CHAPTER 6  PROGRAMMING TECHNICAL UNDER MS/DOS
There are three methods to write application programs to control serial communication.
1. Use device driver name to perform I/O.
2. Use PCCOM library through DOS interrupt routines.
3. Use low level BIOS interrupt routines.

6.1 How to Use Device Driver
After device driver is installed, you may use device name (such as COM5, COM6, ... etc.) as a file to perform I/O operations. Since many high level languages provide several kinds of I/O operations, e.g. in C language, the write statement for low level I/O and fprintf for high level I/O. You must care which I/O routine is used by yourself to prevent the lose of I/O data, if you use high level I/O, you must use flush to dump buffer contents to serial device when necessary.
In the distribution disk, we provide BASIC, C, PASCAL, and FoxPro examples, all of these programs use device driver.
LOOPBACK.BAS
Loopback test.

DTX.BAS
Transfer file.
   
DRX.BAS
Receive file.

LOOPBACK.C
Loopback test.
   
DTERM.C
ASCII terminal emulator.

DCLOCK.C
Receive clock message.

LOOPBACK.PAS
Loopback test.

DTERM.PAS
ASCII terminal emulator.
Back
6.2 PCCOM Library
PCCOM provides C, PASCAL, BASIC, and FOXPRO library routines to let user to write application programs to control serial communication via device drivers.
To write an application program, you must link COMLIBBC.C (Borland C), COMLIBMS.C (MSC), COMLIBV2.PAS (PASCAL), COMLIBGW.BAS (GWBASIC), COMLIBQK.BAS (Quick BASIC), COMLIBQ.BAS (Q BASIC), COMLIBTB.BAS (Turbo BASIC) or COMLIBFP.LIB (Foxpro) file according to your selection language. In the following, we describe the PCCOM library more details.
routine name
 description
cominit
 setup communication parameters
comtxch
 send a character
comrxch
 receive a character
comstat
 check communication status
comactive
 check if port is usable
comsetDTR
 set DTR signal ON or OFF
compeek
 check next char in queue
comsetflow
 set flow control
comtxblk
 send one block data
comrxblk
 receive one block data
combreak
 send break signal
cominstalled
 check if driver installed
comsetport
 set address of serial port
comsetvector
 set interrupt vector
comsetactive
 set active low or high
comsetMCR
 set MCR contents
compurgerxbuf
 remove received buffer data
comtxs
 send a character string
comrxs
 receive a character string
comsettimeout
 set time out
comgettimeout
 get time out
combufstat
 return no. of chars in queue
comgetMCR
 get MCR contents
comgetFC
 get flow control
comgetsets
 get number of PCCOM sets
comgetsetinfo
 get PCCOM set information
comgetinfo
 get port status
comgetports
 get number of PCCOM ports
comdetectport
 detect if the port exist
comgetportinfo
 get one port information
comrxoff
 stop send data
comrxon
 restart send data

1. cominit(portno, baud, db, parity, sb)
int cominit
unsigned int  baud
int  portno, db, parity, sb

The cominit is used to setup several communication parameters.

portno (int)
port number that defined in configuration file.

baud (unsigned int)
baud  rate. Either B38400, B19200, B9600, B4800, B2400, B1200, B600, B300.
    
B38400 (38400) : 38400 baud.
B19200 (19200) : 19200 baud.
B9600  (9600)  :  9600 baud.
B4800  (4800)  :  4800 baud.
B2400  (2400)  :  2400 baud.
B1200  (1200)  :  1200 baud.
B600    (600)  :   600 baud.
B300    (300)  :   300 baud.
     
db (int)
data  bits.  Either CS8,  CS7, CS6, CS5.

CS8 (8) : 8 bits.
CS7 (7) : 7 bits.
CS6 (6) : 6 bits.
CS5 (5) : 5 bits.

parity (int)
parity bit. Either PNONE, PEVEN, PODD.

PNONE (0) : none parity check.
PODD  (1) : odd parity.
PEVEN (2) : even parity.

sb (int)
stop bits. Either ST1 or ST2.

ST1 (1) : one stop bit.
ST2 (2) : two stop bits.
   
return value (int)
0 : success.
1 : fail.

2. comtxch(portno, ch)
int comtxch
int portno
char ch

The comtxch is used to send a character to serial port.

ch (char)
transmitted character.

return value (int)
        0 : success.
        1 : fail.

3. comrxch(portno, ch)
int comrxch
int portno
char *ch
    
The comrxch is used to get a character.

ch (char*)
received character.

return value (int)
        0 : success.
        1 : fail (receive no character).

4. comstat(portno)
int comstat
   int portno

The comstat is used to get serial port status, the status indicator is shown in the following.

return value (int)
  bit 15 : time out error.
  bit 14 : transmitter shift register empty.
  bit 13 : transmitter holding register empty.
  bit 8  : data ready (data available in   
           input buffer).
  bit 7  : received line signal detect (RLSD).
  bit 5  : data set ready (DSR).
  bit 4  : clear to send (CTS).

You may check the parameters to detect which bit is set.

S_CTS 4
 0X0010
S_DSR 5
 0X0020
S_DCD 7
 0X0080
S_RDA 8
 0X0100
S_THRE 13
 0X2000
S_TSRE 14
 0X4000
S_TIMEOUT 15
 0X8000

5. comactive(portno)
int comactive
int portno

Check if the port is active ?

return value (int)
1 : active.
0 : inactive.

6. comsetDTR(portno,n)
void comsetDTR
int portno,n

Set DTR signal ON or OFF.

n (int)
ACTIVE   (1) : set DTR ON.
INACTIVE (0) : set DTR OFF.

7. compeek(portno)
int compeek
int portno

The compeek is used to perform nondestructively read a character. The read character remains in the receive buffer.

return value (int)
0XFFFF : no character.
?    : received character.

8. comsetflow(portno,n)
void comsetflow
int portno, n

To set communication protocol.

n (int)
F_XONXOF (0X09) : set XON/XOFF protocol.
F_RTSCTS  (0X01) : set CTS/RTS protocol.
F_DTRDSR (0X04) : set DSR/DTR protocol.

9. comtxblk(portno, pointer, n)
int comtxblk
int portno,n
void *pointer

To transmit a block of data, it uses one BIOS call to transmit a block of data.

pointer (pointer)
pointer to start of block.

n (int)
block length.

return value (int)
number of characters be sent.

10. comrxblk(portno, pointer, n)
int comrxblk
int portno, n
void *pointer

To receive a block of data, it uses one BIOS call to receive a block of data.

pointer (pointer)
pointer to start of block.

n (int)
number of characters should be received.

return value (int)
number of characters actually received.

11. combreak(portno,brk)
void combreak
int portno, brk

Send the break signal to driver.

brk (int)
ACTIVE    (1) : break the communication.
INACTIVE (0) : unbreak the communication.

12. cominstalled(portno)
int cominstalled
int portno

Check if the driver is installed ?
 
return value (int)
1 : is installed.
0 : not installed.

13. comsetport(portno, addr)
void comsetport
int portno, addr

The comsetport is used to set serial port address.

addr (int)
serial port address.

14. comsetvector(portno, addr)
void comsetvector
int portno, addr

The comsetvector is used to set interrupt vector address. After interrupt vector is set, when interrupt is emerged then the device driver will detect interrupt vector to check which port emerges the interrupt.

addr (int)
interrupt vector address. The address is designed at hardware adapter.

15. comsetactive(portno, active)
void comsetactive
int portno
char active

The comsetactive is used to set interrupt vector is active low or high.

active (char)
0 : active low.
1 : active high.

16. comsetMCR(portno, mcr)
void comsetMCR
int portno, mcr

The comsetMCR is used to control the contents of modem control register. Please see APPENDIX B for more details.

mcr (int)
  bit 7 : 0.
  bit 6 : 0.
  bit 5 : 0.
  bit 4 : loopback.
  bit 3 : OUT2 signal.
  bit 2 : OUT1 signal.
  bit 1 : RTS signal.
  bit 0 : DTR signal.

17. compurgerxbuf(portno)
void compurgerxbuf
int portno

Remove any received buffer data from specify port.

18. comtxs(portno, s)
int comtxs, portno
char *s
 
The comtxs is used to send one string to serial port by calling BIOS for each character.

s (char*)
one character string to be sent.

return value (int)
0 : success.
1 : fail.

19. comrxs(portno, s)
int comrxs, portno
char *s

The comrxs is used to get one string from ring buffer by calling BIOS for each character. This routine gets characters from ring buffer until it find <CR> or ring buffer is empty.

s (char*)
received string.

return value (int)
1 : fail.
0 : success.

20. comsettimeout(portno,timeout)
void comsettimeout
int portno
unsigned int timeout

Set time out value about tx; when transmitter buffer in the specified port is full, the driver will wait until buffer empty or the timeout value is expired.

timeout (unsigned int)
time out value, the unit is 1/18 sec.

21. comgettimeout(portno)
unsigned int comgettimeout
int portno

Get the tx timeout value.

return value (unsigned int)
The time out value, unit is 1/18 sec.

22. combufstat(portno)
int combufstat
int portno

The combufstat is used to count the received characters in buffer.

return value (int)
0xFFFF : buffer overflow.
otherwise : number of characters in received buffer.

23. comgetMCR(portno)
char comgetMCR
int portno

Get the value of MCR.

return value (char)
modem control value (see comsetMCR).

24. comgetFC(portno)
char comgetFC
int portno

Get flow control value.

return value (int)
Flow control value (see comsetflow).

25. comgetsets()
int comgetsets

Get number of PCCOM set which are controlled by device driver.

return value (int)
Number of PCCOM set.

26. comgetsetinfo(portno)
BiosComSet_t *comgetsetinfo
   int portno

     To  get  PCCOM  set  information   about
specified port which belongs to this set.

return value (BiosComSet_t*)
     The set information which is defined as
    typedef struct {
       unsigned char irq;
       unsigned int portno;
       unsigned int groupno;
       unsigned int whichgroup;
       struct {
          unsigned int base;
          unsigned int vector;
          unsigned char active;
          unsigned char portno;
       } group[8];
     } BiosComSet_t;

     Where
irq
irq number of this set.
portno number of serial ports in this set.
groupno number of groups in this set.
whichgroup the specified belongs to which group.
base
base I/O port address.
vector
 interrupt vector address.
active active status.
port
number of ports in this group.
 
27. comgetinfo(portno, baud, db, parity, sb)
void comgetinfo
unsigned int *baud
int portno, *db, *parity, *sb

The comgetinfo is used to get communication parameters of specified port.

baud (unsigned int*)
baud rate.

db (int*)
data bits. Either 5, 6, 7 or 8 data bits.

parity (int*)
parity bit. Either NONE, EVEN, ODD.

NONE (0) : none parity check.
ODD  (1) : odd parity.
EVEN (2) : even parity.

sb (int*)
stop bits. Either 1 or 2 stop bits.

28. comgetports()
int comgetports

Get the number of PCCOM ports controlled by the PCCOM driver.

return value (int)
The number of PCCOM ports.

29. comdetectport(portno)
int comdetectport
int portno

The comdetectport is used to detect the serial port is exist or not.

return value (int)
0 : the port is bad.
1 : the  port  is  ok.

30. comgetportinfo(portno)
BiosComport_t *comgetportinfo
   int portno

Retrieve all information about one specified PCCOM port.

return value (BiosComport_t*)
     The port information which is defined as
    typedef struct {
       unsigned int addr;
       unsigned int baud;
       unsigned char lcr;
       unsigned char mcr;
       unsigned char fc;
       unsigned char rx_qsz;
       unsigned char rx_qfree;
       unsigned char rx_enable;
       unsigned char tx_enable;
     } BiosComport_t;

     Where
addr port address.
baud baud rate.
lcr line control register value.
mcr modem control register value.
fc flow control protocol.
rx_qsz received ring buffer size.
rx_qfree free space in rx ring buffer.
rx_enable flag to notify the port receivable.
tx_enable flag to notify the port transmission.
     
31. comrxoff(portno)
void comrxoff
    int portno

Notify the peer to stop sending data.

32. comrxon(portno)
void comrxon
int portno

Notify the peer to restart sending data.
Back
6.3 Examples
In the distribution disk, we provide file transfer and setup program, all of these programs use PCCOM library.
RSETUP.C
Setup communication parameters.

RTX.C
Transfer file.

RRX.C
Receive file.
Back
6.4 I/O Control Via Software Interrupt
To communicate directly with device drivers, the PCCOM provides a set of sub-functions under extended BIOS INT 14H, and DOS IOCTL INT 21H. All these functions are included in the PCCOM library, however, if you want control I/O through BIOS interrupt by yourself, you need prepare CPU register contents and fork a system call. In the following, we will describe these I/O control functions and teach users to use it.
All BIOS functions are designed as a FOSSIL driver, the specification of FOSSIL driver is shown in the "FOSSIL.DOC" file. Our PCCOM BIOS functions specification are shown in the "PCCOM.SPC" file. The examples of DOS IOCTL   function calls are shown in the "COMDOS.C" file.
Before you perform system call, you must specify sub-function number in ah register.

ah description
00H
 setup communication parameters
01H
 send one character
02H
 receive one character
03H
 get serial port status
04H
 activate port
05H
 deactivate port
06H
 set DTR
07H
 return time tick information
0AH
 purge input buffer
0BH
 send one character with no wait
0CH
 nondestructive read ahead
0FH
 flow control for serial I/O
18H
 block read
19H
 block write
1AH
 break begin or end
1BH
 return information about this driver
30H
 get buffer status
31H
 set port address
32H
 set interrupt vector address
3AH
 get the number of PCCOM sets
3BH
 check port exist
3CH
 get port information
3DH
 stop sending data
3EH
 restart sending data
3FH
 set time out
40H
 get time out value
44H
 IOCTL input/output functions (INT 21H)

1. set up communication parameters
ah register
always set to 0h.

al register
al is used to setup communication parameters.

     bit 7, 6, 5 : setup baud rate.
     bit 4, 3    : setup parity.
     bit 2        : setup stop bits.
     bit 1, 0    : setup data bits.

                 bit contents
     baud rate   7    6    5

        9600     1    1    1
        4800     1    1    0
        2400     1    0    1
        1200     1    0    0
         600      0    1    1
         300      0    1    0
       38400     0    0    1
       19200     0    0    0

                 bit contents
     parity      4    3
     
      even      1    1
      odd        0    1
      none      x    0

                 bit contents
     stop bit    2

        2          1
        1          0

                 bit contents
     data bit    1    0

        8          1    1
        7          1    0
        6          0    1
        5          0    0

dx register
     port number.

return value (ax register)

bit 15 to bit 8 : line status.
bit 7 to bit 0  : modem status.

  bit 15 : time out error.
  bit 14 : transmitter shift register empty.
  bit 13 : transmitter holding register empty.
  bit 12 : 0.
  bit 11 : 0.
  bit 10 : 0.
  bit 9   : input buffer overrun.
  bit 8   : data available in receive buffer.
  bit 7   : data carrier detect.
  bit 6   : ring indicator.
  bit 5   : data set ready.
  bit 4   : clear to send.
  bit 3   : always set to 1 upon return.
  bit 2   : delta data carrier detect.
  bit 1   : delta DSR.
  bit 0   : delta CTS.

2. send a character
ah register
always set to 1h.   

al register
character.

dx register
port number.

return value (ax register)
line status and modem status.

3. receive a character
ah register
always set to 2h.   

al register
received character.

dx register
port number.

return value
ah : line status.
al : receive character.

4. get port status
ah register
always set to 3h.   

dx register
port number.

return value (ax register)
line status and modem status.

5. activate port
ah register
always set to 4h.

dx register
port number.

return value
ax : 1954h if successful.
bl : maximum function number supported.
bh : rev of FOSSIL specification.

6. set DTR
ah register
always set to 6h.

al register
0 : turn off DTR.
1 : turn on DTR.

dx register
port number.

7. nondestructive read ahead
ah register
always set to Ch.

dx register
port number.

return value
ah : 0, if character is available.
al : the read character.
ax : FFFF, if no character is available.
 
The character read by this operation is not removed from input queue. This means that the same character will be obtained in next read operation.

8. set flow control
ah register
always set to Fh.

al register
enable or disable flow control. When set to 1, means enable.

bit 0 : enable / disable receiving XON/XOFF.
bit 1 : enable / disable RTS/CTS.
bit 2 : enable /disable DTR/DSR.
bit 3 : enable / disable transmitting XON/XOFF.

dx register
port number.

9. receive block from a port
ah register
always set to 18h.

dx register
port number.

cx register
length of block.

es:di register
starting address of block.

return value (ax register)
count of received bytes.

10. send block to a port
ah register
always set to 19h.

dx register
port number.

cx register
number of bytes to be sent.

es:di register
starting address for received block.

return value (cx register)
count of sent bytes.

11. send break signal
ah register
always set to 1Ah.

dx register
port number.

al register
1 : send break signal.
0 : stop to send break signal.

12. return information about PCCOM driver
ah register
always set to 1Bh.

dx register
port number.

cx register
length of block.

es:di register
starting address of block.

This function will transfer information about PCCOM and its current status to the caller.

return value (ax register)

The data structure (assembly language style) currently returned by PCCOM is asfollows:
 ;DEFINE BEGIN OF STRUCTURE
      INFO        EQU  $            
 ;SIZE OF THIS STRUC IN BYTES
      STRSIZ     DW   INFO_SIZE   
 ;FOSSIL SPECIFICATION REV
      MAJVER    DB   CURR_FOSSIL
 ;REV OF THIS DRIVER.
      MINVER    DB   CURR_REV   
 ;"FAR" POINTER TO ASCIIZ
 ;DRIVER DESCRIPTION STRING.
      IDENT       DD   ID_STRING    
 ;BYTE SIZE OF THE INPUT BUFFER
      IBUFR       DW   IBSIZE    
 ;NUMBER OF BUFFERED RECV BYTES 
      IFREE       DW   ?           
 ;BYTE SIZE OF THE XMIT BUFFER
      OBUFR     DW   OBSIZE      
 ;NUMBER OF BUFFERED XMIT BYTES
      OFREE     DW   ?        
 ;WIDTH OF DISPLAY SCREEN
      SWIDTH    DB   SCREEN_WIDTH
 ;HEIGHT OF DISPLAY SCREEN 
      SHEIGHT   DB   SCREEN_HEIGHT 
 ;BAUD RATE, COMPUTER TO MODEM
      BAUD        DB   ?

13. get buffer status
ah register
always set to 30h.

dx register
port number.

return value (cx)
number of characters in buffer.
FFFFh : overflow.

14. set port address
ah register
always set to 31h.

dx register
port number.

bx register
port address.

15. set vector address
ah register
always set to 32h.

dx register
port number.

bx register
vector address.

16. set active status in vector
ah register
always set to 33h.

dx register
port number.

al register
0 : active low.
1 : active high.

17. set modem control
ah register
always set to 34h.

dx register
port number.

al register
modem control value.

    bit
     7      0
     6      0
     5      0
     4      loopback
     3      OUT2
     2      OUT1
     1      RTS
     0      DTR

18. get port parameters
ah register
always set to 35h.

dx register
port number.

return value(al)
communication parameters. (see 00H)

19. get modem control value
ah register
always set to 36h.

dx register
port number.

al register
modem control value.

20. get flow control value
ah register
always set to 37h.

dx register
port number.

return value (al)
flow control value. see 0fH.

21. get the total number of PCCOM sets
ah register
always set to 38h.

return value (ax)
total number of PCCOM sets.

22. get information about each PCCOM set
ah register
always set to 39h.

dl register
set number.

es:di register
starting address of block.

return value (ax)
number of bytes returned in buffer point by es:di register.

Data structure :
irq db ;interrupt request
portno dw
 ;total number of ports in
;this set
groupno dw
 ;total number of groups in
;this set
whichgroup dw
 ;specified port in which group
base dw
 ;base address for this com set
vector dw
 ;interrupt vector address
active db
 ;active hi or lo in vector
portno db
 ;total number of ports in
;this com set
:
:
 :
:
:
:
base dw
 ;base address for this com set
vector dw
 ;interrupt vector address
active db
 ;active hi or lo in vector
portno db
 ;total number of ports in
;this com set
               
represented in C:

           typedef struct {
              unsigned char irq;
              unsigned int  portno;
              unsigned int  groupno;
              unsigned int  whichgroup
              struct {
                 unsigned int  base;
                 unsigned int  vector;
                 unsigned char active;
                 unsigned char portno;
              } group[8];
            } BiosComSet_t;
23. get the total number of PCCOM ports
ah register
always set to 3Ah.

return value (ax)
total number of ports.

24. detect whether the the port is existent
ah register
always set to 3Bh.

dx register
port number.

return value (ax)
55AA : this port exists.
    0     : this port not existent.

25. get information of one PCCOM port
ah register
always set to 3Ch.

dx register
port number.

es:di register
pointer to space containing port information.

return value (ax)

Data structure :
addr dw
;port address
baud_rate dw
 ;baud rate
lcr db
 ;line control register value
mcr db
 ;modem control register value
fc db
 ;flow control protocol
q_sz dw
 ;rx ring buffer size
q_free dw
 ;free space in ring buffer
rx_enable db
 ;flag to notify the port
;receivable
tx_enable db
 ;flag to notify the port
;transmission

Represented in C:
           typedef struct {
              unsigned int  addr;
              unsigned int  baud;
              unsigned char lcr;
              unsigned char mcr;
              unsigned char fc;
              unsigned char rx_qsz;
              unsigned int  rx_qfree;
              unsigned char rx_enable;
              unsigned char tx_enable;
           } BiosComPort_t;

26. notify peer to stop sending data
ah register
always set to 3Dh.

dx register
port number.

27. notify peer to restart sending data
ah register
always set to 3Eh.

dx register
port number.

28. set time out value
ah register
always set to 3Fh.

dx register
port number.

cx register
time out value in unit of 1/18 sec.

29. get time out value
ah register
always set to 3Fh.

dx register
port number.

return value (cx)
time out value in unit of 1/18 sec.
Back
6.5 PCCOM DOS IOCTL functions
Under the DOS interrupt 21H and its subfunction 44H, user can use IOCTL to access PCCOM character device.

1. IOCTL input function
Input:
    AH      = 44H
    AL       = 02H
    BX      = handle number
    CX      = number of bytes to transfer
    DS:DX = segment:offset of data buffer
            (viewed as control block)

Output:
    If function is successful:
       Carry flag is clear.
       AX = number of bytes to transfer.
       Data buffer at DS:DX contains data read from device driver.

    If function is not successful:
       Carry flag is set.
       AX = error code:
       01H  invalid function
       05H  access denied
       06H  invalid handle
       0DH  invalid data (bad control block)

The transfer address(DS:DX) points to a control block that is used to communicate with the device driver. The first byte of the control block determines the request that is being made.

Code Bytes Function
0
 3
 get serial port status
1
 3
 get port communication parameters
2
 3
 get modem control value
3
 3
 get flow control value
4
 3
 get the size of available data in rx buffer

- Get Serial Port Status

PortSts db  0  ;control block code
              dw ?  ;serial port status

The format of serial port status is referred to BIOS function 03H.

- Get Port Setting

PortParam  db  1   ;control block code
                     db  ?   ;port communication parameters
                     db  ?   ;not used

The format of serial port setting parameters is referred to BIOS function 00H.

- Get Modem Control Value

PortMC  db  2  ;control block code
               db  ?  ;modem control value
               db  ?  ;not used

The format of modem control value is referred to BIOS function 36H.

- Get Flow Control Value

PortFC  db  3  ;control block code
               db  ?  ;flow control value
               db  ?  ;not used

The format of flow control value is referred to BIOS function 0FH.

- Get the Size of Available data in RX Buffer

PortRXData  db  4  ;control block code
                       dw  ?  ;available data size in rx buffer

2. IOCTL Output functions
Input:
    AH      = 44H
    AL       = 03H
    BX      = handle number
    CX      = number of bytes to transfer
    DS:DX = segment:offset of data buffer
                  (viewed as control block)

Output:

    If function is successful:
    Carry flag is clear.
    AX = number of bytes to transfer.

    If function is not successful:
    Carry flag is set.
    AX = error code:
01H  invalid function
05H  access denied
06H  invalid handle
0DH  invalid data (bad control block)

The transfer address(DS:DX) points to a control block that is used to communicate with the device driver. The first byte of the control block determines the request that is being made.

222
Code Bytes Function
0
 2 set port communication parameters
1
 2 set modem control value
2
 2 set flow control value
3
 2 turn on/off "break"
4
 2 purge rx buffer

- Set Port Communication Parameters

PortParam  db    0   ;control block code
                     db  XX  ;port communication parameters

This function is the same as BIOS function 00H.

- Set Modem Control Value

PortMC  db    1    ;control block code
                db  XX  ;modem control value

This function is the same as BIOS function 34H.

- Set Flow Control Value

PortFC  db    2    ;control block code
               db   XX  ;flow control value

This function is the same as BIOS function 0FH.

- Turn on/off "BREAK"

PortFC    db     2    ;control block code
                 db   XX  ; 1 : start sending 'break'
                                  0 : stop sending 'break'

This function is the same as BIOS function 1AH.

- Purge in RX Buffer

PortRXData  db  4  ;control block code
                       db  ?  ;not used

This function is the same as BIOS function 0AH.