Introduction to xv6

The need for a real OS

• OSTEP textbook gave us concepts: processes, memory, files, concurrency
  • How these are actually implemented?
• Actual OSes are too complex
  • xv6, operating system for teaching.

What is xv6?

• Educational reimplementation of Unix V6 by MIT
• MIT Operating System Engineering: 6.1810
• Approximately 10K lines of C and Assembly
• Clean, modular, readable
• No user-facing features — only a minimal shell and kernel

History of xv6

• The original Unix Version 6, one of the first OS written using C, ran on PDP-11, a 16-bit minicomputer.
  • processes, system calls, pipes, files,..
• xv6 attempts to cover same foundational concepts as Unix V6.
  • First developed on 32-bit x86 architecture in 2006.
• Overtime, x86 became bloated.
  • Legacy support (real mode, segmented memory)
  • Complex bootstrapping
  • Modern system no longer supports x86
    • Not by default, had to install from custom libraries
    • Rockly Linux 10 drops completely.
• In 2020
  • MIT ported xv6 to RISC-V architecture
  • Open source alternative to x86 and ARM

More about xv6

Why RISC-V?

Aspect	x86.	RISC-V
Complexity	Very high	Low
Openness	NDA-heavy	Fully open
Toolchain	Aging (32-bit)	Actively maintained

• Other attributes of RISC-V
  • Well-documented (due to open source nature)
  • Modern and gaining traction from academia and industry
• RISC-V in industry
  • SiFive Chip
  • Espressif Systems for Internet of Things
  • Tenstorrent: AI inference with RISC V
  • Google OpenTitan: Silicon Root of Trust with RISC V
  • …

What are we actually running?

• xv6-riscv runs on a RISC-V CPU, which was emulated by Qemu, which was setup inside a Ubuntu 24:04 container.
• More specifics:
  • Qemu, a machine emulator, starts the simulation of a tiny computer using RISC-V architecture.
  • This computer loads an operating system from scratch, not Linux, but xv6.
  • Once finished loading, xv6 gives you a simple shell, a place to type and run commands like ls, echo, and cat.

Hands-on

• Open a terminal into your container. This can be done by either launch the terminal tab inside your Code browser, or SSH into the container.

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ssh -p 22331 student@localhost #if SSH into container
cd
git clone https://github.com/mit-pdos/xv6-riscv.git
cd xv6-riscv
make
make qemu
 

• You will be booted into the xv6’s shell, as seen below.

• Run the following commands inside the xv6’s shell

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echo hello
cat README
ls
ps
uptime
 

What are the outcomes of the above commands?

• echo hello produces hello.
• cat README prints out the content of the README file.
• ls shows the content of the root directory, which are a list of files/commands.
• ps and uptime failed to run, as you can see they are not available in the / directory.

• To quit qemu, first press Ctrl-A then X.

Default xv6 commands

Command	Purpose
ls	List files in current directory
cat	Print contents of a file
echo	Print arguments to stdout
sh	The xv6 shell
init	First user-space process
kill	Kill a process by PID
sleep	Sleep for a number of ticks
forktest	Test process creation limits
zombie	Test zombie process behavior
stressfs	Stress-test file system
usertests	Runs test suite for xv6 syscalls