diff --git a/src/emulator/instructions/arithmetic.rs b/src/emulator/instructions/arithmetic.rs
index 8b06d44..afeb7c9 100644
--- a/src/emulator/instructions/arithmetic.rs
+++ b/src/emulator/instructions/arithmetic.rs
@@ -1,4 +1,4 @@
-use crate::{EmulatorState, Register, get_register};
+use crate::{EmulatorState, Register, get_register, structs::set_register};
 
 /// Sets the condition code flags according to `result`. `flags` parameter
 /// indicates which flags will be set, 0b1111 will set all (Z, S, C, P)
@@ -12,7 +12,7 @@ fn set_cc(state: &mut EmulatorState, result: u16, flags: u8) {
 
 /// Add values of `register` and `A`
 pub fn add(register: Register, state: &mut EmulatorState) {
-    let result = get_register(register, state) as u16 + state.a as u16;
+    let result = get_register(&register, state) as u16 + state.a as u16;
     set_cc(state, result, 0b1111);
     state.a = (result & 0xff) as u8;
 }
@@ -26,7 +26,7 @@ pub fn adi(byte: u8, state: &mut EmulatorState) {
 
 /// Add values of `register` and `A` and add +1 if carry bit is set
 pub fn adc(register: Register, state: &mut EmulatorState) {
-    let result = get_register(register, state) as u16 + state.a as u16 + if state.cc.c { 1 } else { 0 };
+    let result = get_register(&register, state) as u16 + state.a as u16 + if state.cc.c { 1 } else { 0 };
     set_cc(state, result, 0b1111);
     state.a = (result & 0xff) as u8;
 }
@@ -53,3 +53,17 @@ pub fn dad(register: Register, state: &mut EmulatorState) {
     state.h = (result >> 8) as u8;
     state.l = result as u8;
 }
+
+/// Increase register
+pub fn inr(register: Register, state: &mut EmulatorState) {
+    let (result, _) = get_register(&register, state).overflowing_add(1);
+    set_cc(state, result as u16, 0b1101);
+    set_register(&register, result, state);
+}
+
+/// Decrease register
+pub fn dcr(register: Register, state: &mut EmulatorState) {
+    let (result, _) = get_register(&register, state).overflowing_sub(1);
+    set_cc(state, result as u16, 0b1101);
+    set_register(&register, result, state);
+}
diff --git a/src/emulator/main.rs b/src/emulator/main.rs
index 03518b1..837aece 100644
--- a/src/emulator/main.rs
+++ b/src/emulator/main.rs
@@ -54,7 +54,27 @@ fn tick(state: &mut EmulatorState) {
     match instruction {
         0x00 => {} // NOP
 
-        /* ADD */
+        /* Maths */
+
+        // INR
+        0x04 => arithmetic::inr(Register::B, state),
+        0x0c => arithmetic::inr(Register::C, state),
+        0x14 => arithmetic::inr(Register::D, state),
+        0x1c => arithmetic::inr(Register::E, state),
+        0x24 => arithmetic::inr(Register::H, state),
+        0x2c => arithmetic::inr(Register::L, state),
+        0x34 => arithmetic::inr(Register::M, state),
+        0x3c => arithmetic::inr(Register::A, state),
+
+        // DCR
+        0x05 => arithmetic::dcr(Register::B, state),
+        0x0d => arithmetic::dcr(Register::C, state),
+        0x15 => arithmetic::dcr(Register::D, state),
+        0x1d => arithmetic::dcr(Register::E, state),
+        0x25 => arithmetic::dcr(Register::H, state),
+        0x2d => arithmetic::dcr(Register::L, state),
+        0x35 => arithmetic::dcr(Register::M, state),
+        0x3d => arithmetic::dcr(Register::A, state),
 
         // DAD
         0x09 => arithmetic::dad(Register::B, state),
diff --git a/src/emulator/structs.rs b/src/emulator/structs.rs
index 2f1eaa6..5332ba3 100644
--- a/src/emulator/structs.rs
+++ b/src/emulator/structs.rs
@@ -58,7 +58,7 @@ pub fn register_from_num(b: u8) -> Register {
     }
 }
 
-pub fn get_register(register: Register, state: &EmulatorState) -> u8 {
+pub fn get_register(register: &Register, state: &EmulatorState) -> u8 {
     match register {
         Register::B => state.b as u8,
         Register::C => state.c as u8,
@@ -72,7 +72,7 @@ pub fn get_register(register: Register, state: &EmulatorState) -> u8 {
     }
 }
 
-pub fn set_register(register: Register, value: u8, state: &mut EmulatorState) {
+pub fn set_register(register: &Register, value: u8, state: &mut EmulatorState) {
     match register {
         Register::B => state.b = value,
         Register::C => state.c = value,