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fixed the carry flag in SBC

This commit is contained in:
jborza 2019-04-26 12:14:25 +02:00
parent 0d932b7f61
commit a1259fa544
2 changed files with 7 additions and 6 deletions
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2
cpu.c
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@ -150,7 +150,7 @@ void SBC(State6502 * state, byte operand) {
state->a = result;
state->flags.n = is_negative(state->a);
state->flags.z = state->a == 0;
state->flags.c = result_word > 0xFF;
state->flags.c = result_word <= 0xFF;
}
void ADC(State6502 * state, byte operand) {

11
test6502.c
View File

@ -2078,11 +2078,12 @@ void test_SBC_IMM_(byte a, byte c, byte operand, byte expected_a, byte expected_
void test_SBC_IMM_multiple() {
//A, C, OP => A, N, Z, C, V
test_SBC_IMM_(0x08, 0, /* OP */ 0x01, /*A*/ 0x06, /*N*/ 0, /*Z*/ 0, /*C*/ 0, /*V*/ 0); //regular subtraction without carry bit
test_SBC_IMM_(0x08, 1, /* OP */ 0x01, /*A*/ 0x07, /*N*/ 0, /*Z*/ 0, /*C*/ 0, /*V*/ 0); //regular subtraction with carry bit
test_SBC_IMM_(0x0A, 1, /* OP */ 0x0A, /*A*/ 0x00, /*N*/ 0, /*Z*/ 1, /*C*/ 0, /*V*/ 0); //zero
test_SBC_IMM_(0x04, 1, /* OP */ 0x06, /*A*/ 0xFE, /*N*/ 1, /*Z*/ 0, /*C*/ 1, /*V*/ 0); //borrow from carry bit - 4 + 256 - 6 = 255
test_SBC_IMM_(0x04, 0, /* OP */ 0x06, /*A*/ 0xFD, /*N*/ 1, /*Z*/ 0, /*C*/ 1, /*V*/ 0); //borrow from carry bit - 4 + 256 - 6 = 254
test_SBC_IMM_(0x08, 0, /* OP */ 0x01, /*A*/ 0x06, /*N*/ 0, /*Z*/ 0, /*C*/ 1, /*V*/ 0); //regular subtraction without carry bit
test_SBC_IMM_(0x08, 1, /* OP */ 0x01, /*A*/ 0x07, /*N*/ 0, /*Z*/ 0, /*C*/ 1, /*V*/ 0); //regular subtraction with carry bit
test_SBC_IMM_(0x0A, 1, /* OP */ 0x0A, /*A*/ 0x00, /*N*/ 0, /*Z*/ 1, /*C*/ 1, /*V*/ 0); //zero
test_SBC_IMM_(0x0A, 0, /* OP */ 0x0A, /*A*/ 0xFF, /*N*/ 1, /*Z*/ 0, /*C*/ 0, /*V*/ 0); //negative
test_SBC_IMM_(0x04, 1, /* OP */ 0x06, /*A*/ 0xFE, /*N*/ 1, /*Z*/ 0, /*C*/ 0, /*V*/ 0); //borrow from carry bit - 4 + 256 - 6 = 255
test_SBC_IMM_(0x04, 0, /* OP */ 0x06, /*A*/ 0xFD, /*N*/ 1, /*Z*/ 0, /*C*/ 0, /*V*/ 0); //borrow from carry bit - 4 + 256 - 6 = 254
//source: http://www.righto.com/2012/12/the-6502-overflow-flag-explained.html
//C7 - carry, B - Borrow, S7 - N, V - V
test_SBC_IMM_(0x50, 1, /* OP */ 0xF0, /*A*/ 0x60, /*N*/ 0, /*Z*/ 0, /*C*/ 0, /*V*/ 0); //Unsigned borrow but no signed overflow