What is an OS?
An Operating System is a program that acts as an intermediary between a user of a computer and the computer hardware. It provides a user-friendly environment in which a user may easily develop and execute programs. Otherwise, hardware knowledge would be mandatory for computer programming.
“The one program running at all times on the computer” is the kernel
- The common functions of controlling and allocating resources are then brought together into one piece of software: OS.
- Everything else is either
- a system program (ships with the operating system) , or
- an application program.
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
- Manage resources such as
- Memory
- Processor(s)
- I/O Devices
- Depends on the point of view user and system
- Users want convenience, ease of use and good performance → don’t care about resource utilization
- But shared computer such as mainframe or minicomputer must keep all users happy → Maximize resource utilization.
- Available CPU time, memory, and I/O are used efficiently and that no individual user takes more than her fair share.
Users of dedicated systems such as workstations have dedicated resources but frequently use shared resources from servers.
- Resources like file, compute, and print servers are shared.
- Operating system is designed to compromise between individual usability and resource utilization
- Handheld computers are resource poor, optimized for usability and battery life.
- Some computers have little or no user interface, such as embedded computers in devices and automobiles
Four Components of a Computer System
- 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 users. Word processors, compilers, web browsers, database systems, video games
- Users: People, machines, other computers
System View
- 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
Computer System Operation
- One or more CPUs, device controllers connect through common bus providing access to shared memory
- The CPU and the device controllers can execute oncurrently, competing for memory cycles.
- Memory controller is provided to synchronize access to the memory.
- 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.
- Each device controller has registers for action (like “read character from keyboard”) to take.
- 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
First Boot
When the system is booted, the first program that starts running is a Bootstrap.
- It is stored in read-only memory (ROM) or electrically erasable programmable read-only memory (EEPROM).
- Bootstrap is known by the general term firmware, within the computer hardware.
- It initializes all aspects of the system, from CPU registers to device controllers to memory contents.
- The bootstrap program must know how to load the operating system and how to start executing that system.
- The bootstrap program must locate and load into memory the operating system kernel.
- The first program that is created is init, after the OS is booted. It waits for the occurrence of events.
Trap, Exceptions, Interrupts
Timeline for single running process
- The OS is interrupt driven.
- The interrupt architecture must save the address of the interrupted instruction
- The user can signal a software-generated interrupt, called a trap
| Polling | Vectored |
|---|---|
| CPU constantly checks a set of devices, predefined locations to determine if an interrupt has occurred. | The interrupting device directly informs the CPU via an interrupt |
| CPU will execute an ISR | An ISR will execute |
| Easy to implement but is less efficient | Less memory checks, and allows interrupt passing |
| Volatile | Non Volatile |
|---|---|
| Data is lost when power is switched off | Data persists when power is switched off |
| Cache Memory | HDD, Flash, SSD |
| Registers | ROM, Bootstrap EEPROM |
Von neuman Architecture
Fetch: The CPU fetches or retrieves the next instruction from the memory. The address of the next instruction is stored in the PC.
Decode: Once the instruction is fetched, the CPU decodes it to understand what action is required, this interpretation is done by the Control Unit.
Execute: Based on the opcode and operand. ALU and other functional units are used in this step.
This cycle is repeated until the program terminates.
Storage Device Hierarchy
Tip
Caching: copying information into a faster storage system. In relative terms: main memory can be viewed as cache for secondary storage.
Faster storage (cache) checked first to determine if information is there
- If yes? information is used from that cache
- If no? data is copied to cache and used there
Cache size determines how much size can be stored in the cache. Cache is also really expensive.
Caching Strategies
- LRU: Least Recently used
- LFU: Least Frequently used
- FIFO: Evicts the oldest data
- Random: Evicts randomly chosen data
- Optimal: Evicts data that will be not be used in the future.
I/O structure
- A large portion of the OS is dedicated to managing I/O as reliability is a major concern.
- The CPU ←> Device controllers are connected through a common bus.
- A device-driver is used for each device for managing I/O. It provides a uniform interface between kernel and the controller.
SCSI: Small Computer-Systems Interface enables to connect more devices
After I/O starts
Control returns to user program only after 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
A system call allows a user program to request services from the OS such as waiting for I/O requests to get completed.
- System Call: request to OS to allow user to wait for I/O
- Device Status Table: contains entry for each I/O device including its type, address and state.
- Os indexes into I/O device table to determine device status and to modify table entries to include the interrupt.
Direct Memory Access
- Used for high speed I/O devices, 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 one per byte.