Architecture

Technical documentation for bootOS internals.

Overview

bootOS is a monolithic operating system that fits entirely within a 512-byte boot sector. Despite its tiny size, it provides:

• Complete filesystem with 32-file capacity
• Program execution environment
• Keyboard and screen I/O services
• File management (load, save, delete)
• Self-relocation and memory management

Architecture Documents

System Overview

High-level architecture and design principles of bootOS.

Topics covered:

• Design philosophy
• System components
• Boot process
• Execution model

Memory Layout

Detailed memory organization and address mapping.

Topics covered:

• Memory regions
• Stack and buffers
• Code relocation
• Address constraints

Filesystem Design

How bootOS organizes and manages files on disk.

Topics covered:

• Disk layout
• Directory structure
• File allocation
• Sector mapping

Key Specifications

| Feature | Specification | |---------|--------------| | Code Size | 512 bytes (including boot signature) | | Minimum CPU | 8086/8088 | | Memory Required | 1.5 KB (768 bytes + 512 bytes OS + 256 bytes buffer) | | Max Files | 32 files | | File Size | 512 bytes per file | | Disk Support | 360K to 1.44M floppies | | Load Address | 0x7C00 (BIOS standard) | | Runtime Address | 0x7A00 (relocated) |

Design Constraints

Size Limitations

With only 512 bytes available:

• Boot signature uses 2 bytes (0x55, 0xAA)
• Padding to fill sector uses variable space
• Actual code limited to ~510 bytes

Compatibility Requirements

• 8086/8088 instruction set only
• BIOS interrupt compatibility
• Standard boot sector format
• Floppy disk geometry constraints

System Components

┌─────────────────────────────────────┐
│         BIOS (0x0000-0x03FF)        │  ← Interrupt vectors
├─────────────────────────────────────┤
│          Free Memory                │
├─────────────────────────────────────┤
│  Stack (0x7700) ↓                   │  ← Grows down
├─────────────────────────────────────┤
│  Line Buffer (0x7780)               │  ← 128 bytes
├─────────────────────────────────────┤
│  Directory Sector (0x7800)          │  ← 512 bytes
├─────────────────────────────────────┤
│  bootOS Code (0x7A00)               │  ← 512 bytes
├─────────────────────────────────────┤
│  Program Execution (0x7C00)         │  ← 512 bytes
└─────────────────────────────────────┘

Execution Flow

1. BIOS loads boot sector at 0x7C00
2. bootOS relocates itself to 0x7A00
3. Interrupt vectors installed at 0x0080-0x0094
4. System initializes and shows prompt
5. Command loop waits for user input
6. Programs execute at 0x7C00
7. Services available via interrupts 0x20-0x25

Technical Innovations

Code Reuse

• Shared routines minimize duplication
• Interrupt-based services
• Clever register usage

Memory Efficiency

• Self-relocation frees boot sector space
• Stack and buffers carefully positioned
• No wasted memory regions

Size Optimization

• Compact instruction sequences
• BIOS leveraging for I/O
• Minimal error handling

Further Reading

• System Overview - Detailed system design
• Memory Layout - Memory organization
• Filesystem Design - Disk structure
• API Reference - Programming interface