if AH bit 7 clear
INT 14 - SERIAL - GET PORT STATUS
AH = 03h
DX = port number (00h-03h)
Return:
AH = line status
AL = modem status
Serial port is also accessible via I/O ports. COM1 is accessible via ports
3F8-3FF while COM2 is accessible via 2F8-2FF. The first register at 3F8 (or
2F8 for the other port) is the transmitter holding register if written to and the
receiver buffer register if read from. Other registers of our interest include
3F9 whose bit 0 must be set to enable received data available interrupt and
bit 1 must be set to enable transmitter holding register empty interrupt. Bit 0
of 3FA is set if an interrupt is pending and its bits 1-3 identify the cause of
the interrupt. The three bit causes are as follows.
110
(16550, 82510) timeout interrupt pending
101
(82510) timer interrupt
100
(82510) transmit machine
011
receiver line status interrupt. priority=highest
010
received data available register interrupt. priority=second
001
transmitter holding register empty interrupt. priority=third
000
modem status interrupt. priority=fourth
The register at 3FB is line control register while the one at 3FD is line
status register. The line status register has the same bits as returned in line
status by the get port status BIOS interrupt however the most significant bit
164
img
Computer Architecture & Assembly Language Programming
Course Code: CS401
CS401@vu.edu.pk
is reserved in this case instead of signaling a timeout. The register at 3FC is
the modem control register. Bit 3 of this register must be set to enable
interrupt generation by the serial port.
14.2. SERIAL COMMUNICATION
We give an example where two computers are connected using a serial
cable made just as described above. The program is to be run on both
computers. After that whatever is typed on one computer appears on the
screen of the other.
Example 14.1
001
; a program using serial port to transfer data back and forth
002
[org 0x0100]
003
jmp  start
004
005
screenpos:
dw
0
; where to display next character
006
007
; subroutine to clear the screen
008
clrscr:
push es
009
push ax
010
push cx
011
push di
012
013
mov
ax,
0xb800
014
mov
es,
ax
;
point es to video base
015
xor
di,
di
;
point di to top left column
016
mov
ax,
0x0720
;
space char in normal attribute
017
mov
cx,
2000
;
number of screen locations
018
019
cld
; auto increment mode
020
rep
stosw
; clear the whole screen
021
022
pop
di
023
pop
cx
024
pop
ax
025
pop
es
026
ret
027
028
serial:
push
ax
029
push
bx
030
push
dx
031
push
es
032
033
mov
dx, 0x3FA
;
interrupt identification register
034
in
al, dx
;
read register
035
and
al, 0x0F
;
leave lowerniblle only
036
cmp
al, 4
;
is receiver data available
037
jne
skipall
;
no, leave interrupt handler
038
039
mov
dx, 0x3F8
; data register
040
in
al, dx
; read character
041
042
mov
dx, 0xB800
043
mov
es, dx
; point es to video memory
044
mov
bx, [cs:screenpos] ; get current screen position
045
mov
[es:bx], al
; write character on screen
046
add
word [cs:screenpos], 2 ; update screen position
047
cmp
word [cs:screenpos], 4000 ; is the screen full
048
jne
skipall
; no, leave interrupt handler
049
050
call clrscr
; clear the screen
051
mov  word [cs:screenpos], 0 ; reset screen position
052
053
skipall:
mov
al, 0x20
054
out
0x20, al
; end of interrupt
055
056
pop
es
057
pop
dx
058
pop
bx
059
pop
ax
060
iret
165
img
Computer Architecture & Assembly Language Programming
Course Code: CS401
CS401@vu.edu.pk
061
062
start:
call clrscr
; clear the screen
063
064
mov
ah, 0
;
initialize port service
065
mov
al, 0xE3
;
line settings = 9600, 8, N, 1
066
xor
dx, dx
;
port = COM1
067
int
0x14
;
BIOS serial port services
068
069
xor
ax, ax
070
mov
es, ax
; point es to IVT base
071
mov
word [es:0x0C*4], serial
072
mov
[es:0x0C*4+2], cs ; hook serial port interrupt
073
074
mov
dx,
0x3FC
;
modem control register
075
in
al,
dx
;
read register
076
or
al,
8
;
enable bit 3 (OUT2)
077
out
dx,
al
;
write back to register
078
079
mov
dx,
0x3F9
;
interrupt enable register
080
in
al,
dx
;
read register
081
or
al,
1
;
receiver data interrupt enable
082
out
dx,
al
;
write back to register
083
084
in
al, 0x21
; read interrupt mask register
085
and
al, 0xEF
; enable IRQ 4
086
out
0x21, al
; write back to register
087
088
main:
mov  ah, 0
; read key service
089
int  0x16
; BIOS keybaord services
090
push ax
; save key for later use
091
092
retest:
mov
ah, 3
;
get line status
093
xor
dx, dx
;
port = COM1
094
int
0x14
;
BIOS keyboard services
095
and
ah, 32
;
trasmitter holding register empty
096
jz
retest
;
no, test again
097
098
pop
ax
; load saved key
099
mov
dx, 0x3F8
; data port
100
out
dx, al
; send on serial port
101
102
jmp
main
166