What we will implement

Before starting to build an OS, let's quickly get an overview of the features we will implement.

Features in 1K LoC OS

In this book, we will implement the following major features:

• Multitasking: Switch between processes to allow multiple applications to share the CPU.
• Exception handler: Handle events requiring OS intervention, such as illegal instructions.
• Paging: Provide an isolated memory address space for each application.
• System calls: Allow applications to call kernel features.
• Device drivers: Abstract hardware functionalities, such as disk read/write.
• File system: Manage files on disk.
• Command-line shell: User interface for humans.

Features not implemented

The following major features are not implemented in this book:

• Interrupt handling: Instead, we will use a polling method (periodically check for new data on devices), also known as busy waiting.
• Timer processing: Preemptive multitasking is not implemented. We'll use cooperative multitasking, where each process voluntarily yields the CPU.
• Inter-process communication: Features such as pipe, UNIX domain socket, and shared memory are not implemented.
• Multi-processor support: Only single processor is supported.

Source code structure

We'll build from scratch incrementally, and the final file structure will look like this:

├── disk/     - File system contents
├── common.c  - Kernel/user common library: printf, memset, ...
├── common.h  - Kernel/user common library: definitions of structs and constants
├── kernel.c  - Kernel: process management, system calls, device drivers, file system
├── kernel.h  - Kernel: definitions of structs and constants
├── kernel.ld - Kernel: linker script (memory layout definition)
├── shell.c   - Command-line shell
├── user.c    - User library: functions for system calls
├── user.h    - User library: definitions of structs and constants
├── user.ld   - User: linker script (memory layout definition)
└── run.sh    - Build script

TIP

In this book, "user land" is sometimes abbreviated as "user". Consider it as "applications", and do not confuse it with "user account"!