Lecture Outline

Aims and Objectives
Designing an Operating System

• What should an OS do?
  
• What functions will our OS offer?
  
• What characteristics will our OS have?
  
• Other Desirable Characteristics

• To Design Paper OS to Understand real OSs, By
  • Describing Functions and Characteristics of General Purpose OS
    
  • Introduce concept of Virtual Machine
  
  

• OS as User Interface (User / Hardware)

• Introduce Basic Concepts

• Hierarchical Operating System Model

What should an OS Do?

• Already looked at briefly in lecture 1

• Basic - OS should take instructions and process them

• How?

  By making resources available to enable instructions to be executed

• Stable Environment

• Allow Instructions to have a fair chance of executing (especially in Multi-User / Multi-Process environment)

• Security and Protection

What Functions will our OS Offer?

• Job Sequencing
  
• Job Control Language Interpretation
  
• Error Handling
  
• I/O Handling
  
• Interrupt handling
  
• Scheduling
  
• Resource Control
  
• Protection
  
• User Interface

What characteristics will our OS have?

• Concurrency
  
• Sharing
  
• Long-Term Storage
  
• Nondeterminacy

Other Desirable Characteristics:

• Efficient Performance - Difficult to measure
  
• Reliable - Difficult because of above
  
• Easily Maintainable
  
• Small

• Two Types of User
  • Command-Language User
    
  • System-Call User

• Command-Language user issues commands from terminal (or batch file)

• System-Call Users make run-time system calls. Usually embedded in Programs.

• The User-Interface is character-based (DOS, UNIX) or Graphical (X-Windows, Microsoft Windows, Macintosh) and is used by Command-Language users.

• Raw computer is hardware that is operated by component specific instructions. E.g., COPY command

• Operating System abstracts from hardware, so different OSs can be implemented on same Hardware. E.g., UNIX on DOS PCs.

• VM gives users impression that each has complete control of all resources.

• In reality limits on RAM, Disk space, access to some resources (waiting for others / Unauthorised)

• OS gives impression that computer system is more than the hardware. E.g.,
  • handling I/O
    
  • Memory - OS might support Virtual Memory
    
  • Filing System - access via symbolic name, not actual storage address
    
  • Protection and Error Handling - OS provides, not hardware.
    
  • User control of programs - (E.g., Kill, suspend, run) via command-language
    

• Process - Instance of a program in execution. User or system generated. sequence of actions, one of which is being executed or is capable of being executed.

• Central Processing Unit (CPU/Processor) - executes Processes.

  

• A single processor can execute only one instruction at a time.

• A Multi-Processor has 2 or more processors, each executing 1 process at a time.

Parallel (Concurrent) Processing

• Multiprocessing enables several processes to be in a state of execution concurrently, by sharing system resources.

Resources

• A resource is a component of the computer system for which processes can compete. E.g., CPU, I/O Devices...

• Shareable Resources - Can be used by many Processes at the same time.

• Non-shareable - Can be used by only one process at a time.

The Interrupt & Interrupt Handler

• Current instruction may be to perform a calculation, comparison, Read from/write to I/O, branch, Jump, etc.

• Process cannot always continue executing, leaving CPU unused - inefficient

• Issuing Interrupt forces process to relinquish CPU - OS selects another process.

• Interrupt occurs because current process terminates, performs I/O, makes system call, expiration of time, no longer best process to execute.

• Interrupt Handled by Interrupt Handler - determines source of interrupt and services it.

• Interrupts allows many processes to be between creation and termination \ in execution.

Inter-process Synchronisation

• Nondeterminacy of process execution.

• Processes need to communicate with each other (e.g., to say that they are waiting for data).

Deadlock

• All processes can compete for the same limited resources

• All processes have an equal chance of controlling resources

• The nondeterminacy of the OS means that Deadlock could occur when two or more processes are holding non-shareable resources and are waiting for non-shareable resources that are held by other processes that are waiting for the resources that are held by the other processes in the chain.

  

• The Operating System must identify deadlock and deal with it.

• OS is Built like an onion

Reasons:

• More Modular - Information Hiding

• Easy to maintain - well-defined functions

The Layers

• 1 - The Kernel/Nucleus. Manages processes.

• 2 - Basic I/O. Manages secondary storage devices.

• 3 - Memory-management. Manages Primary memory.

• 4 - File System. Manages collections of data by symbolic names; long-term storage facilities; device Independence.

• 5 - Command-Language Interpreter.

• OS Needs protection because it could be overwritten, modified, etc. which could destroy OS.

• OS is held in primary memory.

• Boundary Registers hold locations of Upper and Lower areas of OS in Memory. User accesses are rejected.

• But User needs to make calls to OS. Done in User Mode/State. Reduced set of OS commands.

• OS or User Program may perform calls to other commands in Supervisor Mode/State.

• Instructions only executable in Supervisor state are collectively known as Privileged Instructions.

• Described Functions and Characteristics of General Purpose OS.

• Introduced the concept of the Virtual Machine

• Looked at OS as a User Interface

• Introduced some Basic Concepts
  Process
  CPU
  Parallel Processing
  Resource
  Interrupt & Interrupt Handler
  Interprocess Communication & Synchronisation
  Deadlock

  Hierarchical OS Model, and the 5 Layers

  Protection

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