Chip8 Instruction Operands: Design and Extensibility

A Chip8 instruction is a 16-bit value, typically structured as an opcode (the upper 4 bits) and one or more operands (the remaining bits). Operands specify registers, immediate values, or addresses that the instruction operates on. Different instructions use different operand types and bit layouts.

Operand Types in Chip8

Common operand types in Chip8 include: - X: 4-bit register index (bits 8–11) - Y: 4-bit register index (bits 4–7) - N: 4-bit immediate value (bits 0–3) - NN: 8-bit immediate value (bits 0–7) - NNN: 12-bit address (bits 0–11)

These operand types are used in various combinations to form the full instruction set.

Operands API Design

The operands API is designed to provide a type-safe, extensible, and convenient way to extract and validate operands from a 16-bit instruction. Each operand type is represented by a template specialization of operand_t<T>, and all operands for an instruction are grouped in the operands_t struct.

Key Components

  • operand_t<T>: A template struct that wraps an operand value of type T (e.g., uint8_t, uint16_t). It provides methods for assignment, retrieval, and validation.

  • operands_t: A struct that holds all possible operand types (X_t, Y_t, N_t, NN_t, NNN_t) as optionals. It provides methods to extract each operand from a 16-bit instruction and to validate them.

Extending for New Operand Types

To support a new operand type:

  1. Define a new type alias: For example, for a 3-bit operand, add: ` using M_t = operand_t<uint8_t>; `

  2. Add a member to operands_t: Add a new std::optional<M_t> _M; and corresponding getter/setter methods:

auto M(std::uint16_t inst) -> void {
    _M = {(inst & 0x00E0) >> 5}; // Example bitmask and shift
}

[[nodiscard]] auto M() const -> std::uint8_t {
    return _M.value_or(M_t{})();
}

[[nodiscard]] auto M_is_valid(u8_acceptable_t acceptable_values = {}) const -> bool {
    return _M.has_value() && _M.value().is_valid(acceptable_values);
}

  1. Use in Opcode Handlers: In your opcode handler, call operands.M(inst) to extract the new operand.

Summary

  • Chip8 instructions are composed of various operand types, each mapped to a specific bit range.

  • The operands API provides a modular and type-safe way to extract, store, and validate operands.

  • To add a new operand type, define a new alias and extend operands_t with extraction and validation logic.

  • This design keeps operand handling clean, extensible, and easy to maintain.

  • Chip8 Opcodes: Overview and API Design

  • The SupportsChip8Ops Concept

  • Core Emulator API Reference