131 lines
4.8 KiB
Markdown
131 lines
4.8 KiB
Markdown
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# xOS
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**A multi-architecture operating system built from scratch targeting x86, ARM, and RISC-V**
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![System Overview][system_overview]
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---
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## What This Is
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An operating system implemented entirely from scratch (no existing kernel), spanning kernel development, driver architecture, filesystems, userland applications, and UI frameworks.
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### Technical Scope
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- **Multi-architecture kernel**: x86/x86-64, ARMv7, Arm64, RISC-V support with unified codebase
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- **Custom bootloaders** for each architecture with ELF validation and secure boot integration
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- **Full userspace stack**: C/C++ standard libraries, Objective-C runtime, UI framework, window server
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- **Production OS features**: Ext2 filesystem, virtual filesystem layer, process management, POSIX signals, TTY subsystem
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---
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## Core Technical Implementation
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### Kernel Architecture
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**Preemptive Multitasking & Scheduling**
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- Priority-based round-robin scheduler with 12 priority levels
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- Per-CPU runqueues for SMP support (multi-core)
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- Conditional kernel preemption (`PREEMPT_KERNEL` flag for real-time responsiveness)
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- Interrupt depth counter preventing race conditions during context switches
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- Idle threads for zero-allocation interrupt handling
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**Memory Management**
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- Virtual memory manager with per-process address spaces
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- Demand paging and swap support
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- Kernel/userspace memory isolation with access validation
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- Shared buffers for zero-copy IPC
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**System Call Interface**
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- 60+ system calls following POSIX conventions (`fork`, `exec`, `read`, `write`, `mmap`, etc.)
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- Architecture-specific syscall dispatch (software interrupts on x86, `svc` on ARM)
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- Kernel-mode syscalls (`ksys1`-`ksys4`) for internal operations
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### Boot Process & Security
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**Custom Bootloader Chain**
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- **Stage 1** (x86): Real-mode to protected-mode transition, memory map collection
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- **Stage 2/Prekernel**: ELF loader, device tree initialization, MMU setup
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- **Kernel validation**: SHA256 + RSA signature verification before execution
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- Passes boot arguments (`boot_args_t`) with memory layout, framebuffer, device tree
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### Filesystem & Device Management
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- **Ext2 filesystem** with full read/write support
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- **Virtual filesystem (VFS)** layer abstracting file operations
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- `/dev` (device nodes) and `/proc` (process/system introspection) filesystems
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- Device tree parsing for ARM/RISC-V hardware discovery
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- Device driver framework with pluggable driver registration
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### Userland & UI
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**Window Server** (compositing manager)
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- Client-server architecture using IPC
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- Desktop and mobile window management modes
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- Hardware-accelerated rendering via LibG
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- Menu bar, dock, popup system
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**Libraries**
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- **LibC/LibCxx**: POSIX-compliant C library and C++ STL
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- **LibObjC**: Objective-C runtime (message dispatch, ARC)
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- **LibFoundation**: Event loop, shared buffers, collections (Cocoa-inspired)
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- **LibG**: 2D graphics (pixel buffers, fonts, PNG/BMP loaders)
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- **LibUI**: High-level UI widgets (buttons, text fields, windows)
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---
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## Key Engineering Decisions
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**1. Multi-Architecture Support**
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Abstracted platform-specific code (`kernel/platform/{x86,arm32,arm64,riscv}`) with common interfaces for MMU, interrupts, context switching. Enables same kernel codebase across all targets.
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**2. Build System**
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GN (Generate Ninja) meta-build system for fast incremental builds. Compiles for multiple architectures from single source tree.
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**3. Bootloader Security**
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Validates kernel ELF binary cryptographically before loading—prevents tampered kernel execution (verified boot).
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**4. Preemptive vs Cooperative**
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Kernel preemption is compile-time optional (`-DPREEMPT_KERNEL`). Enables low-latency scheduling when enabled, or simpler non-preemptive model for easier debugging.
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**5. Window Server Architecture**
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Separates compositing from applications—crash in user app doesn't take down entire UI. Similar to X11/Wayland model.
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---
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## Features
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### Kernel
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* x86/x86-64, ARMv7 and Arm64 kernel with pre-emptive multi-threading
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* Ext2 filesystem
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* /dev and /proc filesystems
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* Local sockets
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* POSIX signals
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* TTY
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* [learn more](https://code.ayaantunio.me/ayaan/Custom-Operating-System/src/branch/master/docs/kernel.md)
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### Libraries
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* Runtime & support: LibC, LibCxx, LibObjC
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* Rich functionality: LibFoundation
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* UI functionality & rendering: LibG, LibUI
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* [learn more](https://code.ayaantunio.me/ayaan/Custom-Operating-System/src/branch/master/docs/libs.md)
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### Userland
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* Composing windows manager for desktop and mobile
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* Simple UI apps
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* Terminal
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### Boot
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* Custom bootloaders
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* Kernel validation during the boot proccess
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* Custom device tree
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* [learn more](https://code.ayaantunio.me/ayaan/Custom-Operating-System/src/branch/master/docs/boot.md)
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## How to build and run xOS?
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See the [build instructions](https://code.ayaantunio.me/ayaan/Custom-Operating-System/src/branch/master/docs/build.md)
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[system_overview]: /Images/sysarch.svg "System Architecture Diagram"
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