Operating System Components and Services, Continued

Tom Kelliher, CS42

Sept. 13, 1996

Definitions

1. Hardware
   1. Uniprocessor --- single CPU

   2. Multiprocessor --- multiple CPU's, shared memory

   3. Distributed system --- multiple CPU's, no shared memory (How is a hypercube classified?)

2. Software
   1. Process (Job, Task) --- Activity generated by a program executed by a sequential processor. Single thread of control. Deterministic.

      Is this definition bullet-proof? Can a process be associated with more than one program? Can a program be associated to more than one process?

   2. Sequential Program --- program executed by a single process

   3. Parallel or Concurrent Program --- program executed by several processes

      If two people run vi, does that make vi a concurrent program? What is sharable there (this distinguishes concurrency, as does working toward a common goal)?

3. O.S. Classification
   1. Based on interaction with the user
      1. Classical --- batch, timesharing

      2. Special Purpose --- database, real-time

   2. Based on number of jobs and processors

      

Example of Process Control --- Unix

Unix shell and commands. Foreground and background processes:

shell:
   while (!EOF)
   {
      read cmd
      parse cmd
      fork cmd
      if (! background command)
         wait cmd
   }

cmd:
   execute
   exit

Pipeline: c1 | c2

set up environment and pipeline
fork c1
fork c2
wait
wait

Operating System Structures

Definitions:

• Layered System --- A system in which pieces are built on top of other pieces, with hardware as a foundation. A layer make calls exclusively to the layer beneath it.

  Advantages:

  1. Well defined structure.
  2. Modular.
  3. Information Hiding.

  Disadvantages:

  1. Poor performance on ``deep'' calls.
  2. Difficulty in creating a ``good'' layered design.

• Micro Kernel --- A small executive that provides only necessary functionality to support threads/processes:
  • CPU scheduling.
  • Process primitives: create, destroy, suspend, activate, change priority, etc.
  • IPC.
  • Virtual Memory.
  • Interrupt handlers.
  • Device driver interface.
  Runs in kernel (supervisor) mode. Everything else runs in user mode.

  Advantages:

  1. Only a small amount of code runs in kernel mode.
  2. Easily extended.
  3. System routines work with an abstract machine model --- portability.

  Disadvantages:

  1. System calls require a context switch (slow).
  2. Not easily extended (traditional kernel).

MS-DOS

Weak layering:

Unix

Traditional kernel:

MS Windows NT 4.0

Micro kernel:

THE

A strictly layered system:

Virtual Machines

Lowest software level (between kernel and hardware) provides a virtual machine interface to multiple, independent kernels. IBM VM.

1. Support for OS development alongside a production system.
2. Virtual user/monitor mode, real user/monitor mode.
3. How is a disk operation carried out?
4. Virtual Machine is a simulation:
   • Simulate one architecture with another.
   • Efficiency?
5. How are physical devices (a disk) ``virtualized?''

Operating System Implementation

1. Implementing OSs in HLLs:
   • Burroughs MCP in Algol (NEWP).
   • Unix in C.
   • A few modules must be written in assembler. Others for efficiency.
2. Running make to build a Unix kernel.
3. BSD/OS boot procedure:
   1. BIOS.
   2. Boot block on 1st hard disk.
   3. /boot.
   4. /bsd.
   5. Autoprobing.
   6. init.
   See boot(8).