yapOS

An OS simulator project to help me learn about the shell interface and basic OS functionalities like process management, memory management, and file system operations. It demonstrates core systems programming concepts including:

• Variable storage and key-value substitution
• Round-robin process scheduling with paging
• Demand-paged memory management with LRU eviction
• A persistent, FAT-style virtual file system
• Script execution, command parsing, and a shell REPL

The design mimics a simplified OS kernel layered over a block device simulator (blank.dsk).

Architecture Overview

Layer	Files	Responsibilities
Shell & Interpreter	shell.c, interpreter.c	Tokenize user input, dispatch commands, stage scripts.
Kernel	kernel.c	Handle ready queue, scheduling, paging, and file operations.
CPU Emulator	cpu.c	Fetch and execute script lines per time slice.
PCB & Scheduling	pcb.c, linked_list.c	Maintain PCBs with program counters, file tables, and page metadata.
Shell Memory	shellmemory.c	Simple variable store with frame allocation.
File System	blank.dsk, kernel.c	Simulate block-based file storage using a FAT and sector map.

Backing Store & Paging

When scripts are loaded via run or exec, they are copied into a temporary backing_store/ directory.
Each script becomes a private file used for demand paging. The system supports a small fixed number of memory frames and uses 3-line pages with LRU replacement. This structure mimics real-world virtual memory backed by secondary storage.

Requirements

• GCC compiler
• Make build system

Building

To build the project, use the make command in the root directory:

make

This will compile all the necessary components and create the myshell executable.

Running

To run yapOS, use the following command:

./myshell blank.dsk

The blank.dsk file serves as the disk image for the operating system.

Configuration

The build process supports the following optional parameters:

• framesize: Size of the frame store (default: 21)
• varmemsize: Size of the variable memory store (default: 10)

You can specify these parameters when running make:

make framesize=30 varmemsize=15

Cleaning

To clean the build artifacts:

make clean

Command Reference

Command	Syntax	Description
help	help	Lists all available commands.
quit, exit	quit / exit	Cleanly shuts down the shell and syncs the disk.
set	set <var> <value...>	Stores a variable with the given value (spaces included).
print	print <var>	Prints the value of a variable.
echo	echo <text> / echo $<var>	Prints text or a substituted variable.
resetmem	resetmem	Clears all stored variables.
run	run <script.txt>	Loads and executes a script sequentially.
exec	exec <s1> [s2] [s3] [s4]	Loads and schedules multiple scripts in round-robin.
ls	ls	Lists files in the virtual root directory.
cat	cat <file>	Prints contents of a file from the beginning.
create	create <file> <bytes>	Creates a new file of given size.
write	write <file> <data...>	Writes data at the current file pointer.
read	read <file> <n>	Reads n bytes from current pointer.
seek	seek <file> <offset>	Sets the current pointer position.
size	size <file>	Shows the file size in bytes.
rm	rm <file>	Deletes a file and frees its blocks.
find_file	find_file <substring...>	Searches for filenames containing the substring.
copy_in	copy_in <host_path>	Imports a file from the host system.
copy_out	copy_out <file>	Exports a virtual file to the host system.
freespace	freespace	Shows free sectors and total free space.
fragmentation_degree	fragmentation_degree	Calculates fragmentation for each file.
defragment	defragment	Rewrites files to be stored contiguously.
recover	recover <0 or 1>	Attempts file system repair (0 = thorough, 1 = fast).

Unknown or malformed commands return standardized error messages prefixed with Bad command:.