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AGENTS.md - Developer Guide for izapple2

This guide provides essential information for AI coding agents working on the izapple2 Apple II emulator project.

Project Overview

izapple2 is an Apple II+/IIe emulator written in Go. The project emulates various Apple II models with support for disk drives, expansion cards, and multiple display modes.

  • Language: Go 1.26.0+
  • Main Package: github.com/ivanizag/izapple2
  • Architecture: Modular card-based system with memory management, CPU emulation, and video rendering

Build Commands

Build the Project

# Build the main package (library only)
go build .

# Build SDL2 frontend
cd frontend/a2sdl
go build .

# Build the experimental SDL3 frontend. Needs no cgo and no SDL developer
# files: SDL3 is embedded in the binary by github.com/Zyko0/go-sdl3
cd frontend/a2sdl3
go build .

# Build WASM frontend
cd frontend/a2wasm
go build .

# Build console frontend
cd frontend/console
go build .

# Build the libretro core, needs a C toolchain
cd frontend/a2libretro
make

Install Dependencies

# Install Go dependencies
go get -v -t -d ./...

# On Linux, install SDL2 development files
sudo apt-get install libsdl2-dev

# On macOS
brew install SDL2

Testing

Run All Tests

# Run all tests
go test ./...

# Run tests with verbose output
go test -v ./...

# Run tests with coverage
go test -cover ./...

Run a Single Test

# Run a specific test by name
go test -v -run TestName

# Run a specific test in a specific package
go test -v -run TestName ./storage

# Examples:
go test -v -run TestPlusBoots
go test -v -run TestNibBackAndForth ./storage
go test -v -run TestConfigurationModel

Run Tests in a Specific Package

# Test only the storage package
go test ./storage

# Test with race detector
go test -race ./...

End-to-End Tests

The project includes E2E boot tests (e2e_boot_test.go, e2e_woz_test.go) that boot various disk images and verify the emulator behavior. These tests use cycle counts and text matching to validate proper operation.

Code Style Guidelines

Package Organization

  • Main emulator code is in the root package izapple2
  • Subpackages include: storage, screen, fujinet, component
  • Frontend implementations are in frontend/ directory
  • frontend/shared has the code more than one frontend needs and the emulator library has no place for, like the screen each frontend shows and the drop targets

Naming Conventions

Types and Structs

  • Use CamelCase for exported types: Apple2, CardDisk2, VideoSource
  • Use camelCase for unexported types: memoryManager, cardBase, trackTracer
  • Card implementations follow pattern: Card<CardName> (e.g., CardDisk2, CardSmartPort)
  • Card builders follow pattern: cardBuilder for the struct

Functions

  • Exported functions: CamelCase starting with uppercase (e.g., NewKeyboardChannel, LoadResource)
  • Unexported functions: camelCase starting with lowercase (e.g., configure, setupCard)
  • Constructor functions: New<Type> (e.g., NewBlockDiskFile, NewSmartPortFujinetNetwork)
  • Factory functions for unexported types: new<Type> (e.g., newVideo, newTraceMonitor)
  • Boolean query methods: start with Is or Has (e.g., IsPaused, isFileWoz)

Variables and Constants

  • Exported constants: CamelCase (e.g., MemoryTypeROM, DiskII)
  • Unexported constants: camelCase (e.g., noCardName, wozMaxTrack)
  • Constants for addresses often use hex: addressLimitZero, ioC8Off

Imports

  • Standard library imports first
  • Third-party imports second
  • Local package imports last
  • Separate groups with blank lines

Example:

import (
    "fmt"
    "strings"

    "github.com/ivanizag/iz6502"
    "golang.org/x/exp/maps"

    "github.com/ivanizag/izapple2/screen"
    "github.com/ivanizag/izapple2/storage"
)

Types and Interfaces

  • Use embedded structs for common functionality (e.g., cardBase embedded in card implementations)
  • Keep interfaces small and focused
  • Define interfaces in the package that uses them, not implements them

Error Handling

  • Return errors rather than panicking in most cases
  • Use fmt.Errorf() for error wrapping with context
  • Check errors immediately after function calls
  • Test functions should use t.Fatal(err) for setup errors, t.Error(err) for assertion failures

Example:

if err != nil {
    return fmt.Errorf("failed to load ROM: %v", err)
}

Comments and Documentation

  • All exported functions, types, and methods must have doc comments
  • Doc comments start with the name being documented
  • Use full sentences with proper punctuation
  • Include references to specifications or documentation when relevant

Example:

// NewCardDisk2 creates a DiskII controller card
// sectors13 enables support for 13-sector disks (DOS 3.2)
func NewCardDisk2(sectors13 bool) *CardDisk2 {

Testing Conventions

  • Test files end with _test.go
  • Test function names: Test<FunctionName> or Test<Feature>
  • Use table-driven tests when testing multiple scenarios
  • Helper functions for tests: test<Purpose> (e.g., testBoots)
  • Use underscores for large numeric literals in tests: 200_000, 100_000_000

Code Formatting

  • Use gofmt (automatic with most Go tooling)
  • Line length: no strict limit, but keep it reasonable (~100-120 chars)
  • Use tabs for indentation (Go standard)

Memory and Hardware Emulation

  • Memory addresses use uint16
  • Use hex notation for hardware addresses: 0xc000, 0xbfff
  • Bit manipulation is common; comment complex operations
  • Cycle counting is important for timing-sensitive operations

Configuration System

  • Configuration uses string-based parameter system
  • Card parameters follow format: cardname,param1=value1,param2=value2
  • Use configuration struct for managing settings
  • Support both command-line and config file configuration

Common Patterns

Card Implementation

  1. Define card struct embedding cardBase
  2. Implement assign() method to set up ROM and memory handlers
  3. Use builder pattern with cardBuilder struct
  4. Register in cardFactory map

Resource Loading

  • Use LoadResource() for files that can be internal, local files, or URLs
  • Internal resources use <internal>/ prefix
  • Support gzip and zip compression transparently

Tracing and Debugging

  • Use executionTracer interface for CPU tracing
  • Implement tracers for different OSes (ProDOS, Pascal, CP/M)
  • Use trace parameter to enable debug output

Continuous Integration

The project uses GitHub Actions (.github/workflows/go.yml) and CircleCI (.circleci/config.yml):

  • Builds are tested on Linux with SDL2
  • Must pass go build and go test ./...
  • Targets Go 1.26+ (currently using 1.26.0)

Additional Resources

  • See README.md for user documentation
  • See doc/features.md for the complete feature list and the default configuration
  • See doc/command_line.md for command-line options
  • See doc/frontend_*.md for a page per frontend: how to build it, how to use it and what it does not do
  • See storage/WozSupportStatus.md for WOZ format support details
  • Reference implementation uses iz6502 CPU emulator: https://github.com/ivanizag/iz6502