Operating System Fundamentals

Operating System Overview

• Operating systems interface the machine with application programs.
• These systems dynamically allocate shared system resources to executing programs.
• Operating system research is concerned with managing and scheduling memory, processes, and devices.
• The interface between operating systems and adjacent levels shifts, with some OS functions migrating to hardware and some application program functions being included in the OS.

What is an Operating System?

• An OS controls the execution of application programs.
• An OS is an interface between applications and hardware.
• An OS provides convenience, efficiency, and the ability to evolve.

Computer Hardware and Software Infrastructure

• The infrastructure includes application programs, libraries/utilities, and the operating system as software.
• Execution hardware, system interconnect (bus), I/O devices, networking, memory translation and main memory are hardware components.

Operating System Services

• Services include, program development, program execution, access to I/O devices, controlled file access, system access, error detection, response, and accounting.

Key OS Functions

• Process Management handles the creation, scheduling, and termination of processes.
• Memory Management tracks each byte in memory and allocates memory as needed.
• File System Management organizes, stores, retrieves, and manages data on storage devices.
• I/O System Management manages input/output operations and device drivers.
• Security and Protection protects system resources and user data from unauthorized access.
• The User Interface provides user interaction either through a CLI or GUI.
• Resource Utilization maximizes the performance of CPU, memory, and I/O devices.

Key Interfaces

• Instruction Set Architecture (ISA) is the interface between hardware and low-level software.
• Application Binary Interface (ABI) is the interface between the compiled application and the OS.
• Application Programming Interface (API) is the interface between the application code and the OS services/libraries.

The Role of an OS

• A computer consists of resources for the movement, storage, and processing of data.
• The OS is responsible for managing these resources.

Operating System as Software

• An OS functions like ordinary computer software.
• An OS is a program or suite of programs executed by the processor.
• An OS often relinquishes control and depends on the processor to regain control.

Evolution of Operating Systems

• A major OS evolves over time due to hardware upgrades, new types of hardware, new services, and fixes.
• Stages of OS evolution include serial processing, simple batch systems, multiprogrammed batch systems, and time-sharing systems.

Serial Processing

• Earliest computers did not have an OS.
• Programmers interacted directly with computer hardware.
• Computers ran from a console with lights, switches, input devices, and a printer.
• Users accessed the computer in "series."
• Problems included scheduling issues and a considerable amount of setup time.
• Scheduling relied on hardcopy sign-up sheets, leading to potential time allocation inefficiencies.

Simple Batch Systems

• Multiple jobs were grouped into a "batch" and ran sequentially without user interaction.
• Early computers were expensive, so maximizing processor utilization was important.
• A monitor was introduced which meant the user no longer had direct access to the processor.
• Jobs are submitted to a computer operator who batches them and places them on an input device.
• Programs branch back to the monitor when finished.
• The monitor controls the sequence of events, which is a resident software always in memory.
• The monitor reads in jobs (from I/O) and gives control, and the job returns control to the monitor.
• "Control is passed to a job" means the processor fetches and executes instructions in a user program.
• "Control is returned to the monitor" means the processor fetches and executes instructions from the monitor program.
• Job Control Language (JCL) provides instructions to the monitor.
• JCL specifies what compiler to use and what data to use.
• $JOB tells the monitor the job type, $DATA signals input data, and $END signals the end of the job.

Desirable Hardware Features

• Memory protection is important for the monitor to prevent the user program from altering the monitor's memory area.
• A timer prevents a job from monopolizing the system.
• Privileged instructions can only be executed by the monitor.
• Interrupts give the OS more flexibility in controlling user programs.
• In user mode, the user program executes and certain instructions may not be executed.
• In kernel mode, the monitor executes, privileged instructions may be executed and protected memory areas accessed.
• The processor time alternates between execution of user programs and execution of the monitor.
• Simple batch systems improve computer utilization despite overhead, as some main memory and processor time must be allocated to the monitor.

Multiprogrammed Batch Systems

• Processors are often idle because I/O devices are slow compared to processors.
• Even automatic job sequencing leads to processor idleness.
• Multiprogramming aims to improve CPU utilization by running multiple programs in memory.
• There must be enough memory for the OS (resident monitor) and one user program.
• When one job waits for I/O, the processor can switch to another job.
• Multiprogramming is also known as multitasking.

Time-Sharing Systems

• Time-sharing may be used to handle multiple interactive jobs.
• Processor time is shared among multiple users.
• Multiple users simultaneously access the system through terminals.
• The OS interleaves the execution of each user program in time slices.

Batch Multiprogramming vs. Time Sharing

• Batch multiprogramming maximizes processor use, while time sharing minimizes response time.
• Batch systems use job control language commands, while time-sharing systems use commands entered at the terminal.

Compatible Time-Sharing Systems

• CTSS was one of the first time-sharing OS.
• CTSS was developed at MIT by Project MAC.
• The System operated on a computer with 32,000 36-bit words of main memory, with the resident monitor consuming 5,000 of those words.
• CTSS uses a system clock to generate interrupts approximately every 0.2 seconds and when an interrupts occur OS regain control and could assign processor to another user
• To simplify the monitor and memory management, a program was always loaded to start at the 5000th word.
• At regular time intervals the current user is preempted and another user loaded in, this is called swappping.
• User programs and data are written out to disk.
• Program code and data are restored in main memory when that program is next given a turn.