subtraction

src/emulator/instructions/arithmetic.rs

@@ -25,6 +25,19 @@ pub fn add(byte: u8, carry: bool, state: &mut EmulatorState) {
     state.a = result;
 }
 
+pub fn sub_reg(register: Register, borrow: bool, state: &mut EmulatorState) {
+    sub(get_register(register, state), borrow, state);
+}
+
+pub fn sub(byte: u8, borrow: bool, state: &mut EmulatorState) {
+    let (a, first) = state.a.overflowing_sub(byte);
+    let (result, second) = a.overflowing_sub(borrow as u8);
+
+    state.cc.c = first != second;
+    set_cc(state, result);
+    state.a = result;
+}
+
 /// Double precision add - Add B&C, D&E or H&L to H&L
 pub fn dad(register: Register, state: &mut EmulatorState) {
     let num = get_register_pair(register, state);

src/emulator/main.rs

@@ -116,6 +116,11 @@ fn tick(state: &mut EmulatorState) {
         0xc6 => arithmetic::add(next_byte(), false, state), // ADI
         0xce => arithmetic::add(next_byte(), state.cc.c, state), // ACI
 
+        0x90..=0x97 => arithmetic::sub_reg(register_from_num(instruction & 0xf), false, state), // SUB
+        0x98..=0x9f => arithmetic::sub_reg(register_from_num(instruction & 0xf), state.cc.c, state), // SBB
+        0xd6 => arithmetic::sub(next_byte(), false, state), // SUI
+        0xde => arithmetic::sub(next_byte(), state.cc.c, state), // SBI
+
         _ => not_implemented(state),
     }