An operating system (OS) is a software that manages computer resources and provides programmers/users with an interface used to access those resources. An operating system processes system data and user input, and responds by allocating and managing tasks and internal system resources as a service to users and programs of the system. An operating system performs basic tasks such as controlling and memory, prioritizing system requests, controlling input/output devices, facilitating computer networking and managing files.
Hardware Protection
- Dual-Mode Operation
- I/O Protection
- Memory Protection
- CPU Protection
Dual-Mode Operation
Sharing system resources requires operating system to ensure that an incorrect program cannot cause other programs to execute incorrectly.
Provide hardware support to differentiate between at least two modes of operations.
1. User mode – execution done on behalf of a user.
2. Monitor mode (also kernel mode or system mode) – execution done on behalf of operating system.
Mode bit added to computer hardware to indicate the current mode: monitor (0) or user (1).
When an interrupt or fault occurs hardware switches to monitor mode.
Privileged instructions can be issued only in monitor mode.
monitor
user
Interrupt/fault
set user mode
I/O Protection
All I/O instructions are privileged instructions.
Must ensure that a user program could never gain control of the computer in monitor mode (I.e., a user program that, as part of its execution, stores a new address in the interrupt vector).
Use of A System Call to Perform I/O
Memory Protection
Must provide memory protection at least for the interrupt vector and the interrupt service routines.
In order to have memory protection, add two registers that determine the range of legal addresses a program may access:
Base register – holds the smallest legal physical memory address.
Limit register – contains the size of the range
Memory outside the defined range is protected.
Hardware Address Protection
Hardware Protection
When executing in monitor mode, the operating system has unrestricted access to both monitor and user’s memory.
The load instructions for the base and limit registers are privileged instructions.
CPU Protection
Timer – interrupts computer after specified period to ensure operating system maintains control.
Timer is decremented every clock tick.
When timer reaches the value 0, an interrupt occurs.
Timer commonly used to implement time sharing.
Time also used to compute the current time.
Load-timer is a privileged instruction.
Storage Hierarchy
Storage systems organized in hierarchy.
Speed
Cost
Volatility
Caching – copying information into faster storage system; main memory can be viewed as a last cache for secondary storage.
Caching
Use of high-speed memory to hold recently-accessed data.
Requires a cache management policy.
Caching introduces another level in storage hierarchy. This requires data that is simultaneously stored in more than one level to be consistent.
Storage Structure
Main memory – only large storage media that the CPU can access directly.
Secondary storage – extension of main memory that provides large nonvolatile storage capacity.
Magnetic disks – rigid metal or glass platters covered with magnetic recording material
Disk surface is logically divided into tracks, which are subdivided into sectors.
The disk controller determines the logical interaction between the device and the computer.
I/O Structure
After I/O starts, control returns to user program only upon 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.
After I/O starts, control returns to user program without waiting for I/O completion.
System call – request to the operating system to allow user to wait for I/O completion.
Device-status table contains entry for each I/O device indicating its type, address, and state.
Operating system indexes into I/O device table to determine device status and to modify table entry to include interrupt.
Interrupt Handling
The operating system preserves the state of the CPU by storing registers and the program counter.
Determines which type of interrupt has occurred:
polling
vectored interrupt system
Separate segments of code determine what action should be taken for each type of interrupt
Common Functions of Interrupts
Interrupt transfers control to the interrupt service routine generally, through the interrupt vector, which contains the addresses of all the service routines.
Interrupt architecture must save the address of the interrupted instruction.
Incoming interrupts are disabled while another interrupt is being processed to prevent a lost interrupt.
A trap is a software-generated interrupt caused either by an error or a user request.
An operating system is interrupt driven.
Computer-System Operation
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.
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.
Distributed Systems
Distribute the computation among several physical processors.
Loosely coupled system – each processor has its own local memory; processors communicate with one another through various communications lines, such as high-speed buses or telephone lines.
Advantages of distributed systems.
Resources Sharing
Computation speed up – load sharing
Reliability
Communications
Real-Time Systems
Often used as a control device in a dedicated application such as controlling scientific experiments, medical imaging systems, industrial control systems, and some display systems.
Well-defined fixed-time constraints.
Hard real-time system.
Secondary storage limited or absent, data stored in short-term memory, or read-only memory (ROM)
Conflicts with time-sharing systems, not supported by general-purpose operating systems.
Soft real-time system
Limited utility in industrial control or robotics
Useful in applications (multimedia, virtual reality) requiring advanced operating-system features.
Parallel Systems
Multiprocessor systems with more than one CPU in close communication.
Tightly coupled system – processors share memory and a clock; communication usually takes place through the shared memory.
Advantages of parallel system:
Increased throughput
Economical
Increased reliability
graceful degradation
fail-soft systems
Time-Sharing Systems
The CPU is multiplexed among several jobs that are kept in memory and on disk (the CPU is allocated to a job only if the job is in memory).
A job is swapped in and out of memory to the disk.
On-line communication between the user and the system is provided; when the operating system finishes the execution of one command, it seeks the next “control statement” not from a card reader, but rather from the user’s keyboard.
On-line system must be available for users to access data and code.
Multiprogrammed Batch Systems
Several jobs are kept in main memory at the same time, and the
CPU is multiplexed among them.If memory can hold several programs, then CPU can switch to another one whenever a program is awaiting for an I/O to completeThis is multiprogramming.
SIMPLE BATCH SYSTEM
- Use of high-level languages, magnetic tapes.
- Jobs are batched together by type of languages.
- An operator was hired to perform the repetitive tasks of loading jobs, starting the computer, and collecting the output (Operator-driven Shop).
- It was not feasible for users to inspect memory or patch programs directly.
Operation of Simple Batch Systems :
- The user submits a job (written on cards or tape) to a computer operator.
- The computer operator place a batch of several jobs on an input device.
- A special program, the monitor, manages the execution of each program in the batch.
- Monitor utilities are loaded when needed.
- Resident monitor is always in main memory and available for execution.
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