( NOTE: Obtained from https://cs61.seas.harvard.edu/site/2018/WeensyOS/

This code illustrates essential OS concepts: x86_64 arch, ELF, loaders, processes, and memory management.

Great code by Dr. Eddie Kohler!

Tested on Ubuntu 16.04 x86_64.

)

Type make run to run our OS using the QEMU emulator. We expect this to work only on CS50 Appliance and other Linux hosts. If you have problems, check out Troubleshooting below.

There are several ways to run the OS.

• make run

  Build the OS and pop up a QEMU window to run it. Close the QEMU window to exit the OS.

• make run-console

  Build the OS and run QEMU in the current terminal window. Press Control-C in the terminal to exit the OS.

In all of these run modes, QEMU also creates a file named log.txt. The code we hand out doesn't actually log anything yet, but you may find it useful to add your own calls to log_printf from the kernel.

Finally, run make clean to clean up your directory.

Real operating systems are big. We have tried to boil down this OS to a minimum, comment it to help you, and separate x86 specifics from more fundamental issues. Here is an overview of the code.

=== Important code ===

• kernel.c: The kernel. Uses functions declared and described in kernel.h and lib.h.
• p-allocator.c, p-fork.c, and p-forkexit.c: The applications. Uses functions declared and described in process.h and lib.h.

=== Support code ===

You may read these if you're interested but you should be able to do the pset using only the code and descriptions in kernel.c, lib.h, and kernel.h.

• bootstart.S, boot.c: The bootloader.
• k-hardware.c: Functions that set up x86 hardware state using programmed I/O and memory-mapped I/O instructions.
• k-exception.S: Kernel assembly code for handling exceptions (interrupts, traps, and faults).
• k-loader.c: Kernel program loader, which loads processes from "image files" into memory.
• process.c: Support code for applications.
• lib.c, lib.h: Support code useful in both the kernel and applications.
• x86-64.h: x86-64 hardware definitions, including functions that correspond to important x86-64 instructions.
• elf.h: ELF support information. (ELF is a format used for executables.)

The main output of the build process is a disk image, weensyos.img. QEMU "boots" off this disk image, but it could also boot on real hardware! The build process also produces other files that you can look at and puts them in the obj/ directory.

• obj/kernel.asm

  This file is the output of objdump -S on the kernel. Use it to see the kernel's assembly code.

• obj/kernel.sym

  This smaller file just lists all the kernel's symbols (i.e., variable names).

• obj/p-allocator.asm, obj/p-allocator.sym, ...

  Similar files are generated for process code.

The OS runs using the QEMU full-system emulator. On CS50 Appliance, our makefiles will install QEMU for you. On your own Linux machine, you will need to install QEMU yourself. On Ubuntu-based hosts, run sudo apt-get install qemu. On Fedora-based hosts, run sudo yum install qemu-system-x86.

If Control-C doesn't work on your QEMU, make sure you are using an actual Control key. On some machines QEMU ignores key remappings (such as swapping Control and Caps Lock).

If Control-C still doesn't work on your QEMU, forcibly close it by running make kill.