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Simple Example Using _bios_serialcom()

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Below is an example of using the C run-time library function_bios_serialcom() to transmit and receive a single character over theserial-communications port COM1.

The _bios_serialcom() routine simply calls the PC's ROM BIOS interrupt14h. On most PCs, these routines are not interrupt-driven or buffered.As a result, characters may be lost if a significant amount ofprocessing occurs between status checks and reads and/or whenreceiving very high-speed transmissions, 1200 baud is the recommendedmaximum. Although Microsoft does not provide interrupt-driven serialI/O routines with our C compilers, there are several third-partiesthat do. A partial listing is available in the Language SupportDirectory that comes with the compiler.

For additional information about ROM BIOS calls, please refer to "IBMROM BIOS" by Ray Duncan, published by Microsoft Press.

If ROM BIOS routines are not operating as expected, you may need tocontact the company that supplied the BIOS for your particularmachine.

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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Article ID: 39501 - Last Review: 01/11/2015 00:48:39 - Revision: 2.0

The C Run-Time (CRT)

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