Lecture No. 05

Dated: 28-04-2025

Browsing UNIX / Linux Directory Structure

Displaying Directory Contents

By default, the ls commands takes . as input.

mzaid@MZaid:/mnt/g/Projects/ChuzaWick420$ ls
README.md  assets

mzaid@MZaid:/mnt/g/Projects/ChuzaWick420$ ls -a
.  ..  .git  README.md  assets

mzaid@MZaid:/mnt/g/Projects/ChuzaWick420$ ls -la
total 4
drwxrwxrwx 1 mzaid mzaid  512 Apr 17 18:12 .
drwxrwxrwx 1 mzaid mzaid  512 Apr 20 20:46 ..
drwxrwxrwx 1 mzaid mzaid  512 Apr 17 18:13 .git
-rwxrwxrwx 1 mzaid mzaid 2848 Apr 17 18:12 README.md
drwxrwxrwx 1 mzaid mzaid  512 Nov 20 14:10 assets

The columns represent the following

1. Entry type (d for directory and - for file) alongside permissions.
2. Number of hard links
3. Owner name
4. Group of the owner
5. Size in bytes
6. Month of last modification
7. Day of last modification
8. Time of last modification
9. Entry name

The files starting with . are called dot files or hidden files.

Creating Directories

mkdir directory name

Deleting Directories

rmdir directory name

Changing Directories

cd directory name

Display Absolute Path of Your Working Directory

pwd

Copying Files

cp file_to_copy destination

Examples

Copy READ.md file to directory src.

cp README.md src/

Copy READ.md file to directory src and name the file test.md.

cp README.md src/test.md

Moving Files

mv file_to_move destination

Examples

Move READ.md file to directory src.

mv README.md src/

Move READ.md file to directory src and name the file test.md.

mv README.md src/test.md

Removing Files

rm file_to_remove

Process Concept

The program can be thought of as a passive entity sitting on the disk meanwhile the process is the active entity in execution, associated with resources.

There are 2 types of processes.

1. IO bound processes are those which spend more time doing the IO than the computations, having many but short CPU bursts.
2. CPU bound processes are those which spend more time doing computations, having longer but less CPU bursts.

Process States

New

The process is being created.

Running

Instructions are being executed.

Waiting

The process is waiting for some event to occur, such as an IO operation.

Ready

The process is waiting to be assigned to a processor.

Terminated

The process has finished execution.

Process state diagram

Process Control Block

Each process is represented in the operating system by a process control block, also called a task control block.
It has following pieces

Process State

The state can be

• New
• Ready
• Running
• Waiting
• Halted

Program counter

The counter indicates the address of the next instruction to be executed for this process.

CPU Registers

Depending on the architecture, these may vary in number of type.

• Accumulators
• Index Registers
• Stack Pointers
• General Purpose Registers
• Program Counter

CPU Scheduling Information

This information includes

• Process Priority
• Pointers to Scheduling routines
• Scheduling parameters

Memory Management Information

This includes

• Base registers
• Limit registers
• Page tables
• Segment tables

Accounting Information

This includes

• Amount of CPU
• Real time used
• Time limits
• Account numbers
• Job or process numbers

IO Status Information

This include

• List of IO devices allocated to the process.
• List of open files.

Process Scheduling

Switching the CPU from one process to another requires us to save the context of currently running process.

Context Switching

Scheduling Queues

Job Queue

As processes enter the system, they are put into a job queue.
This queue consists of all processes in the system.

Ready Queue

This is a linked list, with the header containing the pointers to first and last process control block.
Each process control block includes a pointer to point to next one.

Ready Queue

Device Queue

Each IO device has its own device queue which lists the processes waiting for that device to be available for use.

Queuing Diagram

Schedulers

Long Term (job) Scheduler

Responsible for bringing processes from job pool to ready queue.

Short Term (CPU) Scheduler

It selects which processes need to be executed next and allocates CPU.

---

• Short-term scheduler:
  • Runs frequently (\(\approx\) every 100 ms).
  • Must be fast to avoid CPU overhead.
• Long-term scheduler:
  • Runs infrequently (minutes apart).
  • Controls degree of multiprogramming.
  • Balances I/O bound and CPU bound processes for system efficiency.
• Medium-term scheduler (in some systems):
  • Swaps processes between memory and disk (swap out / swap in).
  • Frees memory or improves job mix.
  • Uses swap space to store swapped-out processes.

Computer system queues, servers, and swapping