diff --git a/Makefile b/Makefile
index cef464a..9cb7b1a 100644
--- a/Makefile
+++ b/Makefile
@@ -1,8 +1,9 @@
 SDL2_CONFIG = sdl2-config
-CFLAGS = -g -Ofast -std=c99 `$(SDL2_CONFIG) --cflags --libs`
+CFLAGS = -g -Ofast -std=c99 -Wall -Wextra `$(SDL2_CONFIG) --cflags --libs`
 TARGET=fox32
 
 CFILES = src/main.c \
+		src/bus.c \
 		src/cpu.c \
 		src/framebuffer.c \
 		src/screen.c
diff --git a/src/bus.c b/src/bus.c
new file mode 100644
index 0000000..208b3ec
--- /dev/null
+++ b/src/bus.c
@@ -0,0 +1,103 @@
+#include <SDL2/SDL.h>
+#include <getopt.h>
+#include <math.h>
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stdint.h>
+#include <stdio.h>
+#include <stdlib.h>
+#include <string.h>
+
+#include "bus.h"
+#include "cpu.h"
+#include "framebuffer.h"
+
+extern fox32_vm_t vm;
+
+int bus_io_read(void *user, uint32_t *value, uint32_t port) {
+    (void) user;
+    switch (port) {
+        case 0x80000000 ... 0x8000031F: { // overlay port
+            uint8_t overlay_number = (port & 0x000000FF);
+            uint8_t setting = (port & 0x0000FF00) >> 8;
+            switch (setting) {
+                case 0x00: {
+                    // overlay position
+                    uint32_t x = overlay_get(overlay_number)->x;
+                    uint32_t y = overlay_get(overlay_number)->y;
+                    *value = (y << 16) | x;
+                    break;
+                };
+                case 0x01: {
+                    // overlay size
+                    uint32_t width = overlay_get(overlay_number)->width;
+                    uint32_t height = overlay_get(overlay_number)->height;
+                    *value = (height << 16) | width;
+                    break;
+                };
+                case 0x02: {
+                    // overlay framebuffer pointer
+                    *value = overlay_get(overlay_number)->pointer;
+                    break;
+                };
+                case 0x03: {
+                    // overlay enable status
+                    *value = overlay_get(overlay_number)->enabled ? 1 : 0;
+                    break;
+                };
+            }
+
+            break;
+        };
+    }
+
+    return 0;
+}
+
+int bus_io_write(void *user, uint32_t value, uint32_t port) {
+    (void) user;
+    switch (port) {
+        case 0x00000000: { // terminal output port
+            putchar((int) value);
+            fflush(stdout);
+            break;
+        };
+
+        case 0x80000000 ... 0x8000031F: { // overlay port
+            uint8_t overlay_number = (port & 0x000000FF);
+            uint8_t setting = (port & 0x0000FF00) >> 8;
+            switch (setting) {
+                case 0x00: {
+                    // overlay position
+                    uint32_t x = value & 0x0000FFFF;
+                    uint32_t y = (value & 0xFFFF0000) >> 16;
+                    overlay_get(overlay_number)->x = x;
+                    overlay_get(overlay_number)->y = y;
+                    break;
+                };
+                case 0x01: {
+                    // overlay size
+                    uint32_t width = value & 0x0000FFFF;
+                    uint32_t height = (value & 0xFFFF0000) >> 16;
+                    overlay_get(overlay_number)->width = width;
+                    overlay_get(overlay_number)->height = height;
+                    break;
+                };
+                case 0x02: {
+                    // overlay framebuffer pointer
+                    overlay_get(overlay_number)->pointer = value;
+                    break;
+                };
+                case 0x03: {
+                    // overlay enable status
+                    overlay_get(overlay_number)->enabled = value != 0;
+                    break;
+                };
+            }
+
+            break;
+        };
+    }
+
+    return 0;
+}
diff --git a/src/bus.h b/src/bus.h
new file mode 100644
index 0000000..e50261f
--- /dev/null
+++ b/src/bus.h
@@ -0,0 +1,4 @@
+#pragma once
+
+int bus_io_read(void *user, uint32_t *value, uint32_t port);
+int bus_io_write(void *user, uint32_t value, uint32_t port);
diff --git a/src/cpu.c b/src/cpu.c
index 0256a46..fbe297b 100644
--- a/src/cpu.c
+++ b/src/cpu.c
@@ -34,16 +34,14 @@ typedef fox32_io_read_t io_read_t;
 typedef fox32_io_write_t io_write_t;
 
 static int io_read_default_impl(void *user, uint32_t *value, uint32_t port) {
-    //return (void) user, (void) value, (int) port;
-    return 0;
+    return (void) user, (void) value, (int) port;
 }
 static int io_write_default_impl(void *user, uint32_t value, uint32_t port) {
     if (port == 0) {
         putchar((int) value);
         fflush(stdout);
     }
-    //return (void) user, (int) port;
-    return 0;
+    return (void) user, (int) port;
 }
 
 static io_read_t *const io_read_default = io_read_default_impl;
diff --git a/src/framebuffer.c b/src/framebuffer.c
index 7bb7096..f29d9a2 100644
--- a/src/framebuffer.c
+++ b/src/framebuffer.c
@@ -8,16 +8,52 @@
 #include <stdlib.h>
 #include <string.h>
 
+#include "bus.h"
 #include "cpu.h"
 #include "framebuffer.h"
 #include "screen.h"
 
-uint32_t PixelBuffer[FRAMEBUFFER_WIDTH * FRAMEBUFFER_HEIGHT];
-
 extern fox32_vm_t vm;
 
-void FramebufferDraw(struct Screen *screen) {
-    SDL_Texture *texture = ScreenGetTexture(screen);
-    memcpy(PixelBuffer, &vm.memory_ram[0x02000000], FRAMEBUFFER_WIDTH * FRAMEBUFFER_HEIGHT * 4);
-    SDL_UpdateTexture(texture, NULL, PixelBuffer, FRAMEBUFFER_WIDTH * 4);
+static uint8_t framebuffer[FRAMEBUFFER_WIDTH * FRAMEBUFFER_HEIGHT * 4];
+static overlay_t overlays[32];
+
+overlay_t *overlay_get(uint32_t index) {
+    if (index >= 32) abort();
+    return &overlays[index];
+}
+
+void draw_framebuffer(struct Screen *screen) {
+    SDL_Texture *texture = ScreenGetTexture(screen);
+
+    memcpy(framebuffer, &vm.memory_ram[0x02000000], FRAMEBUFFER_WIDTH * FRAMEBUFFER_HEIGHT * 4);
+
+    for (size_t i = 0; i < 32; i++) {
+        overlay_t *overlay = &overlays[i];
+        if (!overlay->enabled) continue;
+
+        size_t pointer = overlay->pointer;
+
+        size_t height = overlay->height;
+        size_t width = overlay->width;
+
+        size_t ymin = overlay->y, ymax = ymin + height;
+        size_t xmin = overlay->x, xmax = xmin + width;
+        ymax = ymax < FRAMEBUFFER_HEIGHT ? ymax : FRAMEBUFFER_HEIGHT;
+        xmax = xmax < FRAMEBUFFER_WIDTH ? xmax : FRAMEBUFFER_WIDTH;
+        if (ymin >= ymax) continue;
+        if (xmin >= xmax) continue;
+
+        for (size_t y = ymin; y < ymax; y++) {
+            for (size_t x = xmin; x < xmax; x++) {
+                size_t index_dst = (x + y * FRAMEBUFFER_WIDTH) * 4;
+                size_t index_src = ((x - xmin) + (y - ymin) * width) * 4 + pointer;
+                if (vm.memory_ram[index_src] > 0) {
+                    memcpy(&framebuffer[index_dst], &vm.memory_ram[index_src], 4);
+                }
+            }
+        }
+    }
+
+    SDL_UpdateTexture(texture, NULL, framebuffer, FRAMEBUFFER_WIDTH * 4);
 }
diff --git a/src/framebuffer.h b/src/framebuffer.h
index 9eeb41b..9a38d42 100644
--- a/src/framebuffer.h
+++ b/src/framebuffer.h
@@ -7,4 +7,13 @@
 
 #define VSYNC_INTERRUPT_VECTOR 0xFF
 
-void FramebufferDraw(struct Screen *screen);
+void draw_framebuffer(struct Screen *screen);
+
+typedef struct {
+    uint32_t pointer;
+    uint32_t x, y;
+    uint32_t width, height;
+    bool enabled;
+} overlay_t;
+
+overlay_t *overlay_get(uint32_t index);
diff --git a/src/main.c b/src/main.c
index 87b85e7..245189a 100644
--- a/src/main.c
+++ b/src/main.c
@@ -7,6 +7,7 @@
 #include <stdlib.h>
 #include <string.h>
 
+#include "bus.h"
 #include "cpu.h"
 #include "framebuffer.h"
 #include "screen.h"
@@ -33,6 +34,8 @@ int main(int argc, char *argv[]) {
     }
 
     fox32_init(&vm);
+    vm.io_read = bus_io_read;
+    vm.io_write = bus_io_write;
     vm.halted = false;
     //vm.debug = true;
 
@@ -40,8 +43,7 @@ int main(int argc, char *argv[]) {
 
     ScreenCreate(
         FRAMEBUFFER_WIDTH, FRAMEBUFFER_HEIGHT,
-        "fox32 framebuffer",
-        FramebufferDraw,
+        draw_framebuffer,
         0,
         0,
         0,
diff --git a/src/screen.c b/src/screen.c
index fb9db48..24372ec 100644
--- a/src/screen.c
+++ b/src/screen.c
@@ -180,8 +180,8 @@ struct SDL_Texture *ScreenGetTexture(struct Screen *screen) {
     return screen->Texture;
 }
 
-struct Screen ScreenCreate(
-    int w, int h, char *title,
+void ScreenCreate(
+    int w, int h,
     ScreenDrawF draw,
     ScreenKeyPressedF keypressed,
     ScreenKeyReleasedF keyreleased,
@@ -198,7 +198,6 @@ struct Screen ScreenCreate(
 
     MainScreen.Width = w;
     MainScreen.Height = h;
-    MainScreen.Title = title;
 
     MainScreen.Draw = draw;
     MainScreen.KeyPressed = keypressed;
diff --git a/src/screen.h b/src/screen.h
index 4b3933e..2eb4328 100644
--- a/src/screen.h
+++ b/src/screen.h
@@ -14,9 +14,6 @@ typedef void (*ScreenMouseMovedF)(struct Screen *screen, int dx, int dy);
 struct Screen {
     int Width;
     int Height;
-    void *Context1;
-    void *Context2;
-    char *Title;
     SDL_Texture *Texture;
 
     ScreenDrawF Draw;
@@ -35,8 +32,8 @@ int ScreenProcessEvents();
 
 struct SDL_Texture *ScreenGetTexture(struct Screen *screen);
 
-struct Screen ScreenCreate(
-    int w, int h, char *title,
+void ScreenCreate(
+    int w, int h,
     ScreenDrawF draw,
     ScreenKeyPressedF keypressed,
     ScreenKeyReleasedF keyreleased,