xv6, line by line

xv6 is a small, complete Unix-like operating system written at MIT for teaching. It runs on RISC-V. This site presents every file of it, so you can read the whole thing, from the Makefile to the scheduler, and follow any name to where it is defined and everywhere it is used.

How to read this site

Hover over a highlighted name to see what it is and where it is defined.
Click a name to jump to its definition. The Symbol panel lists every place the name is used, across C, assembly, linker scripts and the Makefile.
Press / to search for any file, function, struct, macro or label.
In an annotated file, click any line to read its notes in the side panel, or press Start reading and step through with j / k. Show notes inline puts the notes between the lines instead.
Dotted underlines are glossary terms; the glossary lists them all.
Your browser's Back button returns you to where you were.

Start with a guided tour

The 52 guided tours walk through the running system step by step, showing the machine state and what the other CPUs are doing. A good path for beginners:

Lost? Four questions locate you at any moment: which privilege mode, which stack, which page table, and which locks this CPU holds (with interrupts on or off). The concept pages explain each one, and every tour step shows all four.

Created byJyotiprakash MishraAuthor of RISC-V Architecture and XV6 (De Gruyter Brill, 2026), the book this site accompanies. Meet the creator →

A suggested reading order

Makefile — How the kernel, the user programs and the disk image are built. annotated
kernel/kernel.ld — Where in memory each part of the kernel is placed. annotated
kernel/entry.S — The first instructions every CPU runs. annotated
kernel/start.c — Machine mode setup, then the switch to supervisor mode. annotated
kernel/main.c — Initializes every kernel subsystem and starts the first process. annotated
kernel/memlayout.h — The physical and virtual memory map. annotated
kernel/vm.c — Page tables: how virtual addresses become physical ones. annotated
kernel/proc.c — Processes, the scheduler, fork/exit/wait. annotated
kernel/trampoline.S — The bridge between user space and the kernel. annotated
kernel/trap.c — Handling system calls, interrupts and faults. annotated
kernel/syscall.c — Dispatching a system call to its handler. annotated
kernel/fs.c — The file system: inodes, directories, path names. annotated

The kernel

File	Language	Lines
kernel/bio.c annotated	C	153
kernel/buf.h annotated	C header	11
kernel/console.c annotated	C	203
kernel/defs.h annotated	C header	186
kernel/elf.h annotated	C header	42
kernel/entry.S annotated	RISC-V assembly	21
kernel/exec.c annotated	C	176
kernel/fcntl.h annotated	C header	5
kernel/file.c annotated	C	179
kernel/file.h annotated	C header	40
kernel/fs.c annotated	C	741
kernel/fs.h annotated	C header	61
kernel/kalloc.c annotated	C	82
kernel/kernel.ld annotated	linker script	45
kernel/kernelvec.S annotated	RISC-V assembly	64
kernel/log.c annotated	C	261
kernel/main.c annotated	C	45
kernel/memlayout.h annotated	C header	63
kernel/param.h annotated	C header	14
kernel/pipe.c annotated	C	143
kernel/plic.c annotated	C	47
kernel/printk.c annotated	C	152
kernel/proc.c annotated	C	703
kernel/proc.h annotated	C header	104
kernel/riscv.h annotated	C header	417
kernel/sleeplock.c annotated	C	55
kernel/sleeplock.h annotated	C header	9
kernel/spinlock.c annotated	C	116
kernel/spinlock.h annotated	C header	8
kernel/start.c annotated	C	66
kernel/stat.h annotated	C header	11
kernel/string.c annotated	C	106
kernel/swtch.S annotated	RISC-V assembly	42
kernel/syscall.c annotated	C	151
kernel/syscall.h annotated	C header	23
kernel/sysfile.c annotated	C	530
kernel/sysproc.c annotated	C	112
kernel/trampoline.S annotated	RISC-V assembly	153
kernel/trap.c annotated	C	220
kernel/types.h annotated	C header	10
kernel/uart.c annotated	C	155
kernel/virtio.h annotated	C header	99
kernel/virtio_disk.c annotated	C	333
kernel/vm.c annotated	C	491
kernel/vm.h annotated	C header	2

User programs and library

File	Language	Lines
user/cat.c annotated	C	43
user/dorphan.c annotated	C	35
user/echo.c annotated	C	19
user/forktest.c annotated	C	56
user/forphan.c annotated	C	40
user/grep.c annotated	C	108
user/grind.c annotated	C	351
user/init.c annotated	C	54
user/kill.c annotated	C	17
user/ln.c annotated	C	15
user/logstress.c annotated	C	48
user/ls.c annotated	C	88
user/mkdir.c annotated	C	23
user/printf.c annotated	C	135
user/rm.c annotated	C	23
user/sh.c annotated	C	499
user/stressfs.c annotated	C	50
user/sync.c annotated	C	10
user/ulib.c annotated	C	162
user/umalloc.c annotated	C	90
user/user.h annotated	C header	50
user/user.ld annotated	linker script	39
user/usertests.c annotated	C	3538
user/usys.S gen annotated	RISC-V assembly	112
user/usys.pl annotated	Perl	45
user/wc.c annotated	C	55
user/zombie.c annotated	C	14

mkfs: builds the disk image

File	Language	Lines
mkfs/mkfs.c annotated	C	307

Build and project files

File	Language	Lines
.gdbinit.tmpl-riscv annotated	GDB script	6
Makefile annotated	Makefile	204
README annotated	text	48
test-xv6.py annotated	Python	224
.clang-format annotated	text	10
.dir-locals.el annotated	Emacs Lisp	4
.editorconfig annotated	text	22
.gitignore annotated	text	12
LICENSE annotated	text	24

83 files · 13,249 lines · 2,457 named symbols. Source: mit-pdos/xv6-riscv @ 06aad25 (MIT license).

© 2026 Jyotiprakash Mishra. Text, tours, questions and labs: CC BY-NC-SA 4.0; site software: GPL-3.0-or-later; xv6 itself: MIT. What covers what. Credit derived work as “xv6, line by line” by Jyotiprakash Mishra, https://xv6.jyotiprakash.org.