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OS

Hardware => Operating system => Application

 OS Provide
- Hardware Abstraction
- Resource management

Abstraction: B/W Application and hardware

- Easy program writing
- Reusable functionality
- Portable

Resource management
- Multiple app but limited resource
- Manage CPU, Memory, Network, Secondary Memory etc
  ---------------

CPU
Each have a unique address

- Memory addrs
- IO Addrs
- Memory Mapped IO addrs

Memory addrs
32 bit process
Range: 0- 2^32-1
 ----------------
Process
Stack
Heap
Data
Text

System call use to access resources of kernel mode
 ------------------

Sharing the CPU - Multi-programming and multi-tasking - Sharing in multi-processor environments * Race conditions and synchronization * Scheduling * Isolation of the OS and user programs * Security mechanisms * Access control * Security assessment

-------------------
CPU Vs RAM
- RAM hold data CPU compute on data

Memory Management
- Fragmentation: Under utilization
    -  Finding Right fit (First fit, Right fit)
- Deallocation: Merge smaller block when process free memory
Modern OS manage above using
- Virtual memory
- Segmentation
-----------
Virtual Memory (Memory Management technique)
- RAM split into fixed size partition called frames
- Page frames typically 4KBs

Split Process into equal block size
Block size = page frame size

Allocate block into page frame

Page table: to maintain map of page and block

- Need not to be contiguous
- Need to lookup page table to access memory

TLV Caches 
- Process block are on user space
- Page table are in kernel space

Demand paging:?
Page fault trap?

Replacement polices
- First in first out
- Last recently used
- least frequent useed

Swap in & swap out

Dirty Bit
Present BIT
 protection Bit

PAGE TABLE
block | Page frame | P | D  | protection Bit

MMU and Virtual memory mapping

---------------------------
Segmentation
Segment Descriptor table
------------------------
Process: program in execution


Process table
block | page frame

Kernel metadata

- Kernel stack
- user stack

PID: process unique id

process state
- new
- ready
- Block
- running

Queue of ready process

Process tree

init.d

Zombies: process terminated but PCB exist

Orphan: parent exit but child running
- Intentional
- daemons

 ELF executable
----------------------------
Event
- Interrupt
- Trap
- Exception

IDTR: Interrupt Descriptor table

CPU Context switching
Multitasking
 -------------------------------
CPU Scheduler
- FCFS (First come first serve)
     - The Convoy Effect
Simple
Fair: all job execute

Waiting time depend
short process stuck

2. SJF (Shortest job first)
- Min average waiting time
Not predictable
starvation

3. SRTF

4. Round robin
Data struct: FIFO (Queue)

Critical process:

Priority based scheduling
starvation

Multilevel queue
------------
Multiprocessor scheduling
 - Global queue
- Each cpu has own queue

Load balance
- Pull migration
- Push migration
------------------------------
Inter process communication
- Shared memory

Signal
- Async unidirectional

Synchronization

Race condition
- Critical section

Critical section
- Mutual exec
- Progress
- No starvation

Locks
Unlock

Critical section solution

Deadlock

Bakery Algorithm
Synchronization N > 2 process
Use to solve critical section problem

Hardware lock
spinlock

Mutexes
Thundering herd problem
priority inversion
priority inheritance

Semaphore
Producer-Consumer problem

             Buffer
P                              c

Dinning philosopher problem

Deadlock
Handle deadlock

Thread
Process can have multiple thread
Thread can have its own stack section, register
Light weight
Efficient communication b/w entities
Efficient context switching

pthread lib

User thread
Kernel thread

Thread pool

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