OS Organization Chapter 2 William Stallings book

A program that controls the execution of application programs

An interface between applications and hardware

• Convenience
• Efficiency
• Ability to evolve

• Instruction set architecture (ISA)
• Application binary interface (ABI)
• Application programming interface (API)

OS is responsible for managing a computer's resources.

A computer is a set of  shared resources, for the movement, storage,and processing of data.

Operating System.

• 1 Operating System Software
• 2 Programs and Data
• 3 Processors
• 4 I/O Controllers
• 5 I/O Devices: Printers, keyboards, cameras etc.
• 6 Memory

• Hardware upgrades
• New types of hardware
• New services
• Fixes

• SerialProcessing
• Simple Batch Systems
• Multiprogrammed Batch Systems
• TimeSharing Systems

• No operating system
• programmers interacted directly with the computer hardware...
• Computers ran from a console with display lights, toggle switches, some form of input device, and a printer...
• Users have access to the computer in “series”...

Scheduling and Set up Time.

• most installations used a hardcopy sign-up sheet to reserve computer time
• time allocations could run short or long, resulting in wasted computer time.

A considerable amount of time was spent just on setting up the program to run

• Because early computers were very expensive, it was important to maximize processor utilization via the concept of a Monitor.
• User no longer has direct access to processor as in Serial Processing
• Jobs are submitted to computer operator who batches them together and places them on an input device.
• Program branches back to the monitor when finished

• Monitor controls the sequenceof events
• Resident Monitor is software always in memory
• Monitor reads in job and gives control
• Job returns control to monitor

• Processor executes instruction from the memory containing the monitor
• Executes the instructions in the user program until it encounters an ending or error condition
• “Control is passed to a job” means processor is fetching and executing instructions in a user program
• “Control is returned to the monitor” means that the processor is fetching and executing instructions from the monitor program

In a simple batch system, “Control is passed to a job”  means that the means processor is fetching and executing instructions in a user program

In a simple batch system it means that the processor is fetching and executing instructions from the monitor program

Special type of programming language used to provide instructions to the monitor

While the user program is executing, it must not alter the memory area containing the monitor

Prevents a job from monopolizing the system

can only be executed by the monitor

gives OS more flexibility in controlling user programs

• user program executes in user mode
• certain areas of memory are protected from user access.
• certain instructions may not be executed.

• monitor executes in kernel mode
• privileged instructions may be executed
• protected areas of memory may be accessed

• Processor time alternates between execution of user programs and execution of the monitor

• Sacrifices:
• -->some main memory is now given over to the monitor...
• -->some processor time is consumed by the monitor...

TRUE

• Central theme of modern OS
• Multiple programs in main memory at the same time
• Needs additional H/W that supports I/O interrupts and DMA (independent I/O processor, I/O channel)

• There must be enough memory to hold the OS (resident monitor) and one user program.
• When one job needs to wait for I/O, the processor can switch to the other job, which is likely not waiting for I/O.

In multiprogramming,also known as multitasking memory is expanded to hold three, four, or more programs and switch among all of them.

• Allows processes to be comprised of a number of fixed-size blocks, called pages.
• Program references a word by means of a virtual address
• Provides for a dynamic mapping between the virtual address used in the program and a real (or physical) address in main memory

• consists of a page number and an offset within the page
• each page may be located anywhere in main memory

The processor spends a certain amount oftime executing, until it reaches an I/O instruction; it must then wait until that I/O instruction concludes before proceeding

• Can be used to handle multiple interactive jobs.
• Processor time is shared among multiple users.
• Multiple users simultaneously access the system through terminals, with the OS interleaving the execution of each user program in a short burst or quantum of computation.

• executable program
• the associated data needed by the program (variables, work space, buffers, etc.)
• the execution context (or “process state”) of the program