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Motivation

Most development projects in operating systems for learning are based on UNIX. The idea is to take a different path and explore new perspectives.

The architecture chosen for this iteration of the project was x86 (32-bit), for the following reasons:

• Easier to find documentation and examples;
• The version of the Oberon compiler I am using only supports the generation of 32-bit objects.

I will use the terms "OS" or "system" to refer to "operating system."

Main goals

In this first iteration, our main goals are:

1. Understand how to interact with hardware
2. Understand how an OS is structured
3. Test the Oberon language in low-level development
4. Test how the Oberon compiler works using components written in different languages (C and Assembly)

Some of the implemented features are incomplete. The implementation focused only on testing different aspects and techniques.

x86 Architecture

Boot

The kernel is compiled with support for Multiboot, so it is possible to use a bootloader such as Grub.

The implementation uses a part written in Assembly (boot.asm).

Runtime

The system uses a combination of interrupts, coroutines, and message passing to perform the different tasks:

1. Drawing the user interface
2. Moving the mouse pointer
3. Processing keyboard input
4. Receiving/sending data via the serial port

Interrupts

The x86 architecture uses an interrupt table where a function is registered and will be invoked when an interrupt occurs. In "interrupts.asm" this table is populated and all calls are redirected to the kernelInterruptHandler function, implemented in "Kernel.ob07".

Message passing

Each task can subscribe to a specific interruption. Thus, when that interruption occurs, the task will receive a message in its "inbox." Messages are dispatched by a main task.

Tasks and coroutines

The system has a main task that runs in a loop and waits for incoming messages. These messages are sent when timer, keyboard, and mouse interrupts occur.

For each message received, the loop checks what type of message it is and performs the corresponding action:

1. KBD_MESSAGE: processes keyboard inputs.
2. MOUSE_MESSAGE: processes mouse input and movement.
3. TICK_MESSAGE: processes timer events. Switches between the coroutines responsible for drawing the screen and updating the mouse position.

Lessons learned

• Difficulty using a high-level language (Oberon) for low-level manipulation;
• x86 architecture requires a lot of backward compatibility;
• Use of components developed in more than one language worked satisfactorily;
• Lack of a use case to guide system modeling.

Download

The project files can be downloaded here.

Instructions on how to compile and run the OS can be found in the README.md document at the root of the package.

References

1. Multiboot specification
2. pcboot
3. Grub bootable disk
4. ODDev Demo
5. Baremetal examples