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rewrite SBC decimal mode - now passes the 2 tests

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This commit is contained in:
Ed Spittles 2010-12-13 13:39:47 +00:00
parent 93d07b88ba
commit 80c9d10a54
1 changed files with 31 additions and 13 deletions
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44
src/py65/devices/mpu6502.py
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@ -373,21 +373,39 @@ class MPU:
data = self.ByteAt(x())
if self.p & self.DECIMAL:
if self.p & self.CARRY:
borrow = 0
else:
borrow = 1
data = convert_to_bin(self.a) - convert_to_bin(data) - borrow
halfcarry = 1
decimalcarry = 0
adjust0 = 0
adjust1 = 0
nibble0 = (self.a & 0xf) + (~data & 0xf) + (self.p & self.CARRY)
if nibble0 <= 0xf:
halfcarry = 0
adjust0 = 10
nibble1 = ((self.a >> 4) & 0xf) + ((~data >> 4) & 0xf) + halfcarry
if nibble1 <= 0xf:
adjust1 = 10 << 4
# the ALU outputs are not decimally adjusted
aluresult = self.a + (~data & 0xFF) + (self.p & self.CARRY)
if aluresult > 0xff:
decimalcarry = 1
aluresult &= 0xff
# but the final result will be adjusted
nibble0 = (aluresult + adjust0) & 0xf
nibble1 = ((aluresult + adjust1) >> 4) & 0xf
self.p &= ~(self.CARRY + self.ZERO + self.NEGATIVE + self.OVERFLOW)
if data == 0:
self.p |= self.ZERO + self.CARRY
elif data > 0:
self.p |= self.CARRY
if aluresult == 0:
self.p |= self.ZERO
else:
self.p |= self.NEGATIVE
data +=100
self.a = convert_to_bcd(data) # throws exception if result is over 99 decimal
self.p |= aluresult & self.NEGATIVE
if decimalcarry == 1:
self.p |= self.CARRY
if ( ~(self.a ^ data) & (self.a ^ aluresult) ) & 0x80:
self.p |= self.OVERFLOW
self.a = (nibble1 << 4) + nibble0
else:
if self.p & self.CARRY:
borrow = 0