OPERATING SYSTEM

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

1. Dual-Mode Operation
2. I/O Protection
3. Memory Protection
4. 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.