Operating System Definitions

1
Operating System Definitions

• Resource Allocator
• Command Interpreter
• Government

2
Operating System Types

• Batch
• Time sharing
• Real-time

3
Process

• A program in execution
• A process stored in memory is executed when the
  contents of an operating system data area called
  the process control block is loaded into the CPU
  register
• PC is the starting address of the program
• The change from one process to another requires
  changing from one page/segment table to another

4
Thread

• The components of both processes and threads are
  executable instructions
• A process can be comprised of one or more threads
• A switch from one thread to another in the same
  process does not cause a change to the
  page/segment table.

5
Context Switch

• The switching from one process to another or one
  thread to another.
• During a context switch, the CPU is performing no
  useful work.

6
Kernel-Level Threads

• Dispatched through the kernel of the operating
  system by calls to the interrupt handler like
  processes
• Switching between kernel-level threads is faster
  than switching between processes, but slower than
  switching between user-level threads

7
User-Level Threads

• Scheduled at the user level, normally by library
  routines
• Context switch time between two user-level
  threads in the same process is very fast

8
Concurrency

• Multiple events competing with each other to
  occur

9
Real Concurrency Example

• Consider 3 secretaries doing the following three
  different jobs
• The first is answering questions asked by
  students who enter the office
• The second is answering phone questions
• The third is preparing a report
• Three secretaries would probably perform these
  tasks faster than one could

10
Apparent Concurrency Example

• One secretary is
• Answering questions when students enter the door
• Answering the phone
• Preparing the report
• In the middle of doing any of the tasks, the
  secretary could be interrupted to do another
• At the end of the day the secretary would
  complain about doing a million tasks at once

11
Synchronization

• Synchronization problems occur when
• two or more processes or threads try to access
  the same shared memory location at the same time,
  where at least one of them is writing to the
  location
• one process/thread must wait until another
  process/thread completes an event

12
Synchronization Problems

• Synchronization is not a problem if only one
  process is executing on a single processor
• Synchronization IS a problem when competing
  processes are executed on multiple processors
  (real concurrency)
• Synchronization is also a problem when multiple
  processes are being executed on a single
  processor when interrupts are possible