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Decimal mode for SBC

This commit is contained in:
Sam M W 2021-01-27 22:44:20 +00:00
parent 3844a730f0
commit 51775fbe95
1 changed files with 36 additions and 5 deletions
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41
src/cpu.rs
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@ -581,9 +581,6 @@ impl CPU {
let a_after = a_before.wrapping_sub(value).wrapping_sub(nc);
// The carry flag is set on unsigned overflow.
let did_carry = (a_after as u8) > (a_before as u8);
// The overflow flag is set on two's-complement overflow.
//
// range of A is -128 to 127
@ -599,12 +596,34 @@ impl CPU {
let mask = Status::PS_CARRY | Status::PS_OVERFLOW;
let bcd1: i8 = if (a_before & 0x0f).wrapping_sub(nc) < (value & 0x0f) {
0x06
} else {
0x00
};
let bcd2: i8 = if (a_after.wrapping_sub(bcd1) as u8 & 0xf0) as u8 > 0x90 {
0x60
} else {
0x00
};
let result: i8 = if self.registers.status.contains(Status::PS_DECIMAL_MODE) {
a_after.wrapping_sub(bcd1).wrapping_sub(bcd2)
} else {
a_after
};
// The carry flag is set on unsigned overflow.
let did_carry = (result as u8) > (a_before as u8);
self.registers.status.set_with_mask(
mask,
Status::new(StatusArgs { carry: did_carry, overflow: did_overflow, ..StatusArgs::none() }),
);
self.load_accumulator(a_after);
self.load_accumulator(result);
}
fn decrement_memory(&mut self, addr: Address) {
@ -786,7 +805,19 @@ mod tests {
assert_eq!(cpu.registers.status.contains(Status::PS_ZERO), true);
assert_eq!(cpu.registers.status.contains(Status::PS_NEGATIVE), false);
assert_eq!(cpu.registers.status.contains(Status::PS_OVERFLOW), true);
cpu.subtract_with_carry(0x48);
assert_eq!(cpu.registers.accumulator as u8, 0x52);
assert_eq!(cpu.registers.status.contains(Status::PS_CARRY), true);
assert_eq!(cpu.registers.status.contains(Status::PS_ZERO), false);
assert_eq!(cpu.registers.status.contains(Status::PS_NEGATIVE), false);
assert_eq!(cpu.registers.status.contains(Status::PS_OVERFLOW), false);
cpu.subtract_with_carry(0x43);
assert_eq!(cpu.registers.status.contains(Status::PS_CARRY), false);
assert_eq!(cpu.registers.status.contains(Status::PS_ZERO), false);
assert_eq!(cpu.registers.status.contains(Status::PS_NEGATIVE), false);
assert_eq!(cpu.registers.status.contains(Status::PS_OVERFLOW), false);
}
#[test]