Introduction

Tom Kelliher, CS42

Sept. 4, 1996

Class Objectives

Why study, write operating systems?

Some things I hope you take with you:

1. An understanding of what an OS does and how it does it.
2. Concurrency (a new concept for you)
3. The design & review process.
4. Practical applications of algorithms and data structures.

OS As Interface

1. High level view: virtual machine abstraction --- convenient ``user'' interface. Abstractions: files, applications. I/O devices integrated into filesystem.
2. Low level: management of real resources: CPU cycles, memory, disk space, device allocations.
3. Secondary concerns: efficiency, fairness.

Abstractions:

1. Multiprogramming, protection and security.
2. Virtual memory.
3. File systems.

Concurrency and Synchronization

Understanding concurrency, pitfalls, mechanisms of control.

Can there really be two processes/threads executing simultaneously?

Example: Banking system where account updates can occur concurrently and independently.

Deposit process:

temp1 = balance;
temp1 += deposit;
balance = temp1;

temp2 = balance;
temp2 -= withdrawal;
balance = temp2;

1. No concurrency, no problem.
2. Consider interleaving.

Layering/Abstraction Within a Computer System

1. Hardware: CPU, memory, I/O.
2. Operating system: kernel, file system, device handlers.
3. Application programs: editors, compilers, workbench tools.
4. Users: people, other programs, computers.

The ``Hello world'' program:

1. Compiled into assembly code.
2. Assembled in machine code.
3. Written to a file.
4. Loaded into memory.
5. Linked against system libraries.
6. Executes
7. Makes supervisor calls to access I/O devices through OS.

Historical Developments

1. Common device drivers:
   1. Reinventing the wheel.
   2. Abstract I/O device interface for software.
2. Resident monitors:
   1. Keep expensive hardware utilized.
   2. Automatic job sequencing --- compile, run user program.
   3. Monitor is always resident in memory.

      

      Protection?

3. System parallelism:
   1. Overlap processing of one job with I/O of another.
   2. Off-line processing (card to tape, vice versa).
   3. Spooling.
4. Multiprogramming (batch):
   1. Overlap processing yields multiple jobs in memory simultaneously --- job pool.

      

   2. CPU idle when job does I/O.
   3. Automatically switch (CPU scheduling) to ``next'' job.
   4. Job runs until completion or I/O.
   5. Protection?
5. Timesharing:
   1. Batch systems aren't productive for program development.
   2. Preemptive CPU scheduling.
   3. Brief quantum for each process.
   4. Response time important.
6. Realtime systems:
   1. Hard deadlines for process completion.
   2. Example: flight surface control on the Space Shuttle.
7. Workstations:
   1. Essentially, a single-user computer.
   2. Mainframe features trickle down
   3. Why multiprogram a workstation?
      1. Increase productivity.
      2. Allows modular system design --- consider the windows in a GUI.
8. Parallel processing:
   1. Tightly coupled multiprocessors.
   2. Shared memory space.
   3. CPU scheduling issues.
   4. Working set locality.
   5. Finding parallelism opportunities within a problem.
9. Distributed systems:
   1. Loosely-coupled, independent systems.
   2. Private memory spaces.
   3. Client/server computing.
   4. Reliability, resource sharing, load balancing, communication, transparency.
   5. Granularity of parallelism.
   6. Network Latency.