MentOS  0.5.2
The Mentoring Operating System
MentOS

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1. What is MentOS

MentOS (Mentoring Operating System) is an open source educational operating system. The goal of MentOS is to provide a project environment that is realistic enough to show how a real Operating System work, yet simple enough that students can understand and modify it in significant ways.

There are so many operating systems, why did we write MentOS? It is true, there are a lot of education operating system, BUT how many of them follow the guideline de fined by Linux?

MentOS aims to have the same Linux's data structures and algorithms. It has a well-documented source code, and you can compile it on your laptop in a few seconds!

If you are a beginner in Operating-System developing, perhaps MentOS is the right operating system to start with.

Parts of MentOS are inherited or inspired by a similar educational operating system called DreamOs written by Ivan Gualandri.

2. Implemented features

Follows the list of implemented features:

Processes and Events

  • [x] Memory protection (User vs Kernel);
  • [x] Processes;
  • [x] Scheduler (synchronous and asynchronous);
  • [x] Interrupts and Exceptions;
  • [x] Signals;
  • [x] Timers and RTC;
  • [x] Wait-queues;
  • [x] System Calls;
  • [ ] Multi-core;

Memory

  • [x] Paging;
  • [x] Buddy System;
  • [x] Slab Allocation;
  • [x] Zone Allocator;
  • [x] Cache Allocator;
  • [x] Heap;
  • [x] Virtual Addressing;

Filesystem

  • [x] Virtual Filesystem (VFS);
  • [x] Initramfs;
  • [x] EXT2;
  • [x] Procfs;

Input/Output

  • [x] Programmable Interrupt Controller (PIC) drivers;
  • [x] Keyboard drivers (IT/ENG layouts);
  • [x] Video drivers;
  • [ ] VGA drivers;

I will try to keep it updated...

3. Prerequisites

MentOS is compatible with the main unix-based operating systems. It has been tested with Ubuntu, WSL1, WSL2, and MacOS.

3.1. Generic Prerequisites

3.1.1. Compile

For compiling the system:

  • nasm
  • gcc
  • make
  • cmake
  • git
  • ccmake (suggested)
  • e2fsprogs (should be already installed)

Under MacOS, for compiling, you have additional dependencies:

  • i386-elf-binutils
  • i386-elf-gcc

3.1.2. Execute

To execute the operating system, you need to install:

  • qemu-system-i386 (or qemu-system-x86)

3.1.3. Debug

For debugging we suggest using:

  • gdb or cgdb

3.2. installation Prerequisites

Under Ubuntu, you can type the following commands:

sudo apt-get update && sudo apt-get upgrade -y
sudo apt-get install -y build-essential git cmake qemu-system-x86 nasm e2fsprogs
sudo apt-get install -y gdb cgdb

Note: Older versions might have qemu-system-i386 instead of qemu-system-x86.

Under MacOS you also need to install the i386-elf cross-compiler. The simplest installation method is through Homebrew package manager. Install Homebrew if you don't already have it, and then type the following commands:

brew update && brew upgrade
brew install i386-elf-binutils i386-elf-gcc git cmake qemu nasm e2fsprogs
brew install gdb cgdb #<- for debug only

4. Compiling MentOS and generating the EXT2 filesystem

Compile MentOS with:

cd <clone_directory>
mkdir build
cd build
cmake ..
make

Then, generate the EXT2 filesystem with:

make filesystem

you just need to generate the filesystem once. If you change a program you need to re-generate the entire filesystem with make filesystem, but this will override any changes you made to the files inside the rootfs.img. In the future I will find a way to update just the /usr/bin directory and the programs.

5. Running MentOS

Boot MentOS with qemu:

make qemu

To login, use one of the usernames listed in files/etc/passwd.

6. Kernel logging

The kernel provides ways of printing logging messages from inside the kernel code to the bash where you executed the make qemu.

These logging functions are:

{C++}
#define pr_emerg(...)
#define pr_alert(...)
#define pr_crit(...)
#define pr_err(...)
#define pr_warning(...)
#define pr_notice(...)
#define pr_info(...)
#define pr_debug(...)
#define pr_default(...)

You use them like you would use a printf:

{C++}
if (fd < 0) {
pr_err("Failed to open file '%s', received file descriptor %d.\n", filename, fd);
return 1;
}

By default only message that goes from pr_notice included down to pr_emerg are displayed.

Each logging function (they are actually macros) is a wrapper that automatically sets the desired log level. Each log level is identified by a number, and declared as follows:

{C++}
#define LOGLEVEL_DEFAULT (-1) ///< default-level messages.
#define LOGLEVEL_EMERG 0 ///< system is unusable.
#define LOGLEVEL_ALERT 1 ///< action must be taken immediately.
#define LOGLEVEL_CRIT 2 ///< critical conditions.
#define LOGLEVEL_ERR 3 ///< error conditions.
#define LOGLEVEL_WARNING 4 ///< warning conditions.
#define LOGLEVEL_NOTICE 5 ///< normal but significant condition.
#define LOGLEVEL_INFO 6 ///< informational.
#define LOGLEVEL_DEBUG 7 ///< debug-level messages.

You can change the logging level by including the following lines at the beginning of your source code:

{C++}
// Include the kernel log levels.
#include "sys/kernel_levels.h"
/// Change the header.
#define __DEBUG_HEADER__ "[ATA ]"
/// Set the log level.
#define __DEBUG_LEVEL__ LOGLEVEL_INFO

This example sets the __DEBUG_LEVEL__, so that all the messages from INFO and below are shown. While __DEBUG_HEADER__ is just a string that is automatically prepended to your message, helping you identifying from which code the message is coming from.

7. Change the scheduling algorithm

MentOS supports scheduling algorithms for aperiodic:

  • Round-Robin (RR)
  • Highest Priority
  • Completely Fair Scheduling (CFS)
  • Aperiodic Earliest Deadline First (AEDF)

It also supports periodic algorithms:

  • Earliest Deadline First (EDF)
  • Rate Monotonic (RM)

If you want to change the scheduling algorithm, to Round-Robin (RR) for instance:

cd build
cmake -DSCHEDULER_TYPE=SCHEDULER_RR ..
make
make qemu

or you can activate one of the others:

# Highest Priority
cmake -DSCHEDULER_TYPE=SCHEDULER_PRIORITY ..
# Completely Fair Scheduling (CFS)
cmake -DSCHEDULER_TYPE=SCHEDULER_CFS ..
# Earliest Deadline First (EDF)
cmake -DSCHEDULER_TYPE=SCHEDULER_EDF ..
# Rate Monotonic (RM)
cmake -DSCHEDULER_TYPE=SCHEDULER_RM ..
# Aperiodic Earliest Deadline First (AEDF)
cmake -DSCHEDULER_TYPE=SCHEDULER_AEDF ..

Otherwise you can use ccmake:

cd build
cmake ..
ccmake ..

Now you should see something like this:

BUILD_DOCUMENTATION ON
CMAKE_BUILD_TYPE
CMAKE_INSTALL_PREFIX /usr/local
DEBUGGING_TYPE DEBUG_STDIO
ENABLE_BUDDY_SYSTEM OFF
SCHEDULER_TYPE SCHEDULER_RR

Select SCHEDULER_TYPE, and type Enter to scroll the three available algorithms (SCHEDULER_RR, SCHEDULER_PRIORITY, SCHEDULER_CFS, SCHEDULER_EDF, SCHEDULER_RM, SCHEDULER_AEDF). Afterwards, you need to

<press c>
<press g>
make
make qemu

8. Use Debugger

If you want to use GDB to debug MentOS, first you need to compile everything:

cd build
cmake ..
make

Then, you need to generate a file called .gdbinit placed inside the build directory, which will tell gdb which object file he needs to read in order to allow proper debugging. To generate the file, just execute:

make gdbinit

Finally, you run qemu in debugging mode with:

make qemu-gdb

If you did everything correctly, you should see an empty QEMU window. Basically, QEMU is waiting for you to connect remotely with gdb. Anyway, running make qemu-gdb will make your current shell busy, you cannot call gdb in it. You need to open a new shell inside the build folder and do a:

cgdb -q -iex 'add-auto-load-safe-path .'

Now you will have:

  1. the QEMU window waiting for you,
  2. the shell where you ran make qemu-gdb also waiting for you,
  3. the debugger that loaded a series of symbol files and the location of their .text section.

By default I placed a breakpoint at the begginning of 1) the bootloader, 2) the kmain function of the kernel.

So, when gdb starts you need to first give a continue:

(gdb) continue

This will make the kernel run, and stop at the first breakpoint which is inside the bootloader:

Breakpoint 1, boot_main (...) at .../mentos/src/boot.c:220
220 {

giving a second continue will get you to the start of the operating system:

This will make the kernel run, and stop at the first breakpoint which is inside the bootloader:

Breakpoint 2, kmain (...) at .../mentos/src/kernel.c:95
95 {

9. Contributors

Project Manager:

Developers:

  • Alessandro Danese, Luigi Capogrosso, Mirco De Marchi
    • Protection ring
    • libc
  • Andrea Cracco
    • Buddy System, Heap, Paging, Slab, Caching, Zone
    • Process Image, ELF
    • VFS: procfs
    • Bootloader
  • Linda Sacchetto, Marco Berti
    • Real time scheduler
  • Daniele Nicoletti, Filippo Ziche
    • Real time scheduler (Asynchronous EDF)
    • Soft IRQs
    • Timer
    • Signals
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