• This repository mainly focuses on projects for the online course "UW-Madison CS 537: Introduction to Operating Systems, Spring 2018" and the textbook Operating Systems: Three Easy Pieces.

• This repo is duplicated from the skeleton repo.

• Read project-descriptions for more informations.

• Docker

# Step1: Build Docker image
./xv6-container build

# Step2: Create Docker container
docker run -v "`pwd`":/home/xv6user/xv6  --name OSTEP -itd xv6env

# Step3: Enter Docker container
docker exec -it OSTEP bash

# Step4:
# * xv6-public # (MIT version)
# * xv6-wisc   # (UW-Madison refactor version)

• Compile xv6

make qemu-nox

• Reference
  • xv6-public
  • xv6-docker

1. (Project 1a) Unix Utilities (cat, grep, zip/unzip)

• ✅ (May 12, 2021)
• Implement Unix utilities on my own, including cat, grep, zip/unzip.

2. (Project 2a) Command Line Interpreter (My Unix Shell)

• ✅ (May 22, 2021)
• Basic shell: The shell supports both interactive mode and batch mode. In interactive mode, when a user types a command, the shell will use the fork(), execv(), and wait() syscalls to create a child process to execute the command. When the execution finishes, the main process will print the prompt again and wait for more user commands.
• Build-in commands: Implement three built-in commands, including exit, cd, and path (search path of the shell).
• Redirection: With the support of redirection, a shell user can send the output of a program to a file rather than to the screen. I implement redirection with open() and dup2() syscalls. Use open() to get the file descriptor of the target file, and then use dup2() to duplicate the file descriptor of STDOUT. Example: ls -l > output.
• Parallel Commands: In this shell, users can run commands in parallel rather than starting a command only when the previous command finishes. Then, after starting all such processes, the shell uses a wait() loop to wait for them to complete. After all processes are done, return control to the user. Example:

  wish> ls -l & echo 123
  

3. (Project 3a) Parallel Zip

• ✅ (June 19, 2021)
• In project 1a, I implemented a simple compression tool based on run-length encoding, known simply as zip.
• In project 3a, I implement something similar, except I use threads to make a parallel version of zip.
• Keywords:
  • Multi-thread: pthread
  • File I/O: open, mmap, munmap

These projects all are to be done inside the xv6 kernel based on an early version of Unix and developed at MIT.

1. (Project 1b) xv6: Introducing a new system call

• ✅ (May 15, 2021)
• Introduce a new system call getreadcount(). This project is very helpful for me to understand the process of calling a syscall.

2. (Project 2b) An xv6 Lottery Scheduler

• ✅ (June 6, 2021)

• Implement a lottery scheduler in xv6. To elaborate, I implement a syscall settickets to set the number of tickets of the calling process. The basic idea is simple: assign each running process a slice of the processor based in proportion to the number of tickets it has; the more tickets a process has, the more it runs. Each time slice, a randomized lottery determines the winner of the lottery; that winning process is the one that runs for that time slice.

  

3. (Project 3b) Virtual Memory (Null Pointer and Read-Only Regions)

• ✅ (July 11, 2021)
• (Task1) Null-pointer Dereference: In xv6's VM system, user code is loaded into the very first part of the address space. Thus, if you dereference a null pointer, you will not see an exception; rather, you will see whatever code is the first bit of code in the program that is running. In Task1, we should make xv6 trap and kill the process that dereferences a null pointer.
• (Task2) Read-only Code: In most operating systems, code is marked read-only instead of read-write. However, in xv6 this is not the case, so a buggy program could accidentally overwrite its own text. In Task2, we need to implement 2 syscalls to avoid the accidents.
  • mprotect(void *addr, int len): Changes the protection bits of the page range starting at addr and of len pages to be read only. Thus, a write to this region should cause a trap (and thus kill the process) after mprotect() finishes.
  • munprotect(void *addr, int len): Sets the region back to both readable and writeable.