diff --git a/game/NOTES.md b/game/NOTES.md
new file mode 100644
index 0000000..a6d7371
--- /dev/null
+++ b/game/NOTES.md
@@ -0,0 +1,7 @@
+Some notes on the game
+
+# Position and velocity
+
+All positions are represented in fixed-point fashion, with a precision of 256.
+
+For instance, a position or distance of 8 pixels is represented as `0x800`. (2048)
\ No newline at end of file
diff --git a/game/game.c b/game/game.c
index 94fa5f0..be9c206 100644
--- a/game/game.c
+++ b/game/game.c
@@ -4,9 +4,20 @@
 #include "map/game_map.h"
 #include "game.h"
 #include "sys/sys.h"
+#include <stdio.h>
 
 uint32_t game_frame;
 
+game_bbox game_player_bbox = {
+    .x=0x9000,
+    // .y=0x5800,
+    .y=0x0000,
+    .w=0x1000,
+    .h=0x1000,
+};
+sys_i32 game_player_dx = 0;
+sys_i32 game_player_dy = 0;
+
 const char* game_title() {
     return "Croc Party!";
 }
@@ -23,6 +34,19 @@ void game_destroy() {
 
 void game_update() {
     game_frame += 1;
+
+    game_player_dy += 1;
+
+    if (device_buttons[0]) { game_player_dx -= 16; }
+    if (device_buttons[1]) { game_player_dx += 16; }
+
+    game_collision collision = game_collision_move_to_contact(
+        &game_player_bbox, 
+        game_player_dx, 
+        game_player_dy
+    );
+    if (collision.collided_x) { game_player_dx = 0; }
+    if (collision.collided_y) { game_player_dy = 0; }
 }
 
 void game_draw() {
@@ -31,7 +55,10 @@ void game_draw() {
     sys_map_draw(map_game_map, spr_game_tiles, 0, 0, 0, 0, 32, 18);
     sys_sprite_draw_ext(
         spr_game_player,
-        (game_frame / 8) % 4 * 2, 144, 88, 2, 2, 
+        (game_frame / 8) % 4 * 2, 
+        game_player_bbox.x / 0x100, 
+        game_player_bbox.y / 0x100, 
+        2, 2, 
         false, false
     );
 }
diff --git a/game/game.h b/game/game.h
index 04fff7b..02e3a79 100644
--- a/game/game.h
+++ b/game/game.h
@@ -1,6 +1,7 @@
 #ifndef CROCPARTY_GAME_H
 #define CROCPARTY_GAME_H
 
+#include "game_collision.h"
 #include "game_palette.h"
 
 const char* game_title();
diff --git a/game/game_collision.c b/game/game_collision.c
new file mode 100644
index 0000000..9b7923e
--- /dev/null
+++ b/game/game_collision.c
@@ -0,0 +1,122 @@
+#include <assert.h>
+#include "sys/sys.h"
+#include "game.h"
+#include "map/game_map.h"
+
+bool game_collision_is_occlusive(sys_maptile tile) {
+    if (tile >= 0 && tile < 30) { return true; }
+    if (tile >= 60 && tile < 90) { return true; }
+    return false;
+}
+
+bool game_collision_can_move(game_bbox body, sys_i32 dx, sys_i32 dy) {
+    sys_i32 x = body.x + dx;
+    sys_i32 y = body.y + dy;
+
+    // the player can never leave the map
+    if (x < 0) { return false; }
+    if (y < 0) { return false; }
+    if (x + body.w > map_game_map.width * TILE_SZ_MICROPIXEL) { 
+        return false; 
+    } 
+    if (y + body.h > map_game_map.height * TILE_SZ_MICROPIXEL) { 
+        return false; 
+    } 
+
+    // TODO: Collide with blocks
+    // If two collides with a block, check if one collides with it.
+    // The collision only matters if one doesn't.
+    return true;
+}
+
+#define round_down(x, incr) (x) - (x) % (incr)
+#define round_up(x, incr) round_down((x) + (incr) - 1, incr)
+
+game_collision game_collision_move_to_contact(
+    game_bbox* body, sys_i32 total_dx, sys_i32 total_dy
+) {
+    sys_i32 src_x = body->x;
+    sys_i32 src_y = body->y;
+    game_collision collision = {
+        .collided_x=false,
+        .collided_y=false,
+        .distance_x=0,
+        .distance_y=0,
+    };
+    sys_i32 tar_x = body->x + total_dx;
+    sys_i32 tar_y = body->y + total_dy;
+
+    while (true) {
+        // calculate how much we still need to move
+        int32_t remaining_dx = tar_x - body->x;
+        int32_t remaining_dy = tar_y - body->y;
+
+        // if we don't need to move, stop
+        if (remaining_dx == 0 && remaining_dy == 0) { 
+            collision.distance_x = body->x - src_x;
+            collision.distance_y = body->x - src_y;
+            return collision; 
+        }
+
+        // figure out where we are
+        sys_i32 body_x0 = body->x;
+        sys_i32 body_y0 = body->y;
+        sys_i32 body_x1 = body->x + body->w;
+        sys_i32 body_y1 = body->y + body->h;
+
+        // figure out what tile we're still in
+        sys_i32 tile_x0 = round_down(body_x0, TILE_SZ_MICROPIXEL);
+        sys_i32 tile_y0 = round_down(body_y0, TILE_SZ_MICROPIXEL);
+        sys_i32 tile_x1 = round_up(body_x1, TILE_SZ_MICROPIXEL);
+        sys_i32 tile_y1 = round_up(body_y1, TILE_SZ_MICROPIXEL);
+
+        // but we want the _next_ tile
+        if (tile_x0 == body_x0) { tile_x0 -= TILE_SZ_MICROPIXEL; }
+        if (tile_y0 == body_y0) { tile_y0 -= TILE_SZ_MICROPIXEL; }
+        if (tile_x1 == body_x1) { tile_x1 += TILE_SZ_MICROPIXEL; }
+        if (tile_y1 == body_y1) { tile_y1 += TILE_SZ_MICROPIXEL; }
+
+        // length of an x axis move
+        int step_dx = 
+            remaining_dx < 0 ? 
+                // line up our left side with the tile to our left
+                // not exceeding dx
+                sys_max_i32(remaining_dx, tile_x0 - body_x0) :
+            remaining_dx > 0 ?  
+                // line up our right side with the tile to our right
+                // not exceeding dx
+                sys_min_i32(remaining_dx, tile_x1 - body_x1) :
+            // dx == 0 
+            INT32_MAX ;
+
+        int step_dy = 
+            remaining_dy < 0 ?
+                // line up our top side with the tile above us
+                // not exceeding dy
+                sys_max_i32(remaining_dy, tile_y0 - body_y0) :
+            remaining_dy > 0 ?
+                // line up our bottom side with the tile below us
+                // not exceeding dy
+                sys_min_i32(remaining_dy, tile_y1 - body_y1) :
+            INT32_MAX;
+
+        // there must be a viable move in some direction
+        assert(step_dx != INT32_MAX || step_dy != INT32_MAX);
+
+        if (sys_abs_i32(step_dx) < sys_abs_i32(step_dy)) {
+            if (game_collision_can_move(*body, step_dx, 0)) {
+                body->x += step_dx; // take the tiny move
+            } else {
+                tar_x = body->x;  // stop trying to move
+                collision.collided_x = true;
+            }
+        } else {
+            if (game_collision_can_move(*body, 0, step_dy)) {
+                body->y += step_dy; // take the tiny move
+            } else {
+                tar_y = body->y;  // stop trying to move
+                collision.collided_y = true;
+            }
+        }
+    }
+}
\ No newline at end of file
diff --git a/game/game_collision.h b/game/game_collision.h
new file mode 100644
index 0000000..1b8d012
--- /dev/null
+++ b/game/game_collision.h
@@ -0,0 +1,27 @@
+#ifndef GAME_COLLISION_H
+#define GAME_COLLISION_H
+
+#include <stdbool.h>
+#include "sys/sys.h"
+
+#define TILE_SZ_MICROPIXEL 0x800
+
+typedef struct {
+    sys_i32 x;
+    sys_i32 y;
+    sys_i32 w;
+    sys_i32 h;
+} game_bbox;
+
+typedef struct {
+    bool collided_x;
+    bool collided_y;
+    sys_i32 distance_x;
+    sys_i32 distance_y;
+} game_collision;
+
+bool game_collision_is_occlusive(sys_maptile tile);
+bool game_collision_can_move(game_bbox body, sys_i32 dx, sys_i32 dy);
+game_collision game_collision_move_to_contact(game_bbox* body, sys_i32 x, sys_i32 y);
+
+#endif
\ No newline at end of file