Parallel Port - Computer Busses - page 272

}
void disable_interrupts(void)
{
int ch;
disable();
outportb(IMR, ch & ~IRQ7);
enable();
}
void reset_vectors(void)
{
setvect(0x0f,oldvect);
}
16.5.4 Program explanation
The initial part of the program enables the interrupt on the parallel port by setting bit 4 of the
control register to 1:
void setup_parallel(void)
{
outportb(CONTROL, inportb(CONTROL) | 0x10); /* Set Bit 4 on control port*/
}
After the serial port has been initialized the interrupt service routine for the IRQ7 line is set to
point to a new 'user-defined' service routine. The primary parallel port LPT1: normally sets
the IRQ7 line active when the ACK line goes from a high to a low. The interrupt associated
with IRQ7 is 0Fh (15). The getvect() function gets the ISR address for this interrupt, which
is then stored in the variable oldvect so that at the end of the program it can be restored.
Finally, in the set_vectors() function, the interrupt assigns a new 'user-defined' ISR (in
this case it is the function pl_interrupt() ):
void set_vectors(void)
{
int int_mask;
disable(); /* disable all ints */
oldvect=getvect(0x0f); /* save any old vector */
setvect (0x0f,pl_interrupt); /* set up for new int serv */
}
At the end of the program the ISR is restored with the following code:
void reset_vectors(void)
{
setvect(0x0f,oldvect);
}
To enable the IRQ7 line on the PIC, bit 5 of the IMR (interrupt mask register) is to be set to a
0 (zero). The statement
ch = inportb(IMR) & 0x7F;
achieves this as it bitwise ANDs all the bits, except for bit 7, with a 1. This is because any bit