CS2005 - Lecture 9 - Operating Systems Structures and Processes

Key services provided by an OS

UI: CLI, GUI

Program Execution: Loads and runs programs

I/O Operations: Manages devices

File-System Manipulation: Creates/deletes files, enforces permissions

Communication: Shared memory or message passing between processes

Error Detection: Handles hardware/software errors

Resource Allocation: Manages CPU, memory

Accounting & Security: Tracks usage and enforces access control.

System call

A programming interface to request OS services (e.g. open(), fork()).

Parameters passed to system calls

Via registers, memory blocks or the stack.

Windows system calls

CreateProcess(), ReadFile().

Unix system calls

fork(), read().

Parts of a process

Text: Code + program counter

Stack: Temporary data

Data: Global variables

Heap: Dynamically allocated memory.

Process states

New: Created

Ready: Waiting for CPU

Running: Executing

Waiting: Blocked

Terminated: Finished

Stored in a PCB

Process state, PC, registers, scheduling info, memory limits, I/O status.

Key queues in process scheduling

Job Queue: All processes

Ready Queue: Processes in memory ready to execute

Device Queue: Processes waiting for I/O

Short-term scheduler

Selects next process for CPU (frequent, fast).

Long-term scheduler

Controls multiprogramming by loading processes into memory (infrequent, slow).

Process Lifecycle example

Ready -> Running: Dispatched by short-term scheduler

Running -> Waiting: I/O request or event wait

Waiting -> Ready: I/O completion

Running -> Terminated: Process exits.