C language program Lecture 3

This same task can be performed by the following program as well. In this case the video text memory is accessed byte by byte.

unsigned char far *scr=(unsigned char far*)0xb8000000;
void main()
{
*scr=0x56;
*(scr+1)=0x07;
*(scr+2)=0x55;
*(scr+3)=0x70;
}

The next example fills whole of the screen with spaces. This will clear the contents of the screen.

unsigned char far *scr=(unsigned char far*)0xb8000000; //corrected
void main()
{
int i; //instruction added
for (i=0;i<2000;i++) scr="0x20;" scr="scr+2;">void interrupt (*old)();
void interrupt newfunc();
char far *scr=(char far* ) 0xb8000000;
void main()
{
old=getvect(0x08);
setvect(0x08,newfunc);
keep(0,1000);
}
void interrupt newfunc ()
{
*scr=0x41; //corrected
*(scr+1)=0x07; //corrected
(*old)();

}

In the above example the timer interrupt is intercepted such that whenever the timer interrupt is invoked (by means of hardware) the memory resident newfunc() is invoked. This function simply displays the ASCII character 0x41 or ‘A’ in the top left corner of the text screen.

Here is another example.
#include
void interrupt (*old)();
void interrupt newfunc();
char far *scr=(char far* ) 0xb8000000;
int j;
void main( )
{
old=getvect(0x08);
setvect(0x08,newfunc); //corrected
keep(0,1000); //corrected
}

void interrupt newfunc ( )
{
for ( j=0;j<4000;j+=2){>
if(*(scr+j)==‘1’){
*(scr+j)=‘9’; }
}
(*old)();
}

This program scans through all the bytes of text display memory when int 8 occurs. It once resident will replace all the ‘1’ on the screen by ‘9’. If even somehow a ‘1’ is displayed on the screen it will be converted to ‘9’ on occurrence of interrupt 8 which occurs 18.2 times every second.

The keyboard Interrupt
Keyboard is a hardware device and it makes use of interrupt number 9 for its input operations. Whenever a key is pressed interrupt # 9 occurs. The operating system processes this interrupt in order to process the key pressed. This interrupt usually reads the scan code from the keyboard port and converts it into the appropriate ASCII code and places the ASCII code in the keyboard buffer in BIOS data area as described I nthe diagram belowLets now experiment on the keyboard interrupt to understand its behavior

Example

#include
void interrupt (*old)( );
void interrupt newfunc( );

void main( )
{
old = getvect(0x09);
setvect(0x09,newfunc);
keep(0,1000);
}
void interrupt newfunc ( )
{
(*old)( );
(*old)( );
(*old)( );
}

This program simply intercepts the keyboard interrupt and places the address of newint in the IVT. The newint simply invokes the original interrupt 9 thrice. Therefore the same character input will be placed in the keyboard buffer thrice i.e three characters will be received for each character input.

Example

#include
void interrupt (*old)( );
void interrupt newfunc( );
char far *scr = (char far* ) 0x00400017;

void main( )
{
old = getvect(0x09);
setvect(0x09,newfunc);
keep(0,1000);
}
void interrupt newfunc ( )
{
*scr = 64;
(*old)( );
}

The above program is quite familiar it will just set the caps lock status whenever a key is pressed. In this case the keyboard interrupt is intercepted.

Example

void interrupt (*old)( );
void interrupt newfunc( );
char far *scr = (char far* ) 0xB8000000;
int j;
void main( )
{
old = getvect(0x09);
setvect(0x09,newfunc);
keep(0,1000);
}
void interrupt newfunc ( )
{ for( j = 0;j <> the keyboard the newfunc functions runs through whole of the test display memory and replaces the ASCII ‘1’ displayed by ASCII ‘9’. Timer & Keyboard Interrupt Program #include
void interrupt (*oldTimer)( ); //corrected
void interrupt (*oldKey)( ); //corrected
void interrupt newTimer ( );
void interrupt newKey ( );
char far *scr = (char far* ) 0xB8000000;
int i, t = 0, m = 0;
char charscr [4000];
void main( )
{
oldTimer = getvect(8);
oldKey = getvect (9);
setvect (8,newTimer);
setvect (9,newKey);
getch();
getch();
getch();
getch();
}

void interrupt newTimer ( )
{
t++;
if ((t >= 182) && (m == 0))
{
for (i =0; i < i ="0;" t =" 0;" m =" 1;" m ="="" w ="0;" m =" 0;">y and fills the screen with spaces and sets a flag m. The newKey function is invoked when a key press occurs. The flag is checked if the it’s set then the screen is restored from the values saved in that character array. Reentrant Procedures & Interrupt If on return of a function the values within the registers are unchanged as compared to the values which were stored in registers on entry into the procedures then the procedure is called reentrant procedure. Usually interrupt procedures are reentrant procedures especially those interrupt procedure compiled using C language compiler are reentrant. This can be understood by the following example
In the above example the function Proc1() is invoked. On invocation the register AX contained the value 1234H, the code within the function Proc1() changes the value in AX to FF55H. On return AX will contain the value 1234H if the function have been implemented as a reentrant procedure i.e a reentrant procedure would restore the values in registers their previous value (saved in the stacked) before returning.
C language reentrant procedures save the registers in stack following the order AX, BX, CX, DX, ES, DS, SI, DI, BP on invocation and restores in reverse order before return.

This fact about reentrant procedures can be analysed through following example.

#include
void interrupt *old();
void interrupt newint()
void main ()
{
old = getvect(0x65);
setvect(0x65,newint);
_AX=0xf00f;
geninterrupt(0x65);
a = _AX
printf(“%x”,a);
}
void interrupt newint()
{
_AX=0x1234;
}


Firstly its important to compile this above and all the rest of the examples as .C files and not as .CPP file. It these codes are compiled using .CPP extension then there is no surety that this program could be compiled.
Again int 65H is used for this experiment. The int 65H vector is made to point at the function newint(). Before calling the interrupt 65H value 0xF00F is placed in the AX register. After invocation of int 65H the value of AX register is changed to 0x1234. But after return if the value of AX is checked it will not be 0x1234 rather it will be 0xF00F indicating that the values in registers are saved on invocation and restore before return and also that the interrupt type procedures are reentrant.