First Level Interrupt Handler (FLIH)

• save registers of current process in PCB
• Determine the source of interrupt
• Initiate service of interrupt - calls interrupt handler
  Hardware dependent - implemented in assembler

Note : FLIH must run with interrupts disabled - Why ?
Enable or disable interrupts by priority level in PSW.

U

Interrupt handler may call kernel to unblock another process (e.g. waiting for I/O) which could result in a different process - rather than interrupted one eventually being run by the dispatcher

Handler could be a kernel procedure to service a normal kernel call, e.g. send a message, delay a process etc.

Nested Interrupts

• This occurs when the interrupt handler runs with interrupts enabled - in what scenarios is this likely to occur ?
• If interrupt handler runs in stack of interrupted procedure, ALL context must be saved on stack. The ALL in this case refers to the level of nesting.
• The stack space of ALL processes must be increased to cope with the additional level of nesting.