diff --git a/SConstruct b/SConstruct
index 22a0aa6..524c08d 100644
--- a/SConstruct
+++ b/SConstruct
@@ -9,4 +9,4 @@ env = Environment(ENV       = { 'TERM' : environ['TERM'] },
                   LINKFLAGS = flags,
                   LIBS      = ['SDL2'])
 
-env.Program('laines', Glob('src/*.cpp'))
+env.Program('laines', Glob('src/*.cpp') + Glob('src/*/*.cpp'))
diff --git a/src/cartridge.cpp b/src/cartridge.cpp
index 4ef87b0..7c085c7 100644
--- a/src/cartridge.cpp
+++ b/src/cartridge.cpp
@@ -6,7 +6,7 @@
 namespace Cartridge {
 
 
-Mapper* mapper;
+Mapper* mapper;  // Mapper chip.
 
 /* PRG-ROM access */
 template <bool wr> u8 access(u16 addr, u8 v)
diff --git a/src/io.cpp b/src/io.cpp
index fb771f0..391f81e 100644
--- a/src/io.cpp
+++ b/src/io.cpp
@@ -4,6 +4,7 @@
 
 namespace IO {
 
+
 // Screen size:
 const unsigned width  = 256;
 const unsigned height = 240;
@@ -18,6 +19,7 @@ u32 pixels[width * height];  // Video buffer.
 u8 joypad_bits[2];  // Joypad shift registers.
 bool strobe;        // Joypad strobe latch.
 
+/* Initialize SDL */
 void init()
 {
     SDL_Init(SDL_INIT_VIDEO);
@@ -37,6 +39,7 @@ void init()
     signal(SIGINT, SIG_DFL);
 }
 
+/* Get the joypad state from SDL */
 u8 get_joypad_state(int n)
 {
     u8 j = 0;
@@ -56,6 +59,7 @@ u8 get_joypad_state(int n)
     return j;
 }
 
+/* Read joypad state (NES register format) */
 u8 read_joypad(int n)
 {
     // When strobe is high, it keeps reading A:
@@ -78,11 +82,13 @@ void write_joypad_strobe(bool v)
     strobe = v;
 }
 
+/* Draw a pixel at the given coordinates */
 void draw_pixel(unsigned x, unsigned y, u32 rgb)
 {
     pixels[y*width + x] = rgb;
 }
 
+/* Put the pixels on screen */
 void flush_screen()
 {
     SDL_UpdateTexture(texture, NULL, pixels, width * sizeof(u32));
diff --git a/src/mapper.cpp b/src/mapper.cpp
index 90e1971..d9ba898 100644
--- a/src/mapper.cpp
+++ b/src/mapper.cpp
@@ -2,6 +2,7 @@
 
 Mapper::Mapper(u8* rom)
 {
+    // Read infos from header:
     prgSize    = rom[4] * 0x4000;
     chrSize    = rom[5] * 0x2000;
     prgRamSize = rom[8] ? rom[8] * 0x2000 : 0x2000;
@@ -9,8 +10,10 @@ Mapper::Mapper(u8* rom)
     this->prg    = rom + 16;
     this->prgRam = new u8[prgRamSize];
 
+    // CHR ROM:
     if (chrSize)
         this->chr = rom + 16 + prgSize;
+    // CHR RAM:
     else
     {
         chrSize = 0x2000;
diff --git a/src/mappers/mapper1.cpp b/src/mappers/mapper1.cpp
new file mode 100644
index 0000000..9f55b8b
--- /dev/null
+++ b/src/mappers/mapper1.cpp
@@ -0,0 +1,76 @@
+#include "mappers/mapper1.hpp"
+
+
+/* Apply the registers state */
+void Mapper1::apply()
+{
+    // Mirroring.
+
+    // 16KB PRG:
+    if (regs[0] & 0b1000)
+    {
+        // 0x8000 swappable, 0xC000 fixed to bank 0x0F:
+        if (regs[0] & 0b100)
+        {
+            map_prg16k(0, regs[3] & 0xF);
+            map_prg16k(1, 0xF);
+        }
+        // 0x8000 fixed to bank 0x00, 0xC000 swappable:
+        else
+        {
+            map_prg16k(0, 0);
+            map_prg16k(1, regs[3] & 0xF);
+        }
+    }
+    // 32KB PRG:
+    else
+        map_prg32k((regs[3] & 0xF) >> 1);
+
+    // 4KB CHR:
+    if (regs[0] & 0b10000)
+    {
+        map_chr4k(0, regs[1]);
+        map_chr4k(1, regs[2]);
+    }
+    // 8KB CHR:
+    else
+        map_chr8k(regs[1] >> 1);
+}
+
+u8 Mapper1::write(u16 addr, u8 v)
+{
+    // PRG RAM write;
+    if (addr < 0x8000)
+        prgRam[addr - 0x6000] = v;
+    // Mapper register write:
+    else if (addr & 0x8000)
+    {
+        // Reset:
+        if (v & 0x80)
+        {
+            writeN   = 0;
+            tmpReg   = 0;
+            regs[0] |= 0x0C;
+            apply();
+        }
+        else
+        {
+            // Write a bit into the temporary register:
+            tmpReg = ((v & 1) << 4) | (tmpReg >> 1);
+            // Finished writing all the bits:
+            if (++writeN == 5)
+            {
+                regs[(addr >> 13) & 0b11] = tmpReg;
+                writeN = 0;
+                tmpReg = 0;
+                apply();
+            }
+        }
+    }
+    return v;
+}
+
+u8 Mapper1::chr_write(u16 addr, u8 v)
+{
+    return chr[addr] = v;
+}
diff --git a/src/mappers/mapper1.hpp b/src/mappers/mapper1.hpp
index 0ef9b30..756b21b 100644
--- a/src/mappers/mapper1.hpp
+++ b/src/mappers/mapper1.hpp
@@ -4,45 +4,11 @@
 
 class Mapper1 : public Mapper
 {
-  private:
     int writeN;
     u8 tmpReg;
     u8 regs[4];
 
-    void apply()
-    {
-        // Mirroring.
-
-        // 16KB PRG:
-        if (regs[0] & 0b1000)
-        {
-            // 0x8000 swappable, 0xC000 fixed to bank 0x0F:
-            if (regs[0] & 0b100)
-            {
-                map_prg16k(0, regs[3] & 0xF);
-                map_prg16k(1, 0xF);
-            }
-            // 0x8000 fixed to bank 0x00, 0xC000 swappable:
-            else
-            {
-                map_prg16k(0, 0);
-                map_prg16k(1, regs[3] & 0xF);
-            }
-        }
-        // 32KB PRG:
-        else
-            map_prg32k((regs[3] & 0xF) >> 1);
-
-        // 4KB CHR:
-        if (regs[0] & 0b10000)
-        {
-            map_chr4k(0, regs[1]);
-            map_chr4k(1, regs[2]);
-        }
-        // 8KB CHR:
-        else
-            map_chr8k(regs[1] >> 1);
-    }
+    void apply();
 
   public:
     Mapper1(u8* rom) : Mapper(rom)
@@ -52,38 +18,6 @@ class Mapper1 : public Mapper
         apply();
     }
 
-    u8 write(u16 addr, u8 v)
-    {
-        if (addr < 0x8000)
-        {
-            prgRam[addr - 0x6000] = v;
-        }
-        else if (addr & 0x8000)
-        {
-            if (v & 0x80)
-            {
-                writeN   = 0;
-                tmpReg   = 0;
-                regs[0] |= 0x0C;
-                apply();
-            }
-            else
-            {
-                tmpReg = ((v & 1) << 4) | (tmpReg >> 1);
-                if (++writeN == 5)
-                {
-                    regs[(addr >> 13) & 0b11] = tmpReg;
-                    writeN = 0;
-                    tmpReg = 0;
-                    apply();
-                }
-            }
-        }
-        return v;
-    }
-
-    u8 chr_write(u16 addr, u8 v)
-    {
-        return chr[addr] = v;
-    }
+    u8 write(u16 addr, u8 v);
+    u8 chr_write(u16 addr, u8 v);
 };