What is an Operating System?

A program that acts as an intermediary between a user of a computer and the computer hardware.

Operating system goals:

• Execute user programs and make solving user problems easier.
• Make the computer system convenient to use.
• Use the computer hardware in an efficient manner.

Why Study Operating Systems?

• Understand the model of operation
  • Easier to see how to use the system
  • Enables you to write efficient code
• Learn to design an OS
• Even so, OS is pure overhead of real work
• Application programs have the real value to person who buys the computer

Computer System Structure

Computer system can be divided into four components:

• Hardware - provides basic computing resources
  • CPU, memory, I/O devices
• Operating system
  • Controls and coordinates use of hardware among various applications and users
• Application programs - define the ways in which the system resources are used to solve the computing problems of the users
  • Word processors, compilers, web browsers, database systems, video games
• Users
  • People, machines, other computers

Four Components of a Computer System

Perspectives of the Computer

Operating System Definition

• OS is a resource allocator
  • Manages all resources
  • Decides between conflicting requests for efficient and fair resource use
• OS is a control program
  • Controls execution of programs to prevent errors and improper use of the computer

Operating System Definition (Cont.)

• No universally accepted definition
• Everything a vendor ships when you order an operating system is good approximation
  • But varies wildly
• The one program running at all times on the computer is the kernel. Everything else is either a system program (ships with the operating system) or an application program

Computer Startup

• bootstrap program is loaded at power-up or reboot
  • Typically stored in ROM or EEPROM, generally known as firmware
  • Initializates all aspects of system
  • Loads operating system kernel and starts execution

Computer System Organization

• Computer-system operation
  • One or more CPUs, device controllers connect through common bus providing access to shared memory
  • Concurrent execution of CPUs and devices competing for memory cycles

Computer-System Operation

• I/O devices and the CPU can execute concurrently.
• Each device controller is in charge of a particular device type.
• Each device controller has a local buffer.
• CPU moves data from/to main memory to/from local buffers
• I/O is from the device to local buffer of controller.
• Device controller informs CPU that it has finished its operation by causing an interrupt.

Common Functions of Interrupts

• Interrupt transfers control to the interrupt service routine generally, through the interrupt vector, which contains the addresses of all the service routines.
• Interrupt architecture must save the address of the interrupted instruction.
• Incoming interrupts are disabled while another interrupt is being processed to prevent a lost interrupt.
• A trap is a software-generated interrupt caused either by an error or a user request.
• An operating system is interrupt driven.

Interrupt Handling

• The operating system preserves the state of the CPU by storing registers and the program counter.
• Determines which type of interrupt has occurred:
  • polling
  • vectored interrupt system
• Separate segments of code determine what action should be taken for each type of interrupt

Interrupt Timeline

I/O Structure

• After I/O starts, control returns to user program only upon I/O completion.
  • Wait instruction idles the CPU until the next interrupt
  • Wait loop (contention for memory access).
  • At most one I/O request is outstanding at a time, no simultaneous I/O processing.
• After I/O starts, control returns to user program without waiting for I/O completion.
  • System call - request to the operating system to allow user to wait for I/O completion.
  • Device-status table contains entry for each I/O device indicating its type, address, and state.
  • Operating system indexes into I/O device table to determine device status and to modify table entry to include interrupt.

Two I/O Methods

Device-Status Table

Direct Memory Access Structure

• Used for high-speed I/O devices able to transmit information at close to memory speeds.
• Device controller transfers blocks of data from buffer storage directly to main memory without CPU intervention.
• Only one interrupt is generated per block, rather than the one interrupt per byte.

Storage Structure

• Main memory - only large storage media that the CPU can access directly.
• Secondary storage - extension of main memory that provides large nonvolatile storage capacity.
• Magnetic disks - rigid metal or glass platters covered with magnetic recording material
  • Disk surface is logically divided into tracks, which are subdivided into sectors.
  • The disk controller determines the logical interaction between the device and the computer.

Storage Hierarchy

• Storage systems organized in hierarchy.
  • Speed
  • Cost
  • Volatility
• Caching - copying information into faster storage system; main memory can be viewed as a last cache for secondary storage.

Storage Device Hierarchy

Caching

• Important principle, performed at many levels in a computer (in hardware, operating system, software)
• Information in use copied from slower to faster storage temporarily
• Faster storage (cache) checked first to determine if information is there
  • If it is, information used directly from the cache (fast)
  • If not, data copied to cache and used there
• Cache smaller than storage being cached
  • Cache management important design problem
  • Cache size and replacement policy

Performance of Various Levels of Storage

• Movement between levels of storage hierarchy can be explicit or implicit

Migration of Integer A from Disk to Register

• Multitasking environments must be careful to use most recent value, not matter where it is stored in the storage hierarchy
  
• Multiprocessor environment must provide cache coherency in hardware such that all CPUs have the most recent value in their cache
• Distributed environment situation even more complex
  • Several copies of a datum can exist
  • Various solutions covered in Chapter 17

Operating System Structure

• Multiprogramming needed for efficiency
  • Single user cannot keep CPU and I/O devices busy at all times
  • Multiprogramming organizes jobs (code and data) so CPU always has one to execute
  • A subset of total jobs in system is kept in memory
  • One job selected and run via job scheduling
  • When it has to wait (for I/O for example), OS switches to another job
• Timesharing (multitasking) is logical extension in which CPU switches jobs so frequently that users can interact with each job while it is running, creating interactive computing
  • Response time should be < 1 second
  • Each user has at least one program executing in memory ⇒ process
  • If several jobs ready to run at the same time ⇒ CPU scheduling
  • If processes don't fit in memory, swapping moves them in and out to run
  • Virtual memory allows execution of processes not completely in memory