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CPU Operation

• The CPU executes one task at a time
• The CPU operates much faster than other computer components
• CPU speed runs at multiples speeds of the system bus
• Compared to CPU cycles, hard drive read and write operations are slow
• CPU utilization is increased by organizing jobs for processors when waiting for disk read/writes or other input/output operations
• Keyboard input or video output, and print jobs can increase CPU utilization
• Multiprogramming, Multitasking, Multiprocessing & Multithreading can increase CPU utilization

CPU Scheduling

• Multiprogramming increases CPU utilization by loading multiple programs into memory
• The operating system switches the processor between programs, so the processor is not idle
• One program runs until it relinquishes control, allowing another program to run
• Cooperative multitasking is a characteristic of multiprogramming
• Multitasking is used on modern operating systems and runs several programs simultaneously
• Multitasking runs other system tasks and assigns time slices to each running process
• Multitasking creates the appearance of several programs running at the same time
• Multiprocessing involves multiple CPUs on a motherboard
• Multiprocessing utilizes multiple processor cores within a single CPU package
• Program instructions are sent to different processors, so multiple instructions are completed simultaneously
• Multithreading requires multiple processors
• Several command strings are directed to different processors for parallel execution in multithreading, speeding up program execution
• Time sharing relies on the processor switching between multiple jobs
• Each process or job is scheduled based on a time slice allotment
• Preemptive multitasking is used
• Preemptive multitasking stops the current process execution and returns to a previous process
• User interaction is slow, so each process runs simultaneously

Operating System Interrupts

• Operating systems are interrupt-driven
• Commands for a process may be executed by the OS, and a packet arrives at the network interface card (NIC)
• An interrupt will be sent from the NIC to the CPU along the control bus
• Hardware interrupts originate from a device controller
• The OS suspends the execution of current process to service the arriving packets
• The OS will sit idle unless a process is running
• The OS waits for an event to initiate action
• An event may be a user opening an executable programs
• A hardware device may require action
• Most actions performed by the OS are triggered by a hardware or software interrupt

Device Controller

• A device controller manages a specific device
• Application programs execute code that interacts with a device
• Applications do not have direct access to devices, a system call is initiated from the application
• The OS issues the appropriate instructions to the CPU
• The CPU sends commands and data to the device controller by writing to local buffers (memory) in the controller
• The device controller has the instructions required to perform CPU-requested actions
• The device controller signals an interrupt to notify the CPU that the requested task is complete
• The CPU stops its current task to service the interrupt
• The interrupt's value is a pointer to a data structure called the interrupt vector
• The interrupt vector points to the memory address of the service routine with instructions to handle the device request
• The service routine with instructions is called an interrupt handler
• The table of interrupt vectors is stored in low memory locations
• The OS passes the results back to the application program
• A device also issues a hardware interrupt when something happens
• The interrupt is a request to the O/S to service the device
• A hardware interrupt may be triggered by pressing a key on a keyboard or when data arrives at the NIC

Software Interrupts

• A trap or exception is a software interrupt from applications or system programs
• Software Interrupts request for a specific service or action to be performed
• Saving data to disk and sending data to printer are functions of the software interrupts
• When a software error has occurred, division by zero, invalid memory access, and attempts by a program to modify another running program's data generate software interrupts

Privilege Levels

• The OS supports multiple privilege levels that protect the system hardware and the OS
• Privilege levels restrict CPU instructions used by programs at a specific level
• Ring 0 has the most privileges and interacts directly with the CPU and memory
• Rings 1 & 2 give privileges to hardware device drivers
• Ring 3 is for User applications
• Programs at ring 3 aren't allowed to access hardware at ring 0
• Operating systems often support two modes of operation
• Ring 0 & Ring 3 are the two rings common to all operating systems
• User mode prevents access to system instructions that could damage hardware, the kernel, or other processes; e.g. Ring 3
• Kernel modes are: Privilege Mode, Supervisor Mode or System Mode, e.g. Ring 0
• Hardware that supports virtualization often uses more levels
• VirtualBox and VMware place the guest kernel code in ring 1
• The guest OS has privileges that surpass user applications, but lack full kernel mode privileges

Protection and Security

• A computer may have multiple concurrent users and processes
• Data access must be controlled to ensure authorized users/processes have access
• Processes only execute in their assigned address space
• Processes and file permissions are tied to a User ID
• Privilege escalation allows a user to change to one with more rights
• Privilege escalation is "Run as administrator" on Windows
• Privilege escalation is "Sudo" on Linux

System Calls

• Operating systems provide many services, like file management and print services
• User applications and system programs access these services via system calls
• A system call is a request by a user program to perform a reserved task
• A system call is a programming interface to the operating system with a request to switch from user to privileged mode
• System calls are typically written in a high-level language like C or C++
• Calls directed to hardware devices could written in low level assembly language
• The CPU uses a mode bit to determine the level of privilege operation
• Kernel Mode = 0 and User Mode = 1
• When a user application is started the mode bit is set to 1
• A user application requires a privileged action to complete a system call
• The OS switches to Kernel Mode 0 to complete a system call
• System calls are programmed directly
• Programming languages compilers and interpreters include libraries to make using system calls easier
• The libraries are called Application Programming Interfaces (APIs)
• The API specifies a library of available function the application programmer can access
• Windows API functions are located in dynamic link libraries (dlls)
• Some Windows API example dlls are Kernel32dll, User32dll, shell32dll, and Gui32.dll
• The operating system interface details are hidden from the programmer by an API
• Systems provide a library or API between programs and the operating system
• On Unix-like systems, the API is part of an implementation of the C library (libc)
• On Windows NT, the API is part of the Native API, ntdll.dll library
• A function call is loading a piece of code into user space to perform a specific task
• When a task (or subroutine) is complete, control is passed back to the main program
• The C standard library provides built-in functions that a program can call
• Functions may complete system calls during execution

Process Management

• A program is a passive entity, a block of instruction code on disk
• A process is an active entity, an executing program
• Each process is given a process id (PID)
• The user may be running several programs at one time, each is a separate process
• Running a word processor, browsing the Internet, and listening to music are all separate processes
• When the process is terminated, assigned memory space is released back to the O/S for other processes
• A register is a small memory area built into a CPU
• Processes load instruction and related data from memory into registers for execution
• Each process has an identification number (PID)
• A program counter is a special register that points to the next instruction for execution
• When context-switching from one process to another, register and counter contents are saved
• Each process has a Process Control Block (PCB)
• The PCB contains information used monitor and manage processes
• A process runs continuously within multi-tasking systems
• When stored in a PCB, program counters and registers can store process values when not actively running
• New status represents the process is being created and assigned Process ID (PID) and memory addresses (stack & heap)
• Running status code indicates instructions are being executed by the Program counter
• Waiting status code means, the process is waiting for an event like an interrupt to occur
• Ready status code represents a processes waiting assignment to a processor for a time slice
• Terminated signifies the process has finished execution

Memory

• The operating system assigns each process id (PID) to a dedicated memory space
• Code segment (text) contains binary executable code (read only)
• The Initial Data segment contains assigned variables for the process
• Block Started by Symbol (BSS) contains variables without a default value
• Heap is memory allocated for use by the process
• Stack i temporary space for functions called by a process
• Based and Limits Registers define private memory addresses for the process
• Each process is allocated dynamic memory addresses called a heap, which stores data calculated by an instruction
• Dynamic memory called a stack (buffers) is assigned at the highest usable memory location and grows downwards
• The stack holds a temporary user's input variables into a program such as a the number entered into a calculator program

User Interface

• The Command Line Interface (CLI) uses a text mode interface
• The CLI allows users to interact with programs via a keyboard/screen with text characters
• Command line interfaces are called shells or command interpreters
• CLI commands operate at Ring level 3, the outer shell, with the kernel at the middle
• Widely uses CLI for system administration due to flexibility and efficiency
• Command line interfaces are available on Windows, Mac OSX and Linux
• Commands used at the CLI can be built into the shell or external commands
• Built-in commands exist always run with shell and cannot be separated
• External commands are separate programs that a shell loads and executes
• Internal and external commands are visibly indistinguishable
• Windows shells include: Command Prompt and PowerShell
• Many Linux servers feature CLI only (no GUI environment)
• CLI only is a common configuration for security and performance
• A Linux desktop with GUI also has CLI access via terminal application
• Bash (Bourne again shell) is the most widely used shell
• Bourne shell (sh) is one of the oldest Unix/Linux shells
• Korn shell (ksh) is a small, limited functionality shell for systems with limited resources
• Graphical User Interface (GUI) utilizes pointing device operations
• Images and icons are used as inputs to OS in GUI
• GUI employs Keyboards for text input
• Multiple windows display at once, but the user is limited to interacting with one at a time
• Mobile systems can combine keyboard, pointing device and basic screen functionality in a touch screen
• CDE is the Common Desktop Environment
• X-Windows include IBM AIX, SUN Solaris
• KDE is the K Desktop Environment
• GNOME is a common desktop environment