Operating Systems Cheat Sheet

OS Functions

Function	Description	Example
Process Management	Manage processes and threads	Process creation, scheduling, synchronization
Memory Management	Manage primary memory allocation	Virtual memory, paging, segmentation
File System Management	Manage files and directories	File creation, deletion, access control
I/O Management	Manage input/output devices	Device drivers, buffering, spooling
Security and Protection	Protect system and user data	Access control, authentication, encryption
Networking	Manage network communication	Network protocols, data transmission
Error Detection	Detect and handle system errors	Hardware failure detection, error recovery

Process States

State	Description	Transition Trigger
New	Process being created	Process creation request
Ready	Process waiting for CPU allocation	Admitted by OS, waiting for scheduler
Running	Process currently executing	Scheduled by CPU scheduler
Waiting	Process waiting for I/O or event	Waiting for I/O completion, resource, signal
Terminated	Process finished execution	Process completed or killed
Blocked	Process waiting for resource	Waiting for unavailable resource

CPU Scheduling Algorithms

Algorithm	Strategy	Advantages	Disadvantages	Use Case
FCFS (First-Come, First-Served)	First arrived, first served	Simple, fair	Convoy effect, poor average waiting time	Batch systems
SJF (Shortest Job First)	Shortest job executes first	Minimum average waiting time	Starvation, hard to predict job length	Theoretical optimization
SRTF (Shortest Remaining Time First)	Preemptive version of SJF	Optimal average waiting time	Starvation, overhead	Real-time systems
Round Robin	Fixed time quantum per process	Good response time, fair	Context switching overhead	Time-sharing systems
Priority Scheduling	Higher priority executes first	Flexible, can prioritize	Starvation possible	Real-time systems
Multilevel Queue	Processes in different priority queues	Different scheduling for different types	Potential starvation	Multiple process types
Multilevel Feedback Queue	Dynamic priority adjustment	Prevents starvation, responsive	Complex implementation	General-purpose systems

Memory Management

Concept	Description	Advantages	Disadvantages
Contiguous Allocation	Each process gets contiguous memory block	Simple, efficient	External fragmentation, memory waste
Paging	Divide memory into fixed-size pages	No external fragmentation, efficient	Internal fragmentation, page table overhead
Segmentation	Divide memory into variable-size segments	Natural program structure, sharing possible	External fragmentation, complex
Virtual Memory	Extend physical memory with disk space	Larger logical address space, process isolation	Performance overhead, complexity
Page Replacement	Replace pages when memory is full	Efficient memory utilization	Page fault overhead

Page Replacement Algorithms

Algorithm	Strategy	Advantages	Disadvantages
Optimal (OPT)	Replace page used farthest in future	Theoretical minimum page faults	Impractical, requires future knowledge
FIFO (First In, First Out)	Replace oldest page	Simple implementation	Belady's anomaly possible
LRU (Least Recently Used)	Replace least recently used page	Good performance, stack algorithm	Implementation overhead
LFU (Least Frequently Used)	Replace least frequently used page	Considers frequency	May keep old popular pages
Clock Algorithm	Approximation of LRU using reference bit	Good performance, simple	Approximation of LRU
Second Chance	FIFO with reference bit check	Better than FIFO	Still FIFO-based

Process Synchronization

Concept	Description	Implementation	Use Case
Critical Section	Code segment accessing shared resources	Entry/exit protocols	Shared resource access
Mutex	Mutual exclusion lock	Binary semaphore	Exclusive access to resource
Semaphore	Integer variable for synchronization	Wait and signal operations	Resource counting, synchronization
Monitor	High-level synchronization construct	Condition variables, locks	Structured synchronization
Producer-Consumer	Coordination between processes	Buffer, semaphores	Data sharing between processes
Reader-Writer	Multiple readers, single writer	Read-write locks	Database access, file sharing
Dining Philosophers	Resource allocation problem	Resource ordering, deadlock prevention	Deadlock avoidance

Deadlock Conditions

Condition	Description	How to Handle	Example
Mutual Exclusion	Resource cannot be shared	Resource sharing where possible	Printer cannot be shared
Hold and Wait	Process holds resources while waiting	Request all resources at once	Process holds disk, waits for printer
No Preemption	Resources cannot be forcibly taken	Allow resource preemption	Cannot take resources from process
Circular Wait	Cycle of processes waiting for each other	Resource ordering	P1 waits for P2, P2 waits for P1

File System Concepts

Concept	Description	Advantages	Disadvantages
Contiguous Allocation	File stored in contiguous blocks	Fast access, simple	External fragmentation, difficult to grow
Linked Allocation	File stored as linked list of blocks	No external fragmentation, easy to grow	No random access, pointer overhead
Indexed Allocation	Index block contains pointers to file blocks	Random access, no external fragmentation	Index block overhead
Free Space Management	Track available disk space	Efficient space utilization	Management overhead
Virtual File System	Abstraction layer for different file systems	Multiple file system support	Additional complexity

Security Concepts

Concept	Description	Implementation	Example
Authentication	Verifying user identity	Passwords, biometrics, certificates	Login with username/password
Authorization	Granting access rights	Access control lists, capabilities	File permissions
Access Control	Controlling resource access	Discretionary, mandatory, role-based	File read/write permissions
Encryption	Protecting data confidentiality	Symmetric, asymmetric encryption	File encryption
Auditing	Monitoring security-related events	Log security events	Access logs, system logs

System Calls

Category	System Call	Purpose	Example
Process Control	fork(), exec(), wait(), exit()	Create, execute, wait, terminate processes	Create child process
File Management	open(), read(), write(), close()	File operations	Read from file
Device Management	ioctl(), read(), write()	Device operations	Read from device
Information Maintenance	getpid(), time(), getuid()	Get system information	Get process ID
Communications	pipe(), shmget(), mmap()	Process communication	Create pipe for IPC
Protection	chmod(), umask(), chown()	Set access permissions	Change file permissions

Virtualization Concepts

Concept	Description	Advantages	Disadvantages
Full Virtualization	Complete simulation of hardware	Runs unmodified OS, good isolation	Performance overhead
Para-virtualization	Guest OS modified to work with hypervisor	Better performance than full virtualization	Requires guest OS modification
Hardware-assisted Virtualization	Hardware support for virtualization	Good performance, security	Requires special hardware
Containerization	OS-level virtualization	Lightweight, fast, efficient	Less isolation than full VM