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reimplemented ADC and SBC based on 2006/2007 code

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James Tauber 2011-08-06 22:35:45 -04:00
parent 32120dffef
commit 6fb2d79cda
1 changed files with 37 additions and 12 deletions
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applepy.py
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@ -586,22 +586,47 @@ class CPU:
def ADC(self, operand_address):
# @@@ doesn't handle BCD yet
assert not self.decimal_mode_flag
a1 = self.accumulator
a2 = self.memory.read_byte(operand_address)
result = a1 + a2 + self.carry_flag
self.accumulator = self.update_nzc(result)
self.overflow_flag = self.carry_flag ^ self.sign_flag
a1 = a2 = self.accumulator
if a1 & 0x80:
a1 = (a1 & 0x7F) - 0x80
m1 = m2 = self.memory.read_byte(operand_address)
if m1 & 0x80:
m1 = (m1 & 0x7F) - 0x80
# twos complement addition
result1 = a1 + m1 + self.carry_flag
# unsigned addition
result2 = a2 + m2 + self.carry_flag
self.accumulator = self.update_nzc(result2)
# perhaps this could be calculated from result2 but result1 is more intuitive
self.overflow_flag = (result1 > 127) | (result1 < -128)
def SBC(self, operand_address):
# @@@ doesn't handle BCD yet
assert not self.decimal_mode_flag
s1 = self.accumulator
s2 = self.memory.read_byte(operand_address)
result = s1 - s2
if not self.carry_flag:
result = result - 1
self.accumulator = self.update_nz(result) # @@@ carry flag?
self.overflow_flag = self.carry_flag ^ self.sign_flag
a1 = a2 = self.accumulator
if a1 & 0x80:
a1 = (a1 & 0x7F) - 0x80
m1 = m2 = self.memory.read_byte(operand_address)
if m1 & 0x80:
m1 = (m1 & 0x7F) - 0x80
# twos complement subtraction
result1 = a1 - m1 - self.carry_flag
# unsigned subtraction
result2 = a2 - m2 - self.carry_flag
self.accumulator = self.update_nz(result2)
self.carry_flag = (result2 >= 0)
# perhaps this could be calculated from result2 but result1 is more intuitive
self.overflow_flag = (result1 > 127) | (result1 < -128)
# BIT