1 Overview and Structure of UNIX.md

The overview of Unix System:

Nothing is better than a birds eye-view 🦅

The Story :

• GE and MIT joined together to develop multics, But it failed.
• Ken Thompson and Dennis Ritchie drew the plan for a new File system.
• Thompson programmed a kernel for it on a GE467 machine.
• Brian Kernighan joined them.
• They created the first version of unix and ran it on PDP-7.
• They moved it to PDP-11.
• Thompson tried to implement the system on Fortran, but created B.
• B was slow, so Ritchie created C.
• The Unix was rewritten in C in 1973
• They published paper in 1974.
• More than 500 Unix websites in 1977.
• They created an amalgamation of all the version as the Unix - III
• Unix - V was created From UNIX - III

Why the hell Unix was popular? 🤔

• Written using C
• A good file system
• Everything in UNIX is treated as a file - Simplicity
• Fast processing speed.
• At the same time:
  • Multiple users can use it
  • Multiple process can run on it.

System Structure:

• UNIX abstracts the hardware to programs.
• Program use system calls to communicate.
• System Calls are also packaged into commands.
• 32/64 System calls are used.
• also a CC which gives a.out file.

👨Users Perspective on:

1. The File System:

• Tree like Structure
• Root - "/"
• path name contains name and /.
• Traverse Using the cd command
• . : current directory
• .. : parent directory
• directory is also an file, but Special.
• all internal nodes are directories
• External nodes (Leaf Nodes) can be a:
  • Directory File
  • Actual File
  • Device File
• cp used to copy files

2. Processing:

• Programs converted into process executing some task.
• Many such process can run at the same time.
• There are calls to create, kill and synchronize them.
• The directory in which a process run is its environment.

3. Shell

• Shell is an program that can execute commands.

• Commands are system calls packaged together.

• This package could be:

  • .exe
  • .sh

• The command is run by the syntax:

    [CommandName] [Options] [Parameters]
  

• When a command is run, the shell forks and exec the command's program file.

• They could be run synchronously or asynchronously to the shell process.

• Outputs of the commands can be:

  1. redirected (> and <) to files
  2. piped (|) between commands

🖥️ UNIX Perspective on:

1. The File System:

• Free Space management
• File Protection

2. Processing:

• Powers of UNIX upon Processes:
  • Create
  • Run
  • Sleep
  • Suspend
  • Kill
• The processes in main memory and Secondary Memory managed by UNIX.
• Schedule their execution.

3. Shell:

• Some UNIX powers given to shell

💾 Hardwares Perspective on:

1) Modes of execution:

• The hardware must follow the two modes of execution.
  • User
  • Kernel
• The process in User mode upgrades to Kernel mode when upon System Call.
• After Execution, they return to User Mode.

2) Interrupt:

• The hardware has devices that may interrupt the UNIX.
• The UNIX pauses current process, services Interrupt and resumes the Process execution.

3) Exception:

• Process executes instructions run by the hardware.
• Sometime they put the machine into a state of confusion, called Exception.
• They are handled by UNIX.

4) Execution Levels:

• Interruption of important processes cannot be tolerated.
• Hence Processes are given levels to avoid critical process be stopped.

5) Memory Management:

• Hardware must allow to storage of processes in Main memory and in Secondary Memory.

The Kernel 🌽

The Structure:

• Three layer architecture.
• communication from User to Kernel level through system calls
  • called by library function
  • called directly
• system call interact with:
  • File system using:
    • open
    • close
    • read
    • write
    • chown
    • chmod
  • Process system using:
    • fork
    • exec
    • exit
    • brk?
    • signal?
  • Hardware implicitly using:
    • File system
    • Process system
• These interactions are based on :
  • File system:
    • Buffer Cache
    • Device Drivers
  • Process System:
    • Scheduling
    • Memory Management
    • IPC
  • Hardware:
    • Interrupt handling

System Concepts:

1) 📁 The File Subsystem:

Perspective of Main Memory:

• There are three structures

  • Inode table
  • File Table
  • User File Descriptor Table

• The disk Inode structure is loaded into main memory.

• Inode table contains pointer to these in-memory copies.

• The File table contains pointer to inodes.

• Descriptor contains pointer to file table entries.

• Processes can use this table to access and modify files.

• NOTE: The root inode ("/") of filesystem is placed into memory when mounting.

Perspective of Disk:

• The disk can be divided into many file systems.
• Each filesystem contain:
• Boot block contain code to load the OS into memory.
• Not all file system has this boot block.
• Super block contains free space locations.
• Inode List for files.
• Data block of files.

2) 🏃‍♂️ Process Subsystem:

Process's Perspective:

Process Necessities:

• Process needs memory for :
  • Text - Machine code
  • Data - to store variable
  • Stack - for function call
• Regions of memory given to the process, to store them.

Relation between process:

• All process in the memory come from forking and exec.
• A process calling fork is parent and the new process is child.
• The exception to this is the process 0

Modes of Process:

• To avoid process in user mode gaining kernel level information, seperate stacks for each mode.

  Stack Frame
  Parameters
  Return Address
  Local Variables
  Address of previous Frame

Memory's Perspective:

Data Structures Involved:

• Four Data Structures:
  • Region Table
  • pregion Table
  • Process Table
  • U area

• Regions are block of memory.

• Region can be:

  • Text
  • Data
  • Stack

• Region Table has region's location and type.

• Pregion Table points to region table.

• process table points to pregion table.

• U area points to process table

• U area also contains information of process such as:

  • Process size
  • Pointer to File Descriptor
  • Current Directory
  • Current Root

• Process Table contains:

  • State Field
  • UID
  • Event Descriptor for suspension. NOTE: Root changes based on file system

Context of Process:

All the activity of process:

• values in CPU register (PC, IR, PS,...)
• u area of the process
• user and kernel stack
• global variables

Process States

• Ready
• Executing in Kernel Mode
• Executing in User Mode
• Sleep

Process Transistions

• The diagram is self-explanatory:

Sleep and wakeup

• Process put to sleep when process needs a event to ocuur.
• If event occurs then the process is woke up.

3) Kernel Data structure:

• All of the before said data structures are fixed size.
• There is a possiblility running out of entries.
• UNIX does not allow this problem, thanks to the way it's structured.

4) Administration With UNIX:

• Administration include:

  • creation of user and groups for different people.
  • Installation of programs.
  • File Permissions
  • Disk formatting

  etc...

• From UNIX perspective, they are user processes.

• A Super-User has priveleges to do these tasks.