2.硬件中断
为了给读者一个直观的印象，我们通过在Windows操作系统中查看COM的资源属性获得某COM对应的中断号，如图2（该对话框中设备管理器中开启）。
 图2 COM中断号 |
实际上COM的确直接对应于一个中断，而系统也按照一定的规律为各类硬件分配了一个较固定的中断号，如表1。
表1 中断向量表
| INT (Hex) | IRQ | Common Uses | | 08 | 0 | System Timer | | 09 | 1 | Keyboard | | 0A | 2 | Redirected | | 0B | 3 | Serial Comms. COM2/COM4 | | 0C | 4 | Serial Comms. COM1/COM3 | | 0D | 5 | Reserved/Sound Card | | 0E | 6 | Floppy Disk Controller | | 0F | 7 | Parallel Comms. | | 70 | 8 | Real Time Clock | | 71 | 9 | Reserved | | 72 | 10 | Reserved | | 73 | 11 | Reserved | | 74 | 12 | PS/2 Mouse | | 75 | 13 | Maths Co-Processor | | 76 | 14 | Hard Disk Drive | | 77 | 15 | Reserved |
通过编写COM对应的中断服务程序，我们也可以操作串口，涉及到的相关函数有：
（1）设置中断向量表
/*dos.h*/
void _Cdecl setvect (int interruptno, void interrupt (*isr) ()); |
例如，COM3对应的中断号是4，那么对应中断向量表中的地址是0x0C，设置0x0C对应中断程序的函数为：
其中的中断服务程序PORT1INT为：
void interrupt PORT1INT()
{
int c;
do
{
c = inportb(PORT1 + 5);
if (c &1)
{
buffer[bufferin] = inportb(PORT1);
bufferin++;
if (bufferin == 1024)
bufferin = 0;
}
}
while (c &1);
outportb(0x20, 0x20);
} |
上述中断服务程序检查是否有字符可接收，其后将其通过inportb(PORT1)语句将其从UART中读出并放入输入buffer。持续的检查UART，以便能在一次中断里读取所有可获得的数据。
最后的"outportb(0x20,0x20);"语句告诉可编程中断控制器（Programmable Interrupt Controller，PIC）中断已经完成。
（2）读取中断向量表
/*dos.h*/
void interrupt (* _Cdecl getvect(int interruptno)) (); |
例如：
| oldport1isr = getvect(INTVECT); |
其中的oldport1isr定义为：
| void interrupt (*oldport1isr)(); |
我们融合setvect()函数、中断服务程序和getvect()函数，给出一个由Craig Peacock编写的完备例程：
/* Name : Sample Comm's Program - 1024 Byte Buffer - buff1024.c */
/* Written By : Craig Peacock <cpeacock@senet.com.au> */
#include <dos.h>
#include <stdio.h>
#include <conio.h>
#define PORT1 0x3F8 /* Port Address Goes Here */
#define INTVECT 0x0C /* Com Port's IRQ here (Must also change PIC setting) */
/* Defines Serial Ports Base Address */
/* COM1 0x3F8 */
/* COM2 0x2F8 */
/* COM3 0x3E8 */
/* COM4 0x2E8 */
int bufferin = 0;
int bufferout = 0;
char ch;
char buffer[1025];
void interrupt(*oldport1isr)();
void interrupt PORT1INT() /* Interrupt Service Routine (ISR) for PORT1 */
{
int c;
do
{
c = inportb(PORT1 + 5);
if (c &1)
{
buffer[bufferin] = inportb(PORT1);
bufferin++;
if (bufferin == 1024)
{
bufferin = 0;
}
}
}
while (c &1);
outportb(0x20, 0x20);
}
void main(void)
{
int c;
outportb(PORT1 + 1, 0); /* Turn off interrupts - Port1 */
oldport1isr = getvect(INTVECT); /* Save old Interrupt Vector of later
recovery */
setvect(INTVECT, PORT1INT); /* Set Interrupt Vector Entry */
/* COM1 - 0x0C */
/* COM2 - 0x0B */
/* COM3 - 0x0C */
/* COM4 - 0x0B */
/* PORT 1 - Communication Settings */
outportb(PORT1 + 3, 0x80); /* SET DLAB ON */
outportb(PORT1 + 0, 0x0C); /* 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 */
outportb(0x21, (inportb(0x21) &0xEF)); /* Set Programmable Interrupt Controller */
/* COM1 (IRQ4) - 0xEF */
/* COM2 (IRQ3) - 0xF7 */
/* COM3 (IRQ4) - 0xEF */
/* COM4 (IRQ3) - 0xF7 */
outportb(PORT1 + 1, 0x01); /* Interrupt when data received */
printf("/nSample Comm's Program. Press ESC to quit /n");
do
{
if (bufferin != bufferout)
{
ch = buffer[bufferout];
bufferout++;
if (bufferout == 1024)
{
bufferout = 0;
}
printf("%c", ch);
}
if (kbhit())
{
c = getch();
outportb(PORT1, c);
}
}
while (c != 27);
outportb(PORT1 + 1, 0);
/* Turn off interrupts - Port1 */
outportb(0x21, (inportb(0x21) | 0x10)); /* MASK IRQ using PIC */
/* COM1 (IRQ4) - 0x10 */
/* COM2 (IRQ3) - 0x08 */
/* COM3 (IRQ4) - 0x10 */
/* COM4 (IRQ3) - 0x08 */
setvect(INTVECT, oldport1isr); /* Restore old interrupt vector */
} |
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