@soapbox.pub/wasmboy/core/cpu/flags.ts

Soapbox fork of Wasmboy.

import { Cpu } from './index';
import { u8Portable, u16Portable } from '../portable/portable';

// Set flag bit on on register F. For instance set zero flag to zero -> (7, 0)
function setFlagBit(flagBit: u8, flagValue: i32): u8 {
  let bitwiseOperand = u8Portable(1 << flagBit);
  if (flagValue > 0) {
    Cpu.registerF = Cpu.registerF | bitwiseOperand;
  } else {
    // XOR out the two ones
    bitwiseOperand = 0xff ^ bitwiseOperand;
    Cpu.registerF = Cpu.registerF & bitwiseOperand;
  }

  return Cpu.registerF;
}

// Overload the set flag bit for ease of use
export function setZeroFlag(value: i32): void {
  setFlagBit(7, value);
}

export function setSubtractFlag(value: i32): void {
  setFlagBit(6, value);
}

export function setHalfCarryFlag(value: i32): void {
  setFlagBit(5, value);
}

export function setCarryFlag(value: i32): void {
  setFlagBit(4, value);
}

// Getters for flags
export function getZeroFlag(): u8 {
  return (Cpu.registerF >> 7) & 0x01;
}

export function getSubtractFlag(): u8 {
  return (Cpu.registerF >> 6) & 0x01;
}

export function getHalfCarryFlag(): u8 {
  return (Cpu.registerF >> 5) & 0x01;
}

export function getCarryFlag(): u8 {
  return (Cpu.registerF >> 4) & 0x01;
}

// Must be run before the register actually performs the add
// amountToAdd i16, since max number can be an u8
export function checkAndSetEightBitHalfCarryFlag(value: u8, amountToAdd: i32): void {
  if (amountToAdd >= 0) {
    // https://robdor.com/2016/08/10/gameboy-emulator-half-carry-flag/
    let result = u8Portable(((<u8>value) & 0x0f) + ((<u8>amountToAdd) & 0x0f)) & 0x10;
    setHalfCarryFlag(<i32>(result !== 0x00));
  } else {
    // From: https://github.com/djhworld/gomeboycolor/blob/master/src/cpu/index.go
    // CTRL+F "subBytes(a, b byte)"
    setHalfCarryFlag(<i32>(<u8>(abs(amountToAdd) & 0x0f) > (value & 0x0f)));
  }
}

export function checkAndSetEightBitCarryFlag(value: u8, amountToAdd: i32): void {
  if (amountToAdd >= 0) {
    let result = u8Portable(value + <u8>amountToAdd);
    setCarryFlag(<i32>(value > result));
  } else {
    setCarryFlag(<i32>(abs(amountToAdd) > <i32>value));
  }
}

// Function to handle 16 bit addition overflow, and set the carry flags accordingly
// i32 on valueTwo to support passing signed immedaite values
export function checkAndSetSixteenBitFlagsAddOverflow(valueOne: u16, valueTwo: i32, useStackPointerBits: boolean): void {
  // need to differentiate between HL and SP
  // HL carries are at 11 and 15, SP carries are at 3 and 7 :p
  if (useStackPointerBits) {
    // Logic from : https://github.com/nakardo/node-gameboy/blob/master/lib/cpu/opcodes.js
    // CTRL+F add_sp_n
    // using the stack pointer bits means we can safely assume the value is signed
    let signedValueOne = <i32>valueOne;
    let result = signedValueOne + valueTwo;
    let flagXor = signedValueOne ^ valueTwo ^ result;

    setHalfCarryFlag(<i32>((flagXor & 0x10) !== 0));
    setCarryFlag(<i32>((flagXor & 0x100) !== 0));
  } else {
    // Logic from: https://github.com/djhworld/gomeboycolor/blob/master/src/cpu/index.go
    // CTRL+F addWords
    // Value two is not signed
    let result = u16Portable(valueOne + <u16>valueTwo);

    // Check the carry flag by allowing the overflow
    setCarryFlag(<i32>(result < valueOne));

    // To check for half carry flag (bit 15), by XOR'ing valyes, and and'ing the bit in question
    let halfCarryXor: u16 = valueOne ^ (<u16>valueTwo) ^ (<u16>result);
    let halfCarryAnd = u16Portable(halfCarryXor & 0x1000);
    setHalfCarryFlag(<i32>(halfCarryAnd !== 0x00));
  }
}