ZEmu6502 - 8-Bit Micro Computer Emulator

Emulator that emulates 6502 instructions with the same clock timing as a native processor.

Initially developed in Rust before being ported over to using Zig.

Development is currently on Linux, has not been tested on other OSs.

Install/Build

The Zig compiler (0.12/nightly) is required to build the project.

Use zig build run -- systems/default.yaml to build and run the emulator.

Status

All but indirect addressing modes are implemented, these are currently a work in progress.

Most instructions are complete (many with unit tests), any incomplete instructions will simply will generate a MicroOpError.NotImplemented result and be logged but otherwise behave like a no-op.

Definition of peripherals and their memory locations is configured via a configuration file, although there is little error handling.

Goals

The initial goal is to run Woz-mon in the emulator to provide a basic test case.

Main MCU

Missing features:

• Handling of NMI and IRQs
• Break instruction

Devices

Devices in development:

• Keyboard

Planned:

• Banked RAM
• Bit-mapped graphics (incorporating a terminal)
• Sound device
• Serial devices
• Emulation of an IO controller
• GPIO via external hardware.

Design

Every instruction is made up of micro-operations, these are the actual operations that are executed on each clock cycle and correspond with the same behaviour of a physical MPU.

Peripherals are implemented with a standard peripheral interface providing:

• read/write functions using a 16bit address and 8bit data
• clock signal (with both rising and falling edges)
• optional load function to load a binary (primarily to initialise ROM)
• optional register dump function for future gdb integration

Future enhancement to include NMI and IRQ status requests.

A system clock provides both positive and negative edges at a configurable frequency used to drive operations of the machine at a fixed rate.

A system module acts as the address bus and decode logic to route address lookup requests between the MCU and peripherals. Along with clock and NMI and IRQ states.

Default Memory Map

The default system can be found in the systems/default.yaml and resolves to:

| Address Range | Peripheral | Notes | |---------------|------------|-----------------------------------------| | 0x0000-0x3FFF | RAM | | | 0x0100-0x01FF | | Stack | | 0x4000-0x7FFF | - | | | 0x8000 | Terminal | Write-only, data treated as characters. | | 0x8001-0x800F | - | | | 0x8010 | Keyboard | Read-only, not currently implemented. | | 0x801F-0xEFFF | - | | | 0xFF00-0xFFFF | ROM | Read-only | | 0xFFFA-0xFFFF | | NMI/Reset/IRQ vectors |

Devices

Builtin

Simple virtual devices not based on a physical component.

• ram - RAM device, defaults to 0xFFFF bytes (actual addressable range based on config)
• rom - ROM device, defaults to 0x8000 bytes (actual addressable range based on config)
  • Use load option to provide an initial ROM image
• terminal - Basic character terminal to output text Apple I style
• keyboard - Keyboard input (in development)

Versatile Interface Adapter (VIA) devices

• w65c22 - WDC W65C22 VIA device (in development currently only provides registers)