Process

Process

1. What is a process?

   • an executing program
   • The activity
   • Code,data and stack
   • program state

2. When is created?

   • System initialization
   • System call

3. When do it end?

   • Voluntary
   • Involuntary

4. What happens on a trap?

   • 1.Hardware saves program counter (on stack or in a special register)
   • 2.Hardware loads new PC, identifies interrupt
   • 3.Assembly language routine saves registers
   • 4.Assembly language routine sets up stack
   • 5.Assembly language calls C to run service routine
   • 6.Service routine calls scheduler
   • 7. Scheduler selects a process to run next (might be the one interrupted...)
   • 8.Assembly language routine loads PC & registers for the selected process

5. Why schedule processes?

   • Bursts of CPU usage alternate with periods of I/O wait
   • Some processes are CPU-bound: they don’t make many I/O requests
   • Other processes are I/O-bound and make many kernel requests

6. When are processes scheduled?

   • At the time they enter the system
   • At relatively fixed intervals (clock interrupts)