Texas Instruments TNETE110A, TNETE211, TNETE100A manual Code that gets executed is

Loading and Unloading an Interrupt Service Routine (ISR)

This routine converts either the eight low hardware interrupts, or the eight high interrupts, or a software interrupt higher than 0xF to the vector table, then makes an O/S call to get the old vector and slips in the new. It returns the pre- vious contents of that table entry so that it can be restored later. The timer inter- rupt is not connected to one of the 15 hardware interrupts, so its vector is high- er than 0xF and is entered explicitly.

The code called is:

void interrupt far TimerIsr()

{

if (timercount)

if (±±timercount == 0) IndicateEvent(EV_TIMEOUT);

if (nic.OldTimer) (*nic.OldTimer)();

}

If a time-out value has been set, nic.OldTimer is decremented. If it reaches 0, it sets an internal program flag that is checked in the main program loop. If the OldTimer value is not 0 (there was a vector saved in this NIC's instance of the structure NICEVN), the routine branches to that code so that whatever else needs to be done on the PC on a timer tick is done.

The next interrupt service routine to be intercepted is the abort routine. If the user tries to kill this program with a control break, the sample program makes an attempt to put back all of the interrupt vectors as they were found before exiting the program. The call to insert code from the sample program is similar to the timer intercept above:

nic.OldAbort = HwSetIntVector((BYTE)0x23, AbortIsr);

The code that gets executed is:

void interrupt far AbortIsr()

{

cleanup();

abort();

}

Interrupt Handling 4-3