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Fix overflow test for ASL.

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This commit is contained in:
Thomas Harte 2022-10-18 11:47:36 -04:00
parent a1ae7c28b2
commit 5560a0ed39
1 changed files with 9 additions and 3 deletions
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12
InstructionSets/M68k/Implementation/PerformImplementation.hpp
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@ -361,10 +361,16 @@ template <Operation operation, typename IntT, typename FlowController> void shif
if(type == Type::LSL) {
status.overflow_flag = 0;
} else {
// Overflow records whether the top bit changed at any point during the operation.
if(shift >= size) {
status.overflow_flag = destination & top_bit<IntT>();
// The result is going to be all bits evacuated through the top giving a net
// result of 0, so overflow is set if any bit was originally set.
status.overflow_flag = destination;
} else {
status.overflow_flag = (destination ^ (destination << shift)) & top_bit<IntT>();
// For a shift of n places, overflow will be set if the top n bits were not
// all the same value.
const auto affected_bits = IntT(~0 << shift);
status.overflow_flag = (destination & affected_bits) && (destination & affected_bits) != affected_bits;
}
}
@ -382,7 +388,7 @@ template <Operation operation, typename IntT, typename FlowController> void shif
} else {
status.carry_flag = status.extend_flag = (destination >> (shift - 1)) & 1;
}
status.overflow_flag = 0;
status.overflow_flag = 0; // The top bit can't change during an ASR, and LSR always clears overflow.
const IntT sign_word =
type == Type::LSR ?