diff --git a/src/constants.rs b/src/constants.rs
new file mode 100644
index 0000000..67a727e
--- /dev/null
+++ b/src/constants.rs
@@ -0,0 +1,15 @@
+/// The size of a ROM bank
+pub const ROM_BANK: usize = 0x4000;
+/// The size of a RAM bank
+pub const RAM_BANK: usize = 0x2000;
+
+// Memory map
+pub const A_ROM_LO: usize = 0x0000;
+pub const A_ROM_HI: usize = 0x4000;
+pub const A_VRAM: usize = 0x8000;
+pub const A_EXT_RAM: usize = 0xa000;
+pub const A_WRAM_LO: usize = 0xc000;
+pub const A_WRAM_HI: usize = 0xd000;
+pub const A_ECHO: usize = 0xe000;
+pub const A_OAM_RAM: usize = 0xfe00;
+pub const A_MMIO: usize = 0xff00;
diff --git a/src/lib.rs b/src/lib.rs
index 08d3986..114e5f7 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -13,7 +13,7 @@ pub mod error {
 
     #[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord)]
     pub struct Error {
-        kind: ErrorKind,
+        pub kind: ErrorKind,
         // any other information about an error
     }
 
@@ -65,6 +65,8 @@ pub mod cpu;
 
 pub mod audio;
 
+pub mod constants;
+
 #[cfg(test)]
 mod tests {
     use crate::memory::{io::*, Cart};
@@ -86,9 +88,7 @@ mod tests {
             let cart = Cart::from(rom);
             let mut cksum = 0u8;
             for index in 0x134..0x14d {
-                cksum = cksum
-                    .wrapping_sub(cart.read(index).unwrap())
-                    .wrapping_sub(1)
+                cksum = cksum.wrapping_sub(cart.read(index).unwrap()).wrapping_sub(1)
             }
             assert_eq!(cksum, cart.headercksum().unwrap())
         }