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Thomas Harte
2022-10-19 14:40:29 -04:00
parent 979bf42541
commit bc9ddacb8d
1 changed files with 4 additions and 2 deletions
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6
InstructionSets/M68k/Implementation/PerformImplementation.hpp
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@@ -357,11 +357,13 @@ template <Operation operation, typename IntT, typename FlowController> void shif
// result of 0, so overflow is set if any bit was originally set. // result of 0, so overflow is set if any bit was originally set.
status.overflow_flag = destination; status.overflow_flag = destination;
} else { } else {
// For a shift of n places, overflow will be set if the top n bits were not // For a shift of n places, overflow will be set if the top n+1 bits were not
// all the same value. // all the same value.
const auto affected_bits = IntT( const auto affected_bits = IntT(
~((top_bit<IntT>() >> shift) - 1) ~((top_bit<IntT>() >> shift) - 1)
); ); // e.g. shift = 1 => ~((0x80 >> 1) - 1) = ~(0x40 - 1) = ~0x3f = 0xc0, i.e. if shift is
// 1 then the top two bits are relevant to whether there was overflow. If they have the
// same value, i.e. are both 0 or are both 1, then there wasn't. Otherwise there was.
status.overflow_flag = (destination & affected_bits) && (destination & affected_bits) != affected_bits; status.overflow_flag = (destination & affected_bits) && (destination & affected_bits) != affected_bits;
} }
} }