Coexistence of IBM Printer Adapter and MDA


Card Parallel port
I/O base address
Parallel port
interrupt
IBM Printer Adapter 378 7
IBM Monochrome Display and Printer Adapter (MDA) 3BC 7


Looking at the above table, one may think that there is a problem - the parallel port on both cards use interrupt 7, an interrupt conflict.
For this particular scenario, there is no problem in practice.
Why is that?

1. Interrupt generation on parallel ports is normally disabled. Even when DOS prints to a parallel printer, it doesn't enable the interrupt 7 generation for the parallel port that the printer is attached to. Only specialist programs will enable the interrupt 7 generation on a parallel port.
2. For there to be a problem, all of the following would need to be true:
* Program/TSR A running - using parallel port at 378 - has enabled interrupt generation on that port.
* Program/TSR B running - using parallel port at 3BC - has enabled interrupt generation on that port.
* Both of those programs/TSRs running at the same time.