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implemented the overflow flag for ADC and SBC

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jborza 2019-04-21 09:38:38 +02:00
parent 80d7e2efea
commit f5fd1d21c5
1 changed files with 7 additions and 5 deletions
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12
cpu.c
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@ -135,23 +135,25 @@ void SBC(State6502* state, byte operand) {
}
word result = state->a - operand_word;
// overflow flag if the the result doesn't fit into the signed byte range -128 to 127
state->flags.v = (state->a ^ operand) & 0x80) && ((state->a ^ (result) & 0x80);
state->a -= result;
state->flags.n = is_negative(state->a);
state->flags.z = state->a == 0;
//todo set overflow flag
//state->flags.v = ??? ;
}
void ADC(State6502* state, byte operand) {
//add operand to A
word result_word = operand + state->a + state->flags.c ? 1 : 0;
byte result = result_word & 0xFF;
//set overflow flag if the result's sign would change - the result doesn't fit into a signed byte
//there is overflow if the inputs do not have different signs and the input sign is different from the output sign
state->flags.v = !((state->a ^ operand) & 0x80) && ((state->a ^ result) & 0x80);
state->a = state->a & 0xFF;
state->flags.n = is_negative(state->a);
state->flags.z = state->a == 0;
state->flags.c = result_word > 0xFF;
//todo set overflow flag if the result's sign would change - the result doesn't fit into a signed byte
//!((M^N) & 0x80) && ((M^result) & 0x80) - there is overflow if the inputs do not have different signs and the input sign is different from the output sign
//state->flags.v = ??? ;
}
void cmp_internal(State6502 * state, byte register_value, byte operand) {