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Miscellenia

As-of-yet unorganized information from lecture.

Deadlock (?)

Example: Terminating a stuck process

Suppose we have a process that is infinitely looping.

To prevent a deadlock, the computer can set a timer to interrupt the computer after some time $n$.

  • Basically, have a counter decremented by the physical clock, and generate an interrupt if it hits 0.
    • (The OS sets the counter via priviledged instruction)

Remember: One of the goals is to distribute resources equally.

Services

We want system calls for stuff like

User Interfaces

User Interface: Almost all OSes have a user interface (UI)

Program Execution: System must be able to load a program into memory, run that program, and end (normally or abnormally).

I/O: A running program may require I/O, which may involve a file or an I/O device.

II.

File System Manipulation: Read/write, create/delete, ls, permissions.

Communication: Exchange info on same computer or between computers.

  • Two major ways: Shared memory, and message passing (packets).

Error Detection: OS should take appropriate action to ensure corret and consistent computing

  • Memory hardware, I/O devices, user programs.
  • Debugging can greatly enhance user and programmer abilities to use the system.

Resource Allocation

Functions for resource sharing.

Resource Sharing: When multiple users/jobs run concurrently.

  • e.g., sharing CPU, main memory, storage, I/O devices

Accounting: Keep track of which users use how much and what kinds of resources.

Protection and Security:

  • Protection: Access to resources controlled.
  • Security: Requires user authentication, extends to external I/O as well.

CLI

Command Interpreter: Allows direct command entry.

  • Outmost layer of the OS.
  • Flavors: Shells
  • Sometimes implemented in kernel.

Example: Built-in and not built-in commands

  1. Some commands may be implemented in the OS, translating straight into system calls.
write(...)
	# implementation here
read(...)
	# implementation here
  1. (More popularly), we modularize the definition, and the OS only provides an interface.
write(...)
	# interface
read(...)
	# interface

GUI

You know what a GUI is.

  • A metaphor for stuff, icons, mouse, windows, et cetera.

System Call Implementation

TODO Add this to previous

System calls can be:

  1. defined in the main body of the kernel, or
  2. loaded in main memory

Typically a number is associated with each system call.

  • The indexing in maintained by the system-call interface.

System-Call Interface:

  • Invokes intended system call in kernel and returns status and any return values.
  • Caller know nothing about implementation. Details are hidden.
  • Caller only needs to obey API.