OperatingSystems

David Goldschmidt, Ph.D.Computer Science

The College of Saint Rose

CIS 432

Operating System Services

An operating system provides services to its users: Program execution

Load programs into memory, run/suspend/halt programs, handle/display errors

I/O operations Seamlessly interact with I/O devices, including

disks, networks connection, etc. Filesystem manipulation

Read/write/traverse filesystem directories,read/write files, enforce permissions, search for files, etc.

Operating System Services

Other services an operating system provides: Inter-Process Communications (IPC)

Processes exchange information via shared memory,message passing, sockets, pipes, files, etc.

Might span multiple computer systems Error detection and recovery

Errors arise in CPU, memory, I/O devices,processes, etc.

Operating system should detect and recover from errors, ensuring correct and consistent operations

Operating System Structure

Using a layered approach, the operating system is divided into N levels or layers Layer 0 is the hardware Layer N is the top-level

user interface Each layer uses functions

and services of lower layers Layer 1 often is the kernel

Operating System Structure

Operating System Kernel

The core program running in anoperating system is the kernel When a computer is switched on,

a bootstrap program executes from ROM Bootstrap program initializes all aspects of

the system, then loads the operating system kernel and starts its execution

User and Kernel Modes

Dual-mode operation provides a user mode anda kernel mode of operation Allows the operating system to protect

itself and other system components

User and Kernel Modes

Kernel relinquishes control to a user process,but may set a timer to ensure a process does not run beyond its allotted time Avoid infinite loops, memory leaks,

memory hogs, etc. Not entirely effective in practice?

How many times have you fought to stop a runaway process?

Kernel also provides securityvia privileged instructions

System Calls via APIs

Operating system services available to usersvia GUI and CLI, which are just software programs System calls are made via programming interface

called an Application Program Interface (API) Common operating system APIs:

Win32 API for Windows POSIX API for POSIX-based systems,

including UNIX, Linux, Mac OS X Java API for Java Virtual Machine C++ Standard Library

System Calls via APIs

Types of system calls include: Process control (e.g. start/suspend/stop a

process) Debugging information, too

File management Device management Information retrieval and maintenance

Current date/time, number of current users, OS version, amount of free memory, process information, etc.

Communications (e.g. IPC, network)

System Calls via APIs

System call sequence for a file copy program:

System Calls via APIs

An API successfully hides the implementation detailsof the operating system Programmers just need to abide by

the API specifications Change the operating system,

but maintain the API such thatit is backwards compatible Deprecate old behavior over

long period of time Why?

hey, the dude abides, man

System Calls via APIs

System Calls via APIs

Example using theprintf() functionfrom C

One API may callanother, which maycall another, etc.

System Calls via APIs

Pass parameters to the operating systemvia registers or a block of data in memory