Class: Amaterasu::GameBoy::Cpu::Instructions::Ld16

Inherits:
Base
  • Object
show all
Defined in:
lib/amaterasu/game_boy/cpu/instructions/ld16.rb,
sig/akane/game_boy/cpu/instructions/ld16.rbs

Overview

Holds the logic of all the 16-bit Load instructions.

Instance Attribute Summary

Attributes inherited from Base

#mnemonic

Instance Method Summary collapse

Methods inherited from Base

#execute, #format_operand

Constructor Details

#initialize(cpu:, target:, source:) ⇒ Ld16 



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# File 'lib/amaterasu/game_boy/cpu/instructions/ld16.rb', line 9

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

  @mnemonic = "LD #{format_operand(target)}, #{format_operand(source)}"
  @logic    = build_logic(target, source)
end

Instance Method Details

#build_logic(target, source) ⇒ Proc 



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# File 'lib/amaterasu/game_boy/cpu/instructions/ld16.rb', line 18

def build_logic(target, source)
  return -> { ld_mem_imm16_sp } if target == :mem_imm16

  case target
  when :bc then -> { @registers.bc = @cpu.fetch_next_word }
  when :de then -> { @registers.de = @cpu.fetch_next_word }
  when :hl
    case source
    when :imm16 then -> { @registers.hl = @cpu.fetch_next_word }
    when :sp_plus_sig8
      lambda do
        unsigned_byte = @cpu.fetch_next_byte
        signed_value = @cpu.sign_value(unsigned_byte)
        sp = @registers.sp
        result = @cpu.add16(@registers.sp, signed_value)

        @registers.clear_flags
        @registers.h_flag = (sp & 0x0F) + (unsigned_byte & 0x0F) > 0x0F
        @registers.c_flag = (sp & 0xFF) + (unsigned_byte & 0xFF) > 0xFF

        @registers.hl = result
      end
    else
      raise ArgumentError, 'Unknown Ld16 source for LD HL'
    end
  when :sp
    case source
    when :imm16 then -> { @registers.sp = @cpu.fetch_next_word }
    when :hl
      lambda do
        @cpu.internal_processing
        @registers.sp = @registers.hl
      end
    else
      raise ArgumentError, 'Unknown Ld16 source for LD SP'
    end
  else
    raise ArgumentError, 'Unknown Ld16 target'
  end
end

#ld_mem_imm16_sp ⇒ void

This method returns an undefined value.

Loads the LSB from SP to the imm16 address. Loads the MSB from SP to the imm16 address + 1.

M-cycle 1: Fetches the opcode. M-cycle 2: Fetches the LSB of the address to be used. M-cycle 3: Fetches the MSB of the address to be used. M-cycle 4: Writes the LSB of SP into the address. M-cycle 5: Writes the MSB of SP into the address + 1.



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# File 'lib/amaterasu/game_boy/cpu/instructions/ld16.rb', line 68

def ld_mem_imm16_sp
  imm16 = @cpu.fetch_next_word
  sp = @registers.sp

  @cpu.bus_write(address: imm16, value: sp & 0xFF)
  @cpu.bus_write(address: imm16 + 1, value: (sp >> 8) & 0xFF)
end