Operating Systems Services Overview

Operating System Services

• Operating systems provide an environment for programs and users
• Helpful services are:
  • User Interface (UI): Command-line (CLI), Graphical User Interface (GUI), touch-screen, batch
  • Program execution: loading programs into memory, running, ending execution (normal or abnormal)
  • I/O operations: reading and writing to files or devices
  • File-system manipulation: creating, deleting, reading, writing, listing files and directories
• Other user-helpful Operating System services:
  • Communications: processes exchange information (same computer, network) via shared memory or message passing
  • Error detection: OS constantly monitors for errors (CPU, memory, I/O devices, user programs)
  • Debugging facilities: improve user and programmer ability to use the system efficiently

System Services

• Resource Allocation: allocating resources (CPU cycles, memory, storage, I/O devices) to multiple users or jobs running concurrently.
• Logging: keeping track of user resource usage.
• Protection and security: controlling access to information and resources, authenticating users, and protecting external I/O devices from invalid access attempts.

System Calls

• Programming interface to OS services
• Typically written in high-level languages (C or C++)
• Mostly accessed by programs via Application Programming Interface (API) instead of direct system calls
• Common APIs:
  • Win32 API (Windows)
  • POSIX API (UNIX, Linux, Mac OS X)
  • Java API (Java Virtual Machine - JVM)

System Call Implementation

• Typically each system call has a unique number for indexing by the system call interface
• System call interface makes a table of the numbers to the intended system calls in the OS kernel.
• The caller needs to know the API and what OS does for the result
• Managed by run-time support library (set of functions built into libraries included with the compiler)

System Call Parameter Passing

• Often more information is required than just identifying the desired system call
• There are 3 general methods to pass parameters to the OS:
  • Simplest approach: parameters passed in registers
  • If too may parameters, they are passed in a block, or table in memory
  • Parameters are placed onto the stack by the program and popped by the OS

Types of System Calls

• Process control: create, terminate, load, execute, get/set attributes, wait for time, wait for event, signal event, allocate/free memory, debugging tools, locks
• File management: create, delete, open, close, read, write, reposition, get/set attributes
• Device management: request, release device, read, write, reposition, get/set attributes, attach/detach devices.
• Information maintenance: get/set time, date, system data, process, file, or device attributes.
• Communications: creating connections, sending/receiving messages, naming processes. shared-memory model.
• Protection: control access to resource, get/set permissions, allow/deny user access.

Linkers and Loaders

• Source code is compiled into object files to be loaded into any physical memory location.
• Linker combines object files into a single binary executable.
• Loader brings the binary executable into memory to execute.
• Relocator: adjusts addresses in the code and data within the program to match the final memory location.
• Modern systems use dynamically linked libraries (like DLLs in Windows) to avoid linking libraries into the executable; libraries are loaded as needed.

System Programs

• Provide a convenient environment for program development and execution
• File manipulation
• Programming language support
• Program loading and execution
• Communications
• Background services
• Application programs