Class: Amaterasu::GameBoy::Cpu::Instructions::CbBit

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

Overview

Holds the logic of all the BIT instructions.

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:, bit_pos:, target:) ⇒ CbBit

Returns a new instance of CbBit.

Parameters:

  • cpu: (Cpu)
  • bit_pos: (Integer)
  • target: (Symbol) 


11
12
13
14
15
16
 
# File 'lib/amaterasu/game_boy/cpu/instructions/cb_bit.rb', line 11

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

  @mnemonic = "BIT #{bit_pos}, #{format_operand(target)}"
  @logic    = build_logic(bit_pos, target)
end

Instance Method Details

#bit_test(bit_pos, target_value) ⇒ void

This method returns an undefined value.

Checks the bit value from a given target at a given position [0-7].

  • Sets the zero flag if bit tested is 0, otherwise clears the flag.
  • Subtraction flag is always cleared.
  • Half Carry flag is always set.
  • Carry flag is untouched.

Parameters:

  • bit (Integer)
  • target_value (Integer) 


41
42
43
44
45
46
47
 
# File 'lib/amaterasu/game_boy/cpu/instructions/cb_bit.rb', line 41

def bit_test(bit_pos, target_value)
  bit_value = bit(target_value, bit_pos)

  @registers.z_flag = bit_value.zero?
  @registers.n_flag = false
  @registers.h_flag = true
end

#build_logic(bit_pos, target) ⇒ Proc

Parameters:

  • bit_pos (Integer)
  • target (Symbol)

Returns:

  • (Proc) 


20
21
22
23
24
25
26
27
28
29
30
31
32
33
 
# File 'lib/amaterasu/game_boy/cpu/instructions/cb_bit.rb', line 20

def build_logic(bit_pos, target)
  case target
  when :b      then -> { bit_test(bit_pos, @registers.b) }
  when :c      then -> { bit_test(bit_pos, @registers.c) }
  when :d      then -> { bit_test(bit_pos, @registers.d) }
  when :e      then -> { bit_test(bit_pos, @registers.e) }
  when :h      then -> { bit_test(bit_pos, @registers.h) }
  when :l      then -> { bit_test(bit_pos, @registers.l) }
  when :mem_hl then -> { bit_test(bit_pos, @cpu.bus_read(address: @registers.hl)) }
  when :a      then -> { bit_test(bit_pos, @registers.a) }
  else
    raise ArgumentError, 'Unknown CbBit target'
  end
end