Module: Badline::InstructionSet::Arithmetic

Included in:
Badline::InstructionSet
Defined in:
lib/badline/instruction_set/arithmetic.rb

Instance Method Summary collapse

Instance Method Details

#adc(_addr, value) ⇒ Object

Add memory with carry to accumulator.

Opcodes:

$61 - indirect_x - 6 cycles
$65 - zeropage   - 3 cycles
$69 - immediate  - 2 cycles
$6D - absolute   - 4 cycles
$71 - indirect_y - 5+ cycles
$75 - zeropage_x - 4 cycles
$79 - absolute_y - 4+ cycles
$7D - absolute_x - 4+ cycles



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# File 'lib/badline/instruction_set/arithmetic.rb', line 17

def adc(_addr, value)
  v = resolve(value)
  result = a + v + status.carry
  status.zero = result.nobits?(0xff)

  if status.decimal?
    result = (a & 0x0f) + (v & 0x0f) + status.carry
    result += 0x06 if result > 0x09
    c = result > 0x0f ? 1 : 0
    result = (a & 0xf0) + (v & 0xf0) + (c << 4) + (result & 0x0f)
    update_calculation_flags(v, result)
    result += 0x60 if result > 0x9f
  else
    update_calculation_flags(v, result)
  end

  status.carry = result > 0xff
  @a = result & 0xff
end

#cmp(_addr, value) ⇒ Object

Compare memory with accumulator.

Opcodes:

$C1 - indirect_x - 6 cycles
$C5 - zeropage   - 3 cycles
$C9 - immediate  - 2 cycles
$CD - absolute   - 4 cycles
$D1 - indirect_y - 5+ cycles
$D5 - zeropage_x - 4 cycles
$D9 - absolute_y - 4+ cycles
$DD - absolute_x - 4+ cycles



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# File 'lib/badline/instruction_set/arithmetic.rb', line 48

def cmp(_addr, value)
  v = resolve(value)
  status.carry = (@a >= v)
  update_number_flags(@a - v)
end

#cpx(_addr, value) ⇒ Object

Compare memory with X register.

Opcodes:

$E0 - immediate  - 2 cycles
$E4 - zeropage   - 3 cycles
$EC - absolute   - 4 cycles



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# File 'lib/badline/instruction_set/arithmetic.rb', line 60

def cpx(_addr, value)
  v = resolve(value)
  status.carry = @x >= v
  update_number_flags(@x - v)
end

#cpy(_addr, value) ⇒ Object

Compare memory with Y register.

Opcodes:

$C0 - immediate  - 2 cycles
$C4 - zeropage   - 3 cycles
$CC - absolute   - 4 cycles



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# File 'lib/badline/instruction_set/arithmetic.rb', line 72

def cpy(_addr, value)
  v = resolve(value)
  status.carry = @y >= v
  update_number_flags(@y - v)
end

#sbc(_addr, value) ⇒ Object

Subtract memory from accumulator with borrow.

Opcodes:

$E1 - indirect_x - 6 cycles
$E5 - zeropage   - 3 cycles
$E9 - immediate  - 2 cycles
$ED - absolute   - 4 cycles
$F1 - indirect_y - 5+ cycles
$F5 - zeropage_x - 4 cycles
$F9 - absolute_y - 4+ cycles
$FD - absolute_x - 4+ cycles



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# File 'lib/badline/instruction_set/arithmetic.rb', line 89

def sbc(_addr, value)
  v = resolve(value)
  v_inv = ~v & 0xff
  carry = status.carry? ? 0 : -1

  result = a + v_inv + status.carry
  status.zero = result.nobits?(0xff)
  status.carry = result > 0xff
  update_calculation_flags(v_inv, result)

  if status.decimal?
    al = (a & 0x0f) - (v & 0x0f) + carry
    al = ((al - 0x06) & 0x0F) - 0x10 if al.negative?

    result = (a & 0xf0) - (v & 0xf0) + al
    result -= 0x60 if result.negative?
  end

  @a = result & 0xff
end