Authors

Marc-Antoine Nadeau
Karl Bridi

A lightweight simulation of a basic operating system shell written in C. It supports command execution, process scheduling, and memory management through demand paging and LRU replacement.

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Features

• Interactive and batch shell modes
• Built-in commands: help, quit, set, print, source, echo
• File and directory utilities: my_ls, my_mkdir, my_touch, my_cd
• One-liner command chaining with ;
• run command for launching external programs using fork-exec-wait
• exec command to run up to 3 concurrent scripts with scheduling
• Supported scheduling policies:
  • FCFS – First Come First Serve
  • SJF – Shortest Job First
  • RR – Round Robin (time slice: 2)
  • AGING – SJF with aging to prevent starvation
  • RR30 – Extended time slice round-robin (30 instructions)
• Background execution with exec ... POLICY #
• Demand paging with 3-line page size
• LRU (Least Recently Used) page replacement policy
• Shared pages between processes executing the same program
• Compile-time configuration of memory limits

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Compilation

You can configure memory size at compile time:

make mysh framesize=12 varmemsize=20

On startup, the shell will display:

Frame Store Size = 12; Variable Store Size = 20

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Usage

Run Shell

• Interactive mode:

  ./mysh
  

• Batch mode:

  ./mysh < script.txt
  

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Example Commands

set x 42
print x
echo $x

my_mkdir demo
my_touch file1
my_ls
my_cd demo

run cat file1.txt
exec prog1 prog2 prog3 RR
exec prog1 RR #

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Paging & Memory Management

• Only first two pages of each program (3 lines per page) are initially loaded.
• Additional pages are loaded on-demand during execution (page faults).
• When memory is full, the least recently used page is evicted.
• Page fault messages and evicted page contents are printed to the terminal.