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Decimal SBC tweak: negative partial results don't cause carry.

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Thomas Harte 2022-06-23 21:58:09 -04:00
parent 2e7afb13c7
commit 4ed3b21bf3
1 changed files with 11 additions and 3 deletions
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14
Processors/65816/Implementation/65816Implementation.hpp
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@ -918,14 +918,22 @@ template <typename BusHandler, bool uses_ready_line> void Processor<BusHandler,
const uint16_t a = registers_.a.full & registers_.m_masks[1];
data_buffer_.value = ~data_buffer_.value & registers_.m_masks[1];
unsigned int result = registers_.flags.carry;
int result = registers_.flags.carry;
uint16_t partials = 0;
#define nibble(mask, adjustment, carry) \
result += (a & mask) + (data_buffer_.value & mask); \
partials += result & mask; \
if(result < carry) result -= adjustment; \
result &= (carry | (carry - 1));
result -= ((result - carry) >> 16) & adjustment; \
result &= (carry & ~(result >> 31)) | (carry - 1);
// i.e. add the next nibble to that in the accumulator, with carry, and
// store it to result. Keep a copy for the partials.
//
// If result is less than carry, subtract adjustment.
//
// Allow onward carry if the bit immediately above this nibble is 1, and
// the current partial result is positive.
nibble(0x000f, 0x0006, 0x00010);
nibble(0x00f0, 0x0060, 0x00100);