32-Bit vs 64-Bit OS

• A 32-bit OS has 32-bit registers, and it can access 2^32 unique memory addresses.
  • i.e., 4GB of physical memory.
• A 64-bit OS has 64-bit registers, and it can access 2^64 unique memory addresses.
  • i.e., 17,179,869,184 GB of physical memory.
• 32-bit CPU architecture can process 32 bits of data & information.
• 64-bit CPU architecture can process 64 bits of data & information.

Advantages of 64-bit over the 32-bit operating system

• Addressable Memory
  • 32-bit CPU -> 2^32 memory addresses, 64-bit CPU -> 2^64 memory addresses
• RAM
  • 32-bit OS can only use 4GB RAM
  • However, upgrade that system with excess RAM to the 64-bit version of Windows, and you’ll notice a difference.
• Performance
  • All calculations take place in the registers.
  • When you’re performing math in your code, operands are loaded from memory into registers.
  • So, having larger registers allow you to perform larger calculations at the same time.
  • 32-bit processor can execute 4 bytes of data in 1 instruction cycle while 64-bit processor can execute 8 bytes of data in 1 instruction cycle.
• Compatibility
  • 64-bit CPU can run both 32-bit and 64-bit OS. While 32-bit CPU can only run 32-bit OS.
• Better Graphics performance
  • 8-bytes graphics calculations make graphics-intensive apps run faster.

Computer Storage

• Computer storage is of two types
  • Primary Memory
  • Secondary Memory
• Internal Memory or Primary Memory
  • This is directly accessible by the processor
  • Main Memory, Cache Memory & CPU registers.
• External Memory or Secondary Memory
  • It is mainly used for the permanent and long-term storage of programs and data

1. Register
   • Smallest unit of storage. It is a part of CPU itself.
   • A register may hold an instruction, a storage address, or any data (such as bit sequence or individual characters).
   • Registers are a type of computer memory used to quickly accept, store, and transfer data and instructions that are being used immediately by the CPU.
2. Cache:
   • Additional memory system that temporarily stores frequently used instructions and data for quicker processing by the CPU.
3. Main Memory
   • RAM
4. Secondary Memory:
   • Storage media, on which computer can store data & programs.
   • Hard Disk, CD, DVD, Pen/Flash drive, SSD, etc

Comparison

1. Cost:
   • Registers is most expensive, then comes cache, then main memory
   • Secondary storages are cheaper than primary.
2. Access Speed:
   • Primary has higher access speed than secondary memory.
   • Registers has highest access speed, then comes cache, then main memory
3. Volatility
   • Primary memory is volatile
   • Secondary memory is non-volatile

Introduction to Process

• Program under execution is called Process
• When a program is loaded into the memory and it becomes a process, it can be divided into four sections ─ stack, heap text and data
• Stack
  • Contains the temporary data such as method/function parameters, return address and local variables.
  • If no Stack area available: Stack overflow
• Heap
  • This is dynamically allocated memory to a process during its run time
  • If no heap area available: Out of memory or Memory insufficient
• Data
  • Contains the global and static variables
• Text/Code
  • Code of the process
  • The value of Program Counter and the contents of the processor’s registers.

How OS creates a process?

• Load the program & static data into memory.
• Allocate runtime stack.
• Heap memory allocation.
• IO tasks.
• OS handoffs control to main ().

Process Table

• All the process are being tracked by OS using a table like data-structure called Process Table
• Each entry in the process table are called Process Control Block

Process Control Block (PCB)

• A PCB is also an table like data-stracture which contains information about the process i.e. registers, quantum, priority, etc

Attributes of a process

• Process-id
  • Process identification: Unique no given to a process
• Program Counter
  • Contains the next instruction to be executed
• Process state
• Priority
• General purpose resisters
  • Store the values of registers used
• List of open files and devices
• Protection

Process Life Cycle(Process States)

• When a process executes, it passes through different states.
• These stages may differ in different operating systems, and the names of these states are also not standardized
• Start/New
  • This is the initial state when a process is first started/created
  • Jobs are stored in Job Queue
• Ready
  • The process is waiting to be assigned to a processor
  • Jobs are sorted and stored in Ready Queue
• Running
  • The processor executes its instructions
• Block or Wait
  • Wating for a resource, during I/O or interupt
  • And at this point jons are stored in Waiting Queue
• Terminate/Completion
  • After Execution is finished process goes to Terminated state

Process Queues

• Job Queue
  • In starting, all the processes get stored in the job queue.
  • Processes are in new/start state
  • It is maintained in the secondary memory.
• Ready Queue
  • Ready queue is maintained in primary memory
  • Processes in Ready state
• Waiting Queue
  • Processes in Wait state.

Schedular

• Job Schedular
  • aka Long term schedular because its seheduling frequency is low, idle time is slightly high.
  • The Job scheduler picks some of the jobs and put them in the primary memory
  • Move processes from: Job Queue(New state)--> Ready Queue(Ready State)
  • LTS controls degree of multi-programming i.e the number of processes in the memory
• CPU Schedular
  • aka Short-term schedular or diapatcher because its seheduling frequency is very, idle time is low.
  • The CPU Schedular picks the job from the ready queue and dispatch to the CPU for the execution.
  • Move processes from: Ready Queue (Ready state)--> Running state
• Medium Term Scheduler
  • Medium-term scheduling is a part of swapping.
  • It removes the processes from the memory.
  • It reduces the degree of multiprogramming.
  • The medium-term scheduler is in-charge of handling the swapped out-processes

Orphan process

• The process whose parent process has been terminated and it is still running.
• Orphan processes are adopted by init process.
• Init is the first process of OS

Zombie process / Defunct process

• A zombie process is a process whose execution is completed but it still has an entry in the process table
• Zombie processes usually occur for child processes, as the parent process still needs to read its child’s exit status.
• Once this is done using the wait system call, the zombie process is eliminated from the process table. This is known as reaping
• It is because parent process maycall wait () on child process for a longer time duration and child process got terminated much earlier.
• As entry in the process table can only be removed, after the parent process reads the exit status of child process.
• Hence, the child process remains a zombie till it is removed from the process table.

Swapping in Operating System

• Swapping is a memory management scheme
• In which any process can be temporarily swapped from main memory to secondary memory
• So that the main memory can be made available for other processes
• Types
  • Swap-in
  • Swap-out

• Swap-out is a method of removing a process from RAM and adding it to the hard disk.
• Swap-in is a method of removing a program from a hard disk and putting it back into the main memory or RAM.
• Swap-out and swap-in is done by Medium Term Scheduler

Context-Switching

• Switching the CPU to another process requires performing a state save of the current process and a state restore of a different process.
• When this occurs, the kernel saves the context of the old process in its PCB and loads the saved context of the new process scheduled to run.
• It is pure overhead, because the system does no useful work while switching.
• Speed varies from machine to machine, depending on the memory speed, the number of registers that must be copied etc.