https://github.com/jyelewis/cpu-6502-emulator

MOS6502 CPU Emulator
https://github.com/jyelewis/cpu-6502-emulator

Last synced: 4 months ago
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MOS6502 CPU Emulator

• Host: GitHub
• URL: https://github.com/jyelewis/cpu-6502-emulator
• Owner: jyelewis
• Created: 2020-07-04T03:51:16.000Z (over 4 years ago)
• Default Branch: master
• Last Pushed: 2020-07-04T04:03:44.000Z (over 4 years ago)
• Last Synced: 2024-09-23T05:18:38.339Z (4 months ago)
• Language: TypeScript
• Size: 36.1 KB
• Stars: 5
• Watchers: 4
• Forks: 3
• Open Issues: 1
• Metadata Files:
  • Readme: readme.md

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README

        
# CPU6502 Emulator

A javascript emulator for the MOS6502 (WDC 65c02 compatible) CPU.

Designed for simulating custom logic boards that use the 6502.

All memory read/writes call out to a delegate function, allowing custom memory logic, I/O logic and address decoding.

## Configuration

```javascript 1.8

const cpu = new CPU6502({

  accessMemory, // function to be called when CPU reads/writes to memory

  logInstructions: false, // output each instruction to console, for advanced debugging

  logInternalState: false, // output internal processor state after each instruction for advanced debugging

  maxInstructions: 50 // automatically pause execution after 50 instructions

});

```

## Usage

### Available methods

```javascript 1.8

const cpu = new CPU6502();

cpu.reset(); // trigger reset sequence

cpu.triggerIRQB(); // trigger IRQB interrupt

cpu.triggerNMIB(); // trigger NMIB interrupt

cpu.pauseClock(); // pause clock, stop executing instructions

cpu.startClock(); // start/resume clock

// accessing internal state

console.log(cpu.reg_a);

console.log(cpu.reg_x);

console.log(cpu.reg_y);

console.log(cpu.programCounter);

console.log(cpu.stackPointer);

console.log(cpu.processorStatus);

```

### Example 1: Executing simple machine code

```javascript 1.8

import { CPU6502, ReadWrite } from "6502-emulator";

// instruction opcodes

const I_NOOP = 0xea;

const I_LDA = 0xa9;

const I_STA = 0x8d;

const I_JMP = 0x4c;

// set up memory

const ram = new Uint8ClampedArray(0xffff); // 64kb ram

ram.fill(I_NOOP); // fill ram with noop instructions

// store reset vector as 0200 (little endian)

ram[0xfffc] = 0x00;

ram[0xfffd] = 0x02;

// create a basic program

ram.set(

  [

    I_LDA, 0x55, // lda 55

    I_STA, 0x00, 0x60, // 55 -> 6000 (output 55 to address 0x6000)

    I_LDA, 0xaa, // lda AA

    I_STA, 0x00, 0x60, // AA -> 6000 (output AA to address 0x6000)

    I_JMP, 0x00, 0x02  // jump back to start of program

  ],

  0x0200

);

const accessMemory = (readWrite, address, value) => {

  // capture a write to 0x6000 as a magic output address, print to console

  if (address === 0x6000 && readWrite === ReadWrite.write) {

    console.log("Output: ", value.toString(16));

    return;

  }

  

  // write value to RAM (processor is reading from [address])

  if (readWrite === ReadWrite.read) {

    return ram[address];

  }

  // store value in RAM (processor is writing [value] to [address])

  ram[address] = value;

};

const cpu = new CPU6502({ accessMemory });

// trigger a reset to start the clock & jump to the reset vector

cpu.reset();

```

### Example 2: Load ROM image from disk

```javascript 1.8

import { CPU6502, ReadWrite } from "6502-emulator";

// load image from disk

const ramImagePath = "./myROMFile";

const ramImage = fs.readFileSync(ramImagePath);

  

// set up memory

const ram = Uint8ClampedArray.from(ramImage);

const accessMemory = (readWrite, address, value) => {

  // capture a write to 0x6000 as a magic output address, print to console

  if (address === 0x6000 && readWrite === ReadWrite.write) {

    console.log("Output: ", value.toString(16));

    return;

  }

  // capture a write to 0x6005 as a magic output address, pause the clock

  if (address === 0x6005 && readWrite === ReadWrite.write) {

    console.log("Exit captured! pausing clock");

    cpu.pauseClock();

    return;

  }

  // write value to RAM (processor is reading from [address])

  if (readWrite === ReadWrite.read) {

    return ram[address];

  }

  // store value in RAM (processor is writing [value] to [address])

  ram[address] = value;

}

const cpu = new CPU6502({ accessMemory });

// trigger a reset to start the clock & jump to the reset vector

cpu.reset();

```