index.md (1661B)
1 +++ 2 title = 'Interrupts' 3 +++ 4 # Interrupts 5 if the program was just waiting for a signal from an I/O device it would be an insane waste of time. that’s why they invented interrupts. 6 7 interrupt request — hardware signal sent by I/O device 8 9 after a processor receives this signal, it responds through an interrupt-service routine 10 11 after interrupt-service routine is done, an interrupt acknowledge signal is sent to the device 12 13 interrupts can be enabled/disabled by setting the IE bit of the status register PS 14 15 btw, also used in exceptions and debugging! 16 17 Example execution: 18 19 1. Device raises an interrupt request, interrupt request arrives during execution of instruction *i* 20 21 2. Processor completes execution of instruction *i* 22 3. Saves contents of PC (counter) and PS (status) 23 4. Interrupts are disabled by setting IE bit of PS to 0 24 25 5. Processor loads PC with address of first instruction of interrupt-service routine, routine is executed 26 27 6. After execution, PC and PS are restored (including setting IE bit to 1) 28 7. Processor continues from instruction *i*+1 29 30  31 32 ## Multiple devices 33 organised in a priority structure 34 35 priority level of processor is priority of program that is being executed 36 37 only interrupts from higher priorities than the processor's are accepted 38 39 option 1 — polling 40 41 - poll all devices 42 - the first to set the IRQ bit of PS to 1 is serviced 43 44 option 2 — vectored interrupts 45 46 - allocate area in memory to hold addresses of interrupt-service routines (interrupt vectors) 47 - each device identifies itself on request 48 - the info provided by requesting device is a pointer into the interrupt-vector table