Class: Amaterasu::GameBoy::Cpu::Instructions::Daa

Inherits:
Base
  • Object
show all
Defined in:
lib/amaterasu/game_boy/cpu/instructions/daa.rb,
sig/akane/game_boy/cpu/instructions/daa.rbs

Overview

Holds the logic for DAA instruction.

Instance Attribute Summary

Attributes inherited from Base

#mnemonic

Instance Method Summary collapse

Methods inherited from Base

#execute, #format_operand

Constructor Details

#initialize(cpu:) ⇒ Daa 



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# File 'lib/amaterasu/game_boy/cpu/instructions/daa.rb', line 9

def initialize(cpu:)
  super

  @mnemonic = 'DAA'
  @logic    = -> { daa }
end

Instance Method Details

#daa ⇒ void

This method returns an undefined value.

Decimal Adjust Accumulator.

Is used after performing a given arithmetic operation (ADD, SUB, ADC, SBC) whose inputs were in Binary-Coded Decimal (BCD). So after running this instruction the numbers would be adjusted to BCD format.

This instruction never clears the Carry flag (1 => 0). It either preserves the value (0 => 0), (1 => 1) or sets it (0 => 1).



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# File 'lib/amaterasu/game_boy/cpu/instructions/daa.rb', line 27

def daa
  acc = @registers.a
  carry_in = @registers.c_flag
  new_carry = carry_in == 1
  bcd_correction = 0x00

  if @registers.n_flag.zero?
    bcd_correction |= 0x06 if @registers.h_flag == 1 || (acc & 0x0F) > 0x09

    if @registers.c_flag == 1 || (acc & 0xFF) > 0x99
      bcd_correction |= 0x60
      new_carry = true
    end

    bcd_adjusted = acc + bcd_correction
  else
    bcd_correction |= 0x06 if @registers.h_flag == 1
    bcd_correction |= 0x60 if @registers.c_flag == 1

    bcd_adjusted = acc - bcd_correction
  end

  @registers.z_flag = bcd_adjusted.nobits?(0xFF)
  @registers.h_flag = false
  @registers.c_flag = new_carry

  @registers.a = bcd_adjusted
end