Below is an example of using the C run-time library function
_bios_serialcom() to transmit and receive a single character over the
serial-communications port COM1.
The _bios_serialcom() routine simply calls the PC's ROM BIOS interrupt
14h. On most PCs, these routines are not interrupt-driven or buffered.
As a result, characters may be lost if a significant amount of
processing occurs between status checks and reads and/or when
receiving very high-speed transmissions, 1200 baud is the recommended
maximum. Although Microsoft does not provide interrupt-driven serial
I/O routines with our C compilers, there are several third-parties
that do. A partial listing is available in the Language Support
Directory that comes with the compiler.
For additional information about ROM BIOS calls, please refer to "IBM
ROM BIOS" by Ray Duncan, published by Microsoft Press.
If ROM BIOS routines are not operating as expected, you may need to
contact the company that supplied the BIOS for your particular
machine.
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Sample Code
/* Compile options needed: none
- The following program is a simple example which sends and receives
one character to/from COM1:
*/
#include <stdio.h>
#include <bios.h>
void main(void)
{
unsigned com1_status;
unsigned com1_send;
unsigned com1_rec;
unsigned com1_init;
int result, mask;
/* open serial port at 1200 baud, 8 data bits,
** No parity, 1 stop bit */
com1_init = _bios_serialcom(_COM_INIT, 0,
_COM_CHR8 | _COM_NOPARITY | _COM_STOP1 | _COM_1200);
printf("Init status: 0x%4.4X\n", com1_init);
/* send an '*' to com1 */
com1_send = _bios_serialcom(_COM_SEND, 0, '*');
printf("Send status: 0x%4.4X\n", com1_send);
mask = 0x6100;
/* value used to mask:
* bits 0-7 are related to modems,
* bits 8-15 are for port status,
* check to see that the following bits are set:
* 8 (data ready)
* 13 (Transmission-hold register empty)
* 14 (Transmission-shift register empty)
*/
/* check the status */
com1_status = _bios_serialcom(_COM_STATUS, 0, 0);
printf("COM1 status: 0x%4.4X\n", com1_status);
/* wait until a character is ready */
do {
/* check the status */
com1_status = _bios_serialcom(_COM_STATUS, 0, 0);
/* mask off the low order byte of com1_status */
com1_status = com1_status & 0xFF00;
} while( (mask & com1_status) == 0);
/* get a character */
com1_rec = _bios_serialcom(_COM_RECEIVE, 0, 0);
printf("Read status: 0x%4.4X\n", com1_rec);
/* print the character we just received */
result = com1_rec & 0x00FF;
printf("Character: 0x%2.2X = %c\n", result, (char)result);
}
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