diff --git a/src/nes.rs b/src/nes.rs
index 537ed49..b390640 100644
--- a/src/nes.rs
+++ b/src/nes.rs
@@ -32,19 +32,24 @@ impl NES {
         }
     }
 
+    // Insert a cartridge into the NES. This inserts the cartridge memory
+    // into the NES address range
     pub fn insert_cartridge(&mut self, cartridge: Cartridge) {
         self.memory.insert_cartridge(cartridge);
     }
 
+    // Initializes the NES CPU programm pointer
     pub fn start(&mut self) {
         self.cpu.find_pc_addr(&self.memory);
     }
 
+    // Reset the CPU
     pub fn reset(&mut self) {
         self.clock_count = 0;
         self.cpu.reset(&self.memory);
     }
 
+    // A single clock on the NES
     pub fn clock(&mut self) {
         self.clock_count += 1;
         if self.clock_count % 3 == 0 {
diff --git a/src/nes/cpu.rs b/src/nes/cpu.rs
index 617ddfe..ccc0079 100644
--- a/src/nes/cpu.rs
+++ b/src/nes/cpu.rs
@@ -1,12 +1,29 @@
 use crate::nes::{Memory,MemoryReader};
 use crate::nes::types::*;
-
-pub mod instructions;
-
 use instructions::{Instruction,Operation,AddrMode};
 use core::fmt::{Debug,Formatter,Result};
 use log::{debug};
 
+pub mod instructions;
+
+// Base address of the stack in memory
+pub const STACK_BASE_ADDR: Addr = 0x0100;
+
+// constants to extract LO and HI from a value. 
+pub const LO: Addr  = 0x00FF;
+pub const HI: Addr  = 0xFF00;
+
+// CPU flags 
+pub const CARRY: Byte = 1 << 0;    // 0000 0001 0x01
+pub const ZERO: Byte = 1 << 1;     // 0000 0010 0x02
+pub const IRQ: Byte = 1 << 2;      // 0000 0100 0x04
+pub const DECIMAL: Byte = 1 << 3;  // 0000 1000 0x08
+pub const BREAK: Byte = 1 << 4;    // 0001 0000 0x10
+pub const UNUSED: Byte = 1 << 5;   // 0010 0000 0x20
+pub const OVERFLOW: Byte = 1 << 6; // 0100 0000 0x40
+pub const NEGATIVE: Byte = 1 << 7; // 1000 0000 0x80
+
+// The NES CPU registers
 pub struct Registers {
     pub a: Byte,
     pub x: Byte,
@@ -18,7 +35,6 @@ pub struct Registers {
 
 impl Registers {
     pub fn new() -> Registers {
-        // TODO true initial state of registers before reset?
         Registers {
             a: 0,
             x: 0,
@@ -37,19 +53,6 @@ impl Debug for Registers {
     } 
 }
 
-pub const STACK_BASE_ADDR: Addr = 0x0100;
-pub const LO: Addr  = 0x00FF;
-pub const HI: Addr  = 0xFF00;
-
-pub const CARRY: Byte = 1 << 0;    // 0000 0001 0x01
-pub const ZERO: Byte = 1 << 1;     // 0000 0010 0x02
-pub const IRQ: Byte = 1 << 2;      // 0000 0100 0x04
-pub const DECIMAL: Byte = 1 << 3;  // 0000 1000 0x08
-pub const BREAK: Byte = 1 << 4;    // 0001 0000 0x10
-pub const UNUSED: Byte = 1 << 5;   // 0010 0000 0x20
-pub const OVERFLOW: Byte = 1 << 6; // 0100 0000 0x40
-pub const NEGATIVE: Byte = 1 << 7; // 1000 0000 0x80
-
 pub struct CPU {
     pub regs: Registers,
     curr_op: Byte,  // current operation
diff --git a/src/nes/memory.rs b/src/nes/memory.rs
index ed99e46..1d1ccc3 100644
--- a/src/nes/memory.rs
+++ b/src/nes/memory.rs
@@ -1,4 +1,3 @@
-// Interface for devices that contain memory that can be accessed by CPU
 use crate::nes::cartridge::Cartridge;
 use crate::nes::types::*;
 
@@ -9,6 +8,7 @@ const PPU_ADDR_RANGE: [Addr; 2] = [0x2000, 0x3fff];
 const PPU_PHYS_RANGE: [Addr; 2] = [0x2000, 0x2007];
 const CART_ADDR_RANGE: [Addr; 2] = [0x4020, 0xffff];
 
+// NES memory: Contains data from RAM, cartridge...
 pub struct NESMemory {
     ram: [Byte; RAM_SIZE], // 2kb
     cartridge: Option<Cartridge>,