akshitgaur2005.tngl.sh
Musings from the mountains
himwant.org
xv6 kernel documentation!
+250
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.github/workflows/astro.yml
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.github/workflows/astro.yml
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# Sample workflow for building and deploying an Astro site to GitHub Pages
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#
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# To get started with Astro see: https://docs.astro.build/en/getting-started/
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#
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name: Deploy Astro site to Pages
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on:
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# Runs on pushes targeting the default branch
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push:
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branches: ["main"]
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# Allows you to run this workflow manually from the Actions tab
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workflow_dispatch:
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# Sets permissions of the GITHUB_TOKEN to allow deployment to GitHub Pages
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permissions:
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contents: read
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pages: write
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id-token: write
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# Allow only one concurrent deployment, skipping runs queued between the run in-progress and latest queued.
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# However, do NOT cancel in-progress runs as we want to allow these production deployments to complete.
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concurrency:
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group: "pages"
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cancel-in-progress: false
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env:
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BUILD_PATH: "." # default value when not using subfolders
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# BUILD_PATH: subfolder
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jobs:
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build:
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name: Build
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runs-on: ubuntu-latest
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steps:
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- name: Checkout
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uses: actions/checkout@v4
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- name: Detect package manager
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id: detect-package-manager
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run: |
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if [ -f "${{ github.workspace }}/yarn.lock" ]; then
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echo "manager=yarn" >> $GITHUB_OUTPUT
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echo "command=install" >> $GITHUB_OUTPUT
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echo "runner=yarn" >> $GITHUB_OUTPUT
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echo "lockfile=yarn.lock" >> $GITHUB_OUTPUT
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exit 0
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elif [ -f "${{ github.workspace }}/package.json" ]; then
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echo "manager=npm" >> $GITHUB_OUTPUT
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echo "command=ci" >> $GITHUB_OUTPUT
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echo "runner=npx --no-install" >> $GITHUB_OUTPUT
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echo "lockfile=package-lock.json" >> $GITHUB_OUTPUT
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exit 0
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else
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echo "Unable to determine package manager"
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exit 1
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fi
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- name: Setup Node
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uses: actions/setup-node@v4
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with:
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node-version: "20"
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cache: ${{ steps.detect-package-manager.outputs.manager }}
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cache-dependency-path: ${{ env.BUILD_PATH }}/${{ steps.detect-package-manager.outputs.lockfile }}
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- name: Setup Pages
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id: pages
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uses: actions/configure-pages@v5
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- name: Install dependencies
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run: ${{ steps.detect-package-manager.outputs.manager }} ${{ steps.detect-package-manager.outputs.command }}
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working-directory: ${{ env.BUILD_PATH }}
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- name: Build with Astro
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run: |
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${{ steps.detect-package-manager.outputs.runner }} astro build \
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--site "${{ steps.pages.outputs.origin }}" \
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--base "${{ steps.pages.outputs.base_path }}"
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working-directory: ${{ env.BUILD_PATH }}
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- name: Upload artifact
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uses: actions/upload-pages-artifact@v3
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with:
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path: ${{ env.BUILD_PATH }}/dist
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deploy:
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environment:
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name: github-pages
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url: ${{ steps.deployment.outputs.page_url }}
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needs: build
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runs-on: ubuntu-latest
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name: Deploy
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steps:
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- name: Deploy to GitHub Pages
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id: deployment
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uses: actions/deploy-pages@v4
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.vscode/extensions.json
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.vscode/extensions.json
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.vscode/launch.json
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.vscode/launch.json
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src/content/home.md
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src/content/home.md
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src/content/posts/hello.md
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src/content/posts/hello.md
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src/content/posts/hello.org
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src/content/posts/hello.org
···
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#+title: Hello
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#+author: Akshit Gaur
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#+description: First Blog!
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#+TAGS: emacs, org, astro
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#+OPTIONS: toc:nil
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* Hello
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** Everyone
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#+begin_src python
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print("HELLO!")
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print("World!")
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#+end_src
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:::caution
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Use caution while using python
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:::
+141
src/content/posts/xv6/index.md
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src/content/posts/xv6/index.md
···
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---
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title: "xv6"
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published: 2025-12-16
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draft: false
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description: 'Documenting the xv6 kernel'
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tags: ["xv6","os"]
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---
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# Why??
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I am currently going through the book ***Operating Systems: Three Easy Pieces***, and the next step after completing the first piece, Virtualisation, is to go through the xv6 kernel. The xv6 kernel had two implementations, the x86 one and RISC-V one. As the x86 one is now unmaintained and on recommendation of a mentor, I am using [xv6-riscv](https://github.com/mit-pdos/xv6-riscv)! So without further ado, let's get started.
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# The xv6 Kernel
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+
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- Based on Unix kernel
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- Created by MIT for educational purposes
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- Implemented for RISC-V
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- Multicore
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- ~6000 lines of code (C and Assembly)
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+
24
+
25
+
## Features
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+
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+
- Processes
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+
- Virtual Address Spaces
29
+
- Page Tables
30
+
- Files and Directories
31
+
- Pipes
32
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- Multitasking
33
+
- Time-Slicing
34
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- 21 system calls
35
+
36
+
37
+
## User Programs
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+
39
+
- sh
40
+
- cat
41
+
- echo
42
+
- grep
43
+
- kill
44
+
- ln
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+
- ls
46
+
- mkdir
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+
- rm
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+
- wc
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+
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+
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## What is missing?
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+
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All the complexity of a "real" operating system is missing. For example, UserID, file protections, IPC, "mount"able filesystem, etc.
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+
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+
56
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## SMP: Shared Memory Multiprocesser
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+
58
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CPU = core = HART
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+
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Main memory (RAM) is shared and is of size 128 Mbytes
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+
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+
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## Supported Devices
64
+
65
+
- UART (Serial Comm. Tx <=> Rx)
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+
- Disk
67
+
- Timer Interrupts
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+
- PLIC: Platform Level Interrupt Controller
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+
- CLINT: Core Local Interrupt Controller
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+
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+
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## Memory Management
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+
74
+
- Page Size = 4096 bytes (`#define PGSIZE`)
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+
- Single Free List
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- No variable sized allocation
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- No "malloc"
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+
- Page Tables-
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- Three Levels
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- One table per process + One table for Kernel
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- Pages marked - R/W/X/U/V
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+
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+
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## Scheduler
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+
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- Round Robin
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- Size of TimeSlice is fixed (1,000,000 cycles)
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- All cores share one "Ready Queue"
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- Next TimeSlice may be on a different core
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+
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+
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## Boot Sequence
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+
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- QEMU
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- Loads kernel code at a fixed address (0x8000-0000)
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- Starts all cores running
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- No bootloader/boot-block/BIOS
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+
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+
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## Locking
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+
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- Spin Locks
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- `sleep()` & `wakeup()`
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+
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+
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## "Param.h"
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+
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- Fixed Limits (like no. of processes, no. of open files, etc)
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- Several Arrays ("`kill(pid)`" => Linear search of Processes array)
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+
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+
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## User Address Space
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+
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+
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+
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Arguments (argc, argv) will be placed on the stack before the program begins execution.
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+
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+
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## RISC-V Virtual Addresses
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+
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- Multiple Schemes are available
122
+
- Sv32 -> Used for 2-level Page Tables
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- Sv39 -> For 3-levels
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- Sv48 -> For 4-levels!
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- xv6 uses Sv39
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- VA Size-
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+
128
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```
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39 bits
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2^39 = 512 GB
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= 0x80-0000-0000
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+
```
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- But xv6 uses only 38 bits for its VAs
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+
135
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```
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38 bits
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2^38 = 256 GB
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= 0x40-0000-0000
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+
```
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- Therefore the allowed range is - 0…0x3F-FFFF-FFFF
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+
+108
src/content/posts/xv6/index.org
+108
src/content/posts/xv6/index.org
···
1
+
#+title: xv6
2
+
#+author: Akshit Gaur
3
+
#+description: Documenting the xv6 kernel
4
+
#+TAGS: xv6, os
5
+
#+OPTIONS: toc:nil
6
+
7
+
* Why??
8
+
I am currently going through the book */Operating Systems: Three Easy Pieces/*, and the next step after completing the first piece, Virtualisation, is to go through the xv6 kernel. The xv6 kernel had two implementations, the x86 one and RISC-V one. As the x86 one is now unmaintained and on recommendation of a mentor, I am using [[https://github.com/mit-pdos/xv6-riscv][xv6-riscv]]! So without further ado, let's get started.
9
+
10
+
* The xv6 Kernel
11
+
- Based on Unix kernel
12
+
- Created by MIT for educational purposes
13
+
- Implemented for RISC-V
14
+
- Multicore
15
+
- ~6000 lines of code (C and Assembly)
16
+
17
+
** Features
18
+
- Processes
19
+
- Virtual Address Spaces
20
+
+ Page Tables
21
+
- Files and Directories
22
+
- Pipes
23
+
- Multitasking
24
+
+ Time-Slicing
25
+
- 21 system calls
26
+
27
+
** User Programs
28
+
- sh
29
+
- cat
30
+
- echo
31
+
- grep
32
+
- kill
33
+
- ln
34
+
- ls
35
+
- mkdir
36
+
- rm
37
+
- wc
38
+
39
+
** What is missing?
40
+
All the complexity of a "real" operating system is missing. For example, UserID, file protections, IPC, "mount"able filesystem, etc.
41
+
42
+
** SMP: Shared Memory Multiprocesser
43
+
CPU = core = HART
44
+
45
+
Main memory (RAM) is shared and is of size 128 Mbytes
46
+
47
+
** Supported Devices
48
+
- UART (Serial Comm. Tx <=> Rx)
49
+
- Disk
50
+
- Timer Interrupts
51
+
- PLIC: Platform Level Interrupt Controller
52
+
- CLINT: Core Local Interrupt Controller
53
+
54
+
** Memory Management
55
+
- Page Size = 4096 bytes (=#define PGSIZE=)
56
+
- Single Free List
57
+
- No variable sized allocation
58
+
- No "malloc"
59
+
- Page Tables-
60
+
+ Three Levels
61
+
+ One table per process + One table for Kernel
62
+
+ Pages marked - R/W/X/U/V
63
+
64
+
** Scheduler
65
+
- Round Robin
66
+
- Size of TimeSlice is fixed (1,000,000 cycles)
67
+
- All cores share one "Ready Queue"
68
+
- Next TimeSlice may be on a different core
69
+
70
+
** Boot Sequence
71
+
- QEMU
72
+
+ Loads kernel code at a fixed address (0x8000-0000)
73
+
+ Starts all cores running
74
+
- No bootloader/boot-block/BIOS
75
+
76
+
** Locking
77
+
- Spin Locks
78
+
- =sleep()= & =wakeup()=
79
+
80
+
** "Param.h"
81
+
- Fixed Limits (like no. of processes, no. of open files, etc)
82
+
- Several Arrays ("=kill(pid)=" => Linear search of Processes array)
83
+
84
+
** User Address Space
85
+
#+CAPTION: Sorry for the bad handwriting!
86
+
[[./ua-space.jpeg]]
87
+
88
+
Arguments (argc, argv) will be placed on the stack before the program begins execution.
89
+
90
+
** RISC-V Virtual Addresses
91
+
- Multiple Schemes are available
92
+
+ Sv32 -> Used for 2-level Page Tables
93
+
+ Sv39 -> For 3-levels
94
+
+ Sv48 -> For 4-levels!
95
+
- xv6 uses Sv39
96
+
- VA Size-
97
+
#+begin_example
98
+
39 bits
99
+
2^39 = 512 GB
100
+
= 0x80-0000-0000
101
+
#+end_example
102
+
- But xv6 uses only 38 bits for its VAs
103
+
#+begin_example
104
+
38 bits
105
+
2^38 = 256 GB
106
+
= 0x40-0000-0000
107
+
#+end_example
108
+
- Therefore the allowed range is - 0...0x3F-FFFF-FFFF
src/content/posts/xv6/ua-space.jpeg
src/content/posts/xv6/ua-space.jpeg
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