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ADC implementation (without the overflow flag, as on SBC)

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jborza 2019-04-21 09:28:07 +02:00
parent a11d260f98
commit 80d7e2efea
1 changed files with 22 additions and 9 deletions
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31
cpu.c
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@ -138,7 +138,20 @@ void SBC(State6502* state, byte operand) {
state->a -= result;
state->flags.n = is_negative(state->a);
state->flags.z = state->a == 0;
//state->flags.v = ??? ; //
//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;
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) {
@ -344,14 +357,14 @@ byte get_byte_indirect_y(State6502 * state) {
int emulate_6502_op(State6502 * state) {
byte* opcode = &state->memory[state->pc++];
switch (*opcode) {
case ADC_IMM: unimplemented_instruction(state); break;
case ADC_ZP: unimplemented_instruction(state); break;
case ADC_ZPX: unimplemented_instruction(state); break;
case ADC_ABS: unimplemented_instruction(state); break;
case ADC_ABSX: unimplemented_instruction(state); break;
case ADC_ABSY: unimplemented_instruction(state); break;
case ADC_INDX: unimplemented_instruction(state); break;
case ADC_INDY: unimplemented_instruction(state); break;
case ADC_IMM: ADC(state, pop_byte(state)); break;
case ADC_ZP: ADC(state, get_byte_zero_page(state)); break;
case ADC_ZPX: ADC(state, get_byte_zero_page_x(state)); break;
case ADC_ABS: ADC(state, get_byte_absolute(state)); break;
case ADC_ABSX: ADC(state, get_byte_absolute_x(state)); break;
case ADC_ABSY: ADC(state, get_byte_absolute_y(state)); break;
case ADC_INDX: ADC(state, get_byte_indirect_x(state)); break;
case ADC_INDY: ADC(state, get_byte_indirect_y(state)); break;
case AND_IMM: AND(state, pop_byte(state)); break;
case AND_ZP: AND(state, get_byte_zero_page(state)); break;
case AND_ZPX: AND(state, get_byte_zero_page_x(state)); break;