Answer to Question #184654 in Software Engineering for Shahrin

Question

If you want to handle 30 interrupts, how many 8259A PIC will you need?

Answer 1

A single 8259A PIC can handle 8 interrupts. Seems like we can use four 8259A PICs as 8x4=32, but this is incorrect.

Remember that cascading requires the reservation of one interrupt, so, actually, you get 7 interrupts handled by a single 8259A PIC.

So, 7+7+7+8=29. Not enough interrupts.

7+7+7+7+8=36. This is how many 8259A PIC we need: 5.