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Add compare(). Add more cases to compare_test().

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This commit is contained in:
Andrew Keeton 2014-11-13 20:08:56 -05:00
parent 233c36ce7e
commit 8b962fd542
1 changed files with 54 additions and 9 deletions
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63
src/machine.rs
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@ -619,6 +619,37 @@ impl Machine {
}
}
// From http://www.6502.org/tutorials/compare_beyond.html:
// If the Z flag is 0, then A <> NUM and BNE will branch
// If the Z flag is 1, then A = NUM and BEQ will branch
// If the C flag is 0, then A (unsigned) < NUM (unsigned) and BCC will branch
// If the C flag is 1, then A (unsigned) >= NUM (unsigned) and BCS will branch
// ...
// The N flag contains most significant bit of the of the subtraction result.
fn compare(&mut self, val: u8) {
let a = self.registers.accumulator;
if a as u8 >= val as u8 {
self.registers.status.insert(PS_CARRY);
} else {
self.registers.status.remove(PS_CARRY);
}
if a as i8 == val as i8 {
self.registers.status.insert(PS_ZERO);
} else {
self.registers.status.remove(PS_ZERO);
}
let diff: i8 = (a as i8) - (val as i8);
if diff < 0 {
self.registers.status.insert(PS_NEGATIVE);
} else {
self.registers.status.remove(PS_NEGATIVE);
}
}
fn push_on_stack(&mut self, val: u8) {
let addr = self.registers.stack_pointer.to_address();
self.memory.set_byte(addr, val);
@ -1044,6 +1075,7 @@ fn compare_test() {
assert!( machine.registers.status.contains(PS_CARRY ));
assert!(!machine.registers.status.contains(PS_NEGATIVE));
machine.execute_instruction(
(instruction::LDA, instruction::UseImmediate(127))
);
@ -1053,14 +1085,6 @@ fn compare_test() {
assert!( machine.registers.status.contains(PS_CARRY ));
assert!(!machine.registers.status.contains(PS_NEGATIVE));
machine.execute_instruction(
(instruction::LDA, instruction::UseImmediate(1))
);
machine.compare(2);
assert!(!machine.registers.status.contains(PS_ZERO ));
assert!(!machine.registers.status.contains(PS_CARRY ));
assert!(!machine.registers.status.contains(PS_NEGATIVE));
machine.execute_instruction(
(instruction::LDA, instruction::UseImmediate(1))
@ -1071,12 +1095,33 @@ fn compare_test() {
assert!(!machine.registers.status.contains(PS_CARRY ));
assert!( machine.registers.status.contains(PS_NEGATIVE));
machine.execute_instruction(
(instruction::LDA, instruction::UseImmediate(20))
);
machine.compare(-50);
assert!(!machine.registers.status.contains(PS_ZERO ));
assert!( machine.registers.status.contains(PS_CARRY ));
assert!(!machine.registers.status.contains(PS_CARRY ));
assert!(!machine.registers.status.contains(PS_NEGATIVE));
machine.execute_instruction(
(instruction::LDA, instruction::UseImmediate(1))
);
machine.compare(-1);
assert!(!machine.registers.status.contains(PS_ZERO ));
assert!(!machine.registers.status.contains(PS_CARRY ));
assert!(!machine.registers.status.contains(PS_NEGATIVE));
machine.execute_instruction(
(instruction::LDA, instruction::UseImmediate(127))
);
machine.compare(-128);
assert!(!machine.registers.status.contains(PS_ZERO ));
assert!(!machine.registers.status.contains(PS_CARRY ));
assert!( machine.registers.status.contains(PS_NEGATIVE));
}