3.硬件查询
通过读取和写入串口UART对应的硬件端口，我们可以控制串口的收发。请看下面的例子：
/* Name : Sample Comm's Program - Polled Version - termpoll.c */
/* Written By : Craig Peacock ＜cpeacock@senet.com.au＞ */
#include ＜dos.h＞
#include ＜stdio.h＞
#include ＜conio.h＞
００００００００００００００００００００#define PORT1 0x3F8
/* Defines Serial Ports Base Address */
/* COM1 0x3F8 */
/* COM2 0x2F8 */
/* COM3 0x3E8 */
/* COM4 0x2E8 */
void main(void)
{
int c;
int ch;
outportb(PORT1 + 1, 0); /* Turn off interrupts - Port1 */
/* PORT 1 - Communication Settings */
outportb(PORT1 + 3, 0x80); /* SET DLAB ON */
outportb(PORT1 + 0, 0x03); /* Set Baud rate - Divisor Latch Low Byte */
/* Default 0x03 = 38,400 BPS */
/* 0x01 = 115,200 BPS */
/* 0x02 = 57,600 BPS */
/* 0x06 = 19,200 BPS */
/* 0x0C = 9,600 BPS */
/* 0x18 = 4,800 BPS */
/* 0x30 = 2,400 BPS */
outportb(PORT1 + 1, 0x00); /* Set Baud rate - Divisor Latch High Byte */
outportb(PORT1 + 3, 0x03); /* 8 Bits, No Parity, 1 Stop Bit */
outportb(PORT1 + 2, 0xC7); /* FIFO Control Register */
outportb(PORT1 + 4, 0x0B); /* Turn on DTR, RTS, and OUT2 */
printf("\nSample Comm's Program. Press ESC to quit \n");
do
{
c = inportb(PORT1 + 5); /* Check to see if char has been */
/* received. */
if (c &1)
{
ch = inportb(PORT1); /* If so, then get Char */
printf("%c", ch);
} /* Print Char to Screen */
if (kbhit())
{
ch = getch(); /* If key pressed, get Char */
outportb(PORT1, ch);
} /* Send Char to Serial Port */
}
while (ch != 27); /* Quit when ESC (ASC 27) is pressed */
}
程序中的
c = inportb(PORT1 + 5); /* Check to see if char has been */
/* received. */
if (c &1)
检查PORT1 + 5端口地址，通过c&1可以判断是否有数据被UART接收到。关于UART对应的端口范围，从图2中也可以直观地看出。
