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Lecture # 16
Sample Program using BIOS routines
Example:
#include<BIOS.H>
#include<DOS.H>
char ch1, ch2;
void initialize (int pno)
_AH=0;
_AL=0x57;
_DX=pno;
geninterrupt(0x14);
char receivechar (int pno)
char ch;
_DX = pno;
_AH = 2;
geninterrupt (0x14);
ch = _AL;
return ch;
The initialize () function initializes the COM port whose number is passed as parameter
using BIOS services. The recievechar() function uses the COM port number to receive a
byte from the COM port using BIOS services.
void sendchar (char ch, int pno) { _DX = pno; _AH = 1; _AL = ch; geninterrupt (0x14); } unsigned int getcomstatus (int pno) { unsigned int temp; _DX = pno; _AH = 03; geninterrupt (0x14); ((char)(&temp)) = _AL; (((char)(&temp)) + 1) = _AH; return temp; }
the sendchar() function sends a character to the COM port using BIOS service whose
number is passed as parameter. And the getcomstatus() function retrieves the status of the
COM port whose number has been specified and returns the modem and line status in an
unsigned int.
void main() { while(1) { i = getcomstatus (0); if ((((((char)(&i)) + 1)&0x20) == 0x20) && (kbhit())) { ch1 = getche(); sendchar (ch1, 0); } if (((((char)(&i)) +1) & 0x01) == 0x01) { ch2 = receivechar (0); putch (ch2); } if ((ch1 == 27) || (ch2 ==27)) break; } }
Let’s suppose two UARTs are interconnected using a NULL modem
In the main () function there is a while loop which retrieves the status of the COM port.
Once the status has been retrieved it checks if a byte can be transmitted, if a key has been
void SelfTestOff(unsigned int far * com)
outportb( (com)+4, inport((com)+4) & 0xEf);
void writechar( char ch, unsigned int far * com)
while ( !((inportb((*com)+5) & 0x20) == 0x20));
outport(*com,ch);
char readchar( unsigned int far *com)
while (!((inportb((*com)+5) & 0x01)==0x01));
return inportb(*com);
The SelfTestOff() function turns this facility off. The writechar() function writes the a
byte passed to this function on the data port. The readchar() function reads a byte from
the data port.
unsigned int far com=(unsigned int far) 0x00400000; void main () { char ch = 0; int i = 1;int j= 1; char ch2='A'; initialize( com); SelfTestOn(com); clrscr(); while (ch!=27) { if (i==80) { j++; i=0; }
The main function after initializing and turning the self test mode on enters a loop which
will terminate on input of the escape character. This loop also controls the position of the
cursor such the cursor goes to the next line right after a full line has been typed.
if (j==13) j=0;
gotoxy(i,j); ch=getche(); writechar(ch,com); ch2=readchar(com); gotoxy(i,j+14); putch(ch2); i++; } SelfTestOff (com); }
All the input from the keyboard is directed to the output of the UART and all the input
from the UART is also directed to the lower part of the screen. As the UART is in self
test mode the output becomes the input. And hence the user can see output send to the
UART in the lower part of the screen as shown in the slide below
hello how r u? whats new about systems programming?
hello how r u? whats new about systems programming?
Sample Program using interrupt driven I/O
unsigned int far com=(unsigned int far) 0x00400000; unsigned char far scr=(unsigned char far) 0xB8000000; int i =0,j=0;char ch;int k; void interrupt (oldint)(); void interrupt newint(); void main () { initialize(com); SelfTestOn(com); oldint = getvect(0x0c); setvect(0x0c,newint); outport((com)+1,1); outport(0x21,inport(0x21)&0xEF); keep(0,1000); }
This si program is also quite similar to the previous one. The only difference is that in
this the I/O is performed in an interrupt driven patter using the Int 0x0C as the COM
uses IRQ4. Also to use it in this way IRQ4 must be unmasked from the IMR register in
PIC. Also before returning from the ISR the PIC must be signaled an EOI code.
void interrupt newint() { ch= readchar(com); if (i==80) { j++; i=0; } if (j==13) j=0; k = i2+(j+14)80*2; *(scr+k)=ch; i++; outport(0x20,0x20); }
• C:>DEBUG
-o 3f8 41
-o 3f8 42
-o 3f8 56
-o 3f8 55
-q
C:>
#include <bios.h>
#include <dos.h>
void interrupt (*oldint)();
void interrupt newint();
unsigned char far *scr= (unsigned char far *)0xB8000000;
void initialize (unsigned int far *com)
outportb ( (com)+3, inport ((com)+3) | 0x80);
outportb ( (*com),0x80);
outportb( (*com) +1, 0x01);
outportb ( (*com)+3, 0x1b);
