• MentOS (Mentoring Operating System)
  • Table of Contents
  • What is MentOS
  • Implemented features
  • Prerequisites
    • Installing the prerequisites
  • Compiling MentOS
  • Generating the EXT2 filesystem
  • Running MentOS
  • Running MentOS from GRUB
  • Running and adding new programs to MentOS
    • Create a new program
    • Add the new program to the list of compiled sources
    • Running a program or a test
  • Kernel logging
  • Change the scheduling algorithm
  • Debugging the kernel
  • Contributors

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.

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Follows the list of implemented features:

Processes and Events

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

Memory

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

Filesystem

• Virtual Filesystem (VFS);
• Initramfs;
• EXT2;
• Procfs;

Input/Output

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

Inter-Process Communication (IPC)

• Semaphore
• Message queue
• Shared memory
• Named pipe

I will try to keep it updated...

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

For compiling the system we need:

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

Under MacOS, for compiling, you have additional dependencies:

• i386-elf-binutils
• i386-elf-gcc

For executing the operating system we need:

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

For debugging we suggest using:

• gdb or cgdb

Under Ubuntu, you can type the following commands:

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

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

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Compile MentOS with:

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

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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.

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Boot MentOS with qemu:

make qemu

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

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For booting MentOS from GRUB in QEMU we need the following tools:

• xorriso
• grub-mkrescue (from grub-common)

We also need grub-pc-bin, otherwise GRUB won't start in QEMU.

Which can be installed in Ubuntu with the following command:

sudo apt-get update && sudo apt-get upgrade -y
sudo apt-get install -y grub-common grub-pc-bin xorriso

Boot MentOS with qemu through GRUB by calling:

make qemu-grub

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This section explains how to add a new program to MentOS, and also how to run programs in mentos. It also explains how to add new tests, which are located in the programs/tests folder.

Head to the programs (or the programs/tests) folder. Create and open a new program, for instance a file called hello_world.c, with your preferred editor, and add this content to the file:

#include <stdio.h>

int main(int argc, char *argv[])
{
    printf("Hello, World!\n\n");
    return 0;
}

Now we can add the program to the list of files which are compiled and placed inside MentOS filesystem. The following procedure is the same for both programs and programs/tests, what changes is which CMakeLists.txt file we modify.

You need to modify the CMakeLists.txt file, either programs/CMakeLists.txt or programs/tests/CMakeLists.txt, and add your program to the list of files to be compiled:

# Add the executables (manually).
set(PROGRAMS
    init.c
    ...
    hello_world.c
)

or

# Add the executables (manually).
set(TESTS
    t_mem.c
    ...
    hello_world.c
)

That's it, the hello_world.c file will be compiled and will appear inside the /bin or /bin/tests folder of MentOS.

Once you login into MentOS, if you placed your source code in programs, you can execute the program by simply typing:

hello_world

because the file resides in /bin, and that folder is listed in the PATH environment variable.

Now, if you placed your source code inside the programs/tests folder, the executable will end up inside the /bin/tests folder. However, the /bin/tests folder is not listed in PATH, so, if you want to execute a test from that folder you need to specify the full path:

/bin/tests/hello_world

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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:

#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:

    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:

#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:

// 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
// Include the debuggin header.
#include "io/debug.h"

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.

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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

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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. Basically, it provides for each binary file, the location of their .text section. 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:

gdb --quiet --command=gdb.run

or

cgdb --quiet --command=gdb.run

Now you should have in front of you:

1. the QEMU window waiting for you;
2. the first shell where you ran make qemu-gdb, which is also waiting for you;
3. the second shell where gdb is runnign and, you guessed it, is waiting for you.

By default I placed a breakpoint at the begginning of (1) the bootloader and (2) the kernel itself.

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 second breakpoint which is inside the kernel:

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

There is also a launch configuration for vscode in .vscode/launch.json, called (gdb) Attach, which should allow you to connect to the running process.

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Project Manager:

• Enrico Fraccaroli

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
• And many other valuable contributors
  • rouseabout
  • seekbytes
  • fischerling

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