Acorn Emu

Acorn Emu is an x86 emulation project.

Key Concepts

These are all defined in src/config/config.h

Memory Map

We need to know where things need to live in memory - a BIOS expects to live in a certain place and the bootloader is expecting to find itself at 0x7C00.

• 0x00000000 - 0x000003FF - real mode interrupt vector table
• 0x00007C00 - this is where the bootloader will be loaded (512 bytes)
• 0x000F0000 - BIOS ROM space (64KB)
• 0x00100000 - 1 MB mark - this is where protected mode starts
• Beyond this - this is where the OS kernel will live and space for application memory

Max Interrupts

x86 supports 256 interrupt vectors (indexed 0-255)

CPU Ring Levels

We'll be defining two ring levels for the CPU:

• Level 0 - this is where the kernel operates - no userspace code will be allowed to run here.
• Level 3 - this is where the userspace code will live.

x86 Instruction Lengths

• x86 instructions have a maximum length of 15 (made up of prefix + opcode + operands)
• REX prefix extends instructions to 64-bit

Directory Structure

• src - the source code
  • config - system wide constants like memory sizes, addresses etc.
  • cpu - this is the CPU core - registers, flags, execution
  • memory - memory management systems
  • decoder - instruction decoding (reading x86 bytecode)
  • io - input and output devices
• include - public headers
• tests - test files