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Fix ADD/SUBX carry.

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This commit is contained in:
Thomas Harte 2022-10-19 22:17:51 -04:00
parent df7f94f362
commit ec728ad573
1 changed files with 17 additions and 6 deletions
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23
InstructionSets/M68k/Implementation/PerformImplementation.hpp
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@ -55,10 +55,23 @@ template <bool is_add, bool is_extend, typename IntT>
static void add_sub(IntT source, IntT &destination, Status &status) {
static_assert(!std::numeric_limits<IntT>::is_signed);
const IntT extend = (is_extend && status.extend_flag) ? 1 : 0;
const IntT result = is_add ?
(destination + source + extend) :
(destination - source - extend);
IntT result = is_add ?
destination + source :
destination - source;
status.carry_flag = is_add ? result < destination : result > destination;
// If this is an extend operation, there's a second opportunity to create carry,
// which requires a second test.
if(is_extend && status.extend_flag) {
if constexpr (is_add) {
++result;
status.carry_flag |= result == 0;
} else {
status.carry_flag |= result == 0;
--result;
}
}
status.extend_flag = status.carry_flag;
// Extend operations can reset the zero flag only; non-extend operations
// can either set it or reset it. Which in the reverse-logic world of
@ -68,8 +81,6 @@ static void add_sub(IntT source, IntT &destination, Status &status) {
} else {
status.zero_result = Status::FlagT(result);
}
status.extend_flag =
status.carry_flag = is_add ? result < destination : result > destination;
status.set_negative(result);
status.overflow_flag = overflow<is_add>(source, destination, result);
destination = result;