Class: Amaterasu::GameBoy::Cpu::Instructions::CbSwap

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

Overview

Holds the logic of all the SWAP instructions.

  • SWAP r8
  • SWAP [HL]

Instance Attribute Summary

Attributes inherited from Base

#mnemonic

Instance Method Summary collapse

Methods included from Utils::BitOps

bit, clear_bit, #self?.bit, #self?.clear_bit, #self?.set_bit, set_bit

Methods inherited from Base

#execute, #format_operand

Constructor Details

#initialize(cpu:, target:) ⇒ CbSwap

Returns a new instance of CbSwap.

Parameters:

  • cpu: (Cpu)
  • target: (Symbol) 


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# File 'lib/amaterasu/game_boy/cpu/instructions/cb_swap.rb', line 14

def initialize(cpu:, target:)
  super(cpu:)

  @mnemonic = "SWAP #{format_operand(target)}"
  @logic    = build_logic(target)
end

Instance Method Details

#build_logic(target) ⇒ Proc

Parameters:

  • target (Symbol)

Returns:

  • (Proc) 


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# File 'lib/amaterasu/game_boy/cpu/instructions/cb_swap.rb', line 23

def build_logic(target)
  case target
  when :b      then -> { @registers.b = swap(@registers.b) }
  when :c      then -> { @registers.c = swap(@registers.c) }
  when :d      then -> { @registers.d = swap(@registers.d) }
  when :e      then -> { @registers.e = swap(@registers.e) }
  when :h      then -> { @registers.h = swap(@registers.h) }
  when :l      then -> { @registers.l = swap(@registers.l) }
  when :mem_hl then -> { swap_mem_hl }
  when :a      then -> { @registers.a = swap(@registers.a) }
  else
    raise ArgumentError, 'Unknown CbSwap target'
  end
end

#swap(target) ⇒ void

This method returns an undefined value.

Swaps the positions of the Upper and Lower 4 bits.

11110000 -> 00001111

Parameters:

  • reg8 (Integer) 


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# File 'lib/amaterasu/game_boy/cpu/instructions/cb_swap.rb', line 42

def swap(target)
  upper4 = (target >> 4) & 0x0F
  result = (target << 4) | upper4

  @registers.clear_flags
  @registers.z_flag = result.nobits?(0xFF)

  result
end

#swap_mem_hl ⇒ void

This method returns an undefined value.

Takes 2 extra cycles due to the Bus read and write operations.



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# File 'lib/amaterasu/game_boy/cpu/instructions/cb_swap.rb', line 53

def swap_mem_hl
  byte = @cpu.bus_read(address: @registers.hl)
  upper4 = (byte >> 4) & 0x0F
  result = (byte << 4) | upper4

  @registers.clear_flags
  @registers.z_flag = result.nobits?(0xFF)

  @cpu.bus_write(address: @registers.hl, value: result)
end