Process states, how interrupts change priorities

Three types of process states

• Ready

• Running

• Blocked

Conditions under which a process could change from running state to ready state

• Time slice of a running process expires (in round robin)

  • And there is a process with a higher priority in the ready queue, the running process is pre-empted (interrupted)

• When an interrupt arrives at the CPU

  • The process running on the CPU gets pre-empted

Outline how the kernel of an OS acts as an interrupt handler

• Kernel receives a signal when an interrupt is generated

• Kernel checks priority and reviews the status/priority of the current of the current interrupts

• System enters kernel mode if the type of interrupt is of higher priority than the current process

• Kernel consults the interrupt dispatch table

  • And saves the state of the interrupted process

• Kernel restores the process state (contents of register once the interrupt is serviced)

Why a process could be in running state

• CPU time as been allocated and the process is being executed

Why a process could be in ready state

• The process is waiting in the ready queue for a slice of CPU time, could run

Why a process could be in blocked state

• Process is waiting for an I/O operation / some event to take place and be completed