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