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uni/year2/semester1/logseq-stuff/pages/System Software & Operating Systems.md

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- #[[CT213 - Computer Systems & Organisation]]
- **Previous Topic:** [[Programming Models]]
- **Next Topic:** [[Process Management]]
- **Relevant Slides:** ![Lecture 3.pdf](../assets/Lecture_3_1663453274726_0.pdf)
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- # Types of Software
- What is **Application Software**? #card
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- **Application Software** is a computer program designed to perform a group of coordinated functions for the benefit of the user.
- It is designed solve a specific problem.
- What is **System Software**? #card
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- **System Software** is a program dedicated to ^^managing the computer.^^
- It provides a platform to other software & a general programming environment.
- There are two main types of system software:
- **Operating System**
- **Utility Software**
- What is the **Operating System (OS)**? #card
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- The **Operating System** provides functions used by the **application software**.
- It provides the mechanisms for application software to **share** the hardware in an orderly fashion to:
- **Increase the overall performance** by allowing different application software to use different parts of the computer at the same time.
- **Decrease the time to execute** a collection of programs and, again, increase the overall performance
- The OS interacts directly with the hardware to provide an interface to other system software and with application software whenever the application software wants to use the system's resources.
- The OS is **application-domain independent**, provides **resource abstraction**, and provides **resources sharing** through strict resource management policies.
- What is **Utility Software**? #card
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- **Utility Software** is system software designed to help **analyse**, **configure**, **optimise**, or **maintain** a computer.
- Examples include data compression, disk cleaners, disk defragmentation, registry cleaners, or system monitors.
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- # Resources
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- What are **Resources**? #card
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- Different hardware components that a program may access are referred to as **resources**.
- Any particular resource, such as a hard disk, has a generic interface that defines how the programmer can make the resource perform a desired operation.
- What is **Resource Abstraction**? #card
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- Resource Abstraction is done by providing an ^^abstract model of the operation of the hardware components.^^
- Abstraction generalises the hardware behaviour but ^^restricts its flexibility.^^
- With abstraction, certain operations become easy to perform, while others may become ^^impossible^^, such as specific hardware control.
- An abstraction can be made to be much simpler than the actual resource interface.
- Simpler resources can be abstracted to a ^^common abstract resource interface.^^
- ## Resource Sharing
- Abstract & physical resources can be shared among a set of concurrently executing programs.
- What is **Space Multiplex Sharing**? #card
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- **Space Multiplex Sharing** is when resources can be divided into two or more **distinct units** of the resource that can be used independently.
- E.g., Memory, HDD.
- What is **Time Multiplex Sharing**? #card
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- **Time Multiplex Sharing** is when a process is allocated ^^exclusive control of an entire resource for a short period of time^^ (not **spatially divisible**).
- E.g., the Processor Resource.
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- # Operating Systems
- ## OS Organisation
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- ![image.png](../assets/image_1663456127205_0.png)
- What is the **Process & Resource Manager**? #card
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- The **Process & Resource Manager** handles ^^resource allocation.^^
- It uses the abstraction provided by the other managers.
- What is the **Memory Manager**? #card
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- Besides other functions, the **Memory Manager** is in charge of the ^^implementation of the **virtual memory**.^^
- It is classically a separate part of the operating system.
- What does the **File Manager** do? #card
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- The **File Manager** abstracts device I/O operations into a relatively simple operation.
- What is the **Device Manager**? #card
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- The **Device Manager** handles the details of ^^reading & writing to the physical devices.^^
- The Device Manager is implemented within the **device driver**.
- ## OS Evolution
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- ### Computers with no OS
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- Programming in machine language.
- Lack of I/O devices.
- ### Rudimentary OS
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- Programming done in Assembly.
:LOGBOOK:
CLOCK: [2022-09-18 Sun 00:46:32]
:END:
- Some basic I/O devices.
- Some I/O control modules - assembler, debugger, loader, linker.
- ### Batch Processing System
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- Service a collection of **jobs**, called a **batch**, from a **queue**.
- What is a **Job**? #card
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- A **Job** is a predefined sequence of commands, programs, and data combined into a single unit.
- Job Control Language and monitor batch (Interpreter for JCL).
- The user doesn't interact with programs while they operate.
- Process Scheduling: **FIFO**.
- Memory is divided into two parts: **system memory** and **program memory**.
- No special jobs for I/O management, as a job has exclusive access to the I/O devices.
- File management.
- ### Operating Systems that use **Multiprogramming**
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- What is **Multiprogramming**? #card
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- **Mulitprogramming** is the technique of loading multiple programs into **space-multiplexed memory** while **time-mulitplexing** the processor.
- **Timesharing Systems**, **Real-Time Operating Systems**, and **Distributed Operating Systems** all use multiprogramming.
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- #### Common Features of Multiprogramming Systems #card
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- **Multitasking:** multiple processes sharing machine resources.
- **Hardware Support** for memory protection & I/O devices.
- **Multi-user** & **multi-access** support (through time-sharing mechanisms).
- Optional support for **real-time operations** (based on efficient usage of multitasking support).
- Interactive user interface.
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- #### Time Sharing Systems #card
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- **Time Sharing Systems** have **multiprogramming** & **multi-user** support.
- They use **time slice** (round robin) processor scheduling.
- They have protection & inter-process communication support for Memory Management.
- They have support for the protection & sharing of I/O between users.
- The File Management has protection support & sharing support between users.
- ![image.png](../assets/image_1663504895182_0.png)
- #### Real-Time Operating Systems #card
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- **Real-Time Operating Systems** are used whenever a large number of ^^critical external events^^ have to be treated in a short or ^^limited time interval^^ .
- They have support for multiprogramming / multi-tasking.
- The main goal of Real-Time Operating Systems is the **minimisation of response time** to service the external events.
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- Processor Scheduling: Priority-based pre-emptive.
- Memory Management:
- Concurrent processes are loaded into the memory.
- Support for protection & inter-process communication.
- I/O Management:
- Critical in time.
- Processes dealing with I/O are directly connected to the **interrupt vectors** (for handling interrupt requests).
- File Management:
- File Management may not be present.
- If it exists, it should comply with the requirements for **Time Sharing** & **Real-Time** systems.
- ![image.png](../assets/image_1663504922118_0.png)
- #### Distributed Operating Systems #card
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- Multiprogramming induces a ^^strong **centralisation** tendency.^^
- **Distributed Operating Systems** aim for ***decentralisation***.
- Distributed Operating Systems are based on computer **network** technologies, with different communication & synchronisation protocols.
- **Client-Server** application architecture.
- **Security** & **Protection** are the primary concerns.
- ![image.png](../assets/image_1663586535352_0.png){:height 313, :width 318}
- ### Modern Operating Systems
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- **Modern Operating Systems** take attributes from Batch, Timesharing, Real-Time, & Distributed Operating Systems.
- Batch OS -> Constant load on the processor on low-priority tasks.
- Timesharing OS -> Interactive processes are treated on a time share.
- Real-Time OS-> Critical Processes (i.e., network drivers) are tread according to real-time constraints.
- Distributed OS -> Client-Server model protocols.
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- ## OS Implementation Considerations
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- ### Monolithic Operating System #card
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- **Monolithic Operating Systems** try to achieve the functional requirements by ^^executing all the code in the same address space^^ to increase the performance of the system.
- They are often too complex to manage.
- ### Hierarchical Operating System #card
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- **Hierarchical Operating Systems** run most of their services in the **user space**, aiming to improve maintainability & modularity of the codebase.
- Suitable for Object-Oriented Programming, the levels are very well-defined.
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- **Multiprogramming:** The illusion that multiple programs are running simultaneously.
- **Protection:** Access to shared system resources.
- **Processor Modes:** Different privilege levels.
- Restrictions on the operations that can be run.
- **Kernels:** Complete control over everything in the system (i.e., supervisor).
- ### Multi-Programming #card
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- **Multiprogramming** is a technique that allows the system to present the illusion that multiple programs are running on the computer simultaneously.
- It is achieved by ^^switching rapidly between programs.^^
- Each program is allowed to execute for a fixed amount of time called a **timeslice**.
- When a program timeslice ends, the OS stops it, removes it, and gives another program control over the processor - this is a **context switch**.
- To do a context switch, the OS copies the contents of the current program register file into memory, restores the contents of the next program's register file into the processor, and starts executing the next program.
- From the program POV, they can't tell that a context switch has been performed.
- **Protection** between multi-programmed programs is very important.
- Many multiprogrammed computers are **multi-user** - they allow multiple persons to be logged on at a time.
- Besides Protection, data **privacy** is also important.
- ### Protection #card
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- The result of any program running on a multiprogrammed computer ^^must be the same^^ as if the program was the only program running on the computer.
- Programs ^^must not be able to access other program's data^^ and must be confident that their data will not be modified by other programs.
- Programs ^^must not interfere with other program's use of I/O devices.^^
- **Protection** is achieved by having the ^^OS have full control over the resources of the system^^ (processor, memory, & I/O devices).
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- **Virtual Memory** is one of the techniques used to achieve protection between programs.
- Each program operates as if it were the only program on the computer, occupying a full set of the address space as in its virtual space.
- The OS is **translating** memory addresses that the program references into physical addresses used by the memory system.
- As long two program's addresses are not translated to the same address space, programs can be written as if they were the only ones running on the machine.
- ### Processor Modes
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- What are **Processor Modes**? #card
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- **Processor Modes** are operating modes for the CPU that ^^place restrictions^^ on the operations that can be performed by the currently running process.
- Hardware supported CPU modes help the OS to ^^enforce rules^^ that would prevent viruses, spyware, & similar malware from running.
- Only very specific & limited "kernel" code would run unrestricted.
- Any other software (including portions of the OS), would run restricted and would have to ask the "kernel" for permission to modify anything that could compromise the system.
- Multiple mode levels could be designed.
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- The **mode bit** is used to define the **execution capability** of a program on a processor.
- The mode bit may be logically extended to define areas of memory to be used when the processor is in supervisor mode, versus when it is in user mode.
- #### Supervisor Mode #card
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- The processor can execute ^^any instruction.^^
- Instructions that can be executed only in supervisor mode are called *supervisor*, *privileged*, or *protected* instructions. (E.g., I/O instructions).
- The executing process has access on both memory spaces.
- #### User Mode #card
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- The processor can execute only a ^^subset of the instruction set.^^
- The executing process has access only to the ^^user space.^^
- Some processors do not differentiate between protected mode & user mode.
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- #### Privileged Mode
- To ensure that the OS is the only one that can control the physical resources, it executes in **Privileged Mode**.
- The OS is also responsible for **Low-Level UI**
- When keys are pressed, the OS is responsible to determine which program should receive the input.
- When a program wants to display some output, the user program executes some system call that displays the data.
- User programs execute in **User Mode**
- When User Mode programs want to execute something that requires privileged rights, it sends a request to the OS, known as a **system call**, that asks the OS to do the operation for them.
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- ## Kernel
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- What is the **Kernel**? #card
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- The **Kernel** (sometimes called the **nucleus**) is the part of the operating system that executes in supervisor mode.
- The Kernel operates as **trusted** software
- It implements protection mechanisms that cannot be changed through the actions of un-trusted software executing in user mode.
- It provides the lowest level abstraction layer for resources (memory, processors, and IO devices).
- A fundamental design decision: Should a given function of the OS be incorporated in the kernel or not?
- Protection issues.
- Performance issues.
- ## Methods for Requesting System Services
- 1. Through a **Command Line Interface**
- By calling a specific command.
- Using a command interpreter known as a **shell**.
- #### Command Execution Mechanism #card
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- A key pressed by the user generates a hardware interrupt.
- A specialised module of the OS reads the keyed character and stores it in a special command line buffer.
- There are special characters (e.g., to edit the command line) that are not stored in the command line buffer.
- Control is taken by the shell (command interpreter) when an **end of line** is detected.
- The command is **analysed** (with error or success).
- If successful, the shell decides if it is an **internal** or **external** command (for another module).
- If **internal:** the shell tries the execution, which can end successfully or with an error.
- If **external:** the shell looks for the corresponding executable file & executes it with the detected parameters from the previous phrase.
- 2. From user processes requesting services from the OS
- #### System Call
- The parameters of the call are passed according to the **specific OS convention** & hardware architecture.
- Switch to **protected (supervisor) mode** using a specific mechanism.
- E.g., software interrupt, trap, special instruction of type "call supervisor".
- This mechanism is different to a normal call.
- A **special module** takes over, that will analyse the parameters & their access rights.
- This module can reject the system call.
- If accepted, the **corresponding routine** from the OS is executed and the **result** is returned to the user.
- Upon return, the user mode is returned.
- #### Message
- The user process ^^constructs a **message** (A) that describes a desired service.^^
- It uses the **send** function to pass the message to a trusted operating system process.
- The send function checks the message, switches the processor to **protected mode**, & then delivers the message to the process that implements the target function.
- Meanwhile, the user waits for a result with a **message receive** operation.
- When the kernel finishes processing the request, it sends a message (B) back to the user process.
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