diff --git a/inc/util.inc b/inc/util.inc
index 21cdfc0..8c4a726 100644
--- a/inc/util.inc
+++ b/inc/util.inc
@@ -1,5 +1,5 @@
-; Stores a 16-bit register into the address stored in HL
-; \1 The register to store
+; Stores a 16-bit value into the address stored in HL
+; \1 The value to store (immediate or register)
 MACRO STORE16
     ld [hl], HIGH(\1)
     inc hl
diff --git a/src/bg.s b/src/bg.s
index 0a4d878..a768d9b 100644
--- a/src/bg.s
+++ b/src/bg.s
@@ -5,18 +5,22 @@ SECTION "BG Data", WRAM0
 
 BG_COLLISION_DATA:: dw
 BG_MAP_WIDTH:: db
+BG_MAP_HEIGHT:: db
 
 SECTION "BG Code", ROM0
 
 MACRO update_map_info
     ld hl, BG_COLLISION_DATA
     ld a, HIGH(\1)
-    ld [hli], a
+    ld [hl+], a
     ld a, LOW(\1)
-    ld [hli], a
+    ld [hl+], a
 
     ld a, \2
-    ld [hli], a
+    ld [hl+], a
+
+    ld a, \3
+    ld [hl+], a
 ENDM
 
 BG_Init::
@@ -47,6 +51,6 @@ BG_Init::
     ld d, intro_NUM_TILES
     call memcpy
 
-    update_map_info intro_COLLISION, intro_WIDTH
+    update_map_info intro_COLLISION, intro_WIDTH, intro_HEIGHT
 
     ret
diff --git a/src/collision.s b/src/collision.s
index fb4f7fb..ad791c2 100644
--- a/src/collision.s
+++ b/src/collision.s
@@ -35,18 +35,18 @@ player_bg_collides::
 
     ; top left
     call can_move_to
-    ret z
+    ret nz
 
     ; top right
     inc c
     call can_move_to
-    ret z
+    ret nz
 
     ; bottom left
     dec c
     inc b
     call can_move_to
-    ret z
+    ret nz
 
     ; bottom right
     inc c
@@ -58,7 +58,7 @@ player_bg_collides::
 ; @param b y-coordinate
 ; @param c x-coordinate
 ; @destroy a, d, e
-; @note Z = false, NZ = true
+; @note NZ = false, Z = true
 can_move_to:
     ; todo: should be aware of scx/scy
     push bc
@@ -86,7 +86,7 @@ can_move_to:
 
     ; check [hl] = 1
     ld a, [hl]
-    cp 0
+    cp 1
 
     pop hl
     pop bc
diff --git a/src/player.s b/src/player.s
index 9946141..161af50 100644
--- a/src/player.s
+++ b/src/player.s
@@ -15,7 +15,6 @@ PLAYER_DIR:: db
 
 PLAYER_JUMPING:: db
 PLAYER_VY:: dw
-PLAYER_OLD_Y: db
 
 Section "Player Code", ROM0
 
@@ -26,6 +25,7 @@ DEF SPRITE_OAM_IDX EQU 0
 DEF SPRITE_IDX EQU 32 ; tile index, rename later
 DEF SPRITE_WIDTH EQU 2
 DEF SPRITE_HEIGHT EQU 2
+DEF PLAYER_SPEED EQU 2
 
 DEF GRAVITY EQU (0 << 8) | 8
 DEF INIT_VY EQU (2 << 8) | 40
@@ -63,13 +63,13 @@ Player_Update::
     ; check for jump
     ld a, b
     and BTN_UP
-    jr z, .update_jump
+    jr z, .jump_update_check
 
     ; initialize jump state if not already jumping
     ld hl, PLAYER_JUMPING
     ld a, [hl]
     or a
-    jr nz, .update_jump
+    jr nz, .jump_update
 
     ld [hl], 1
 
@@ -78,12 +78,14 @@ Player_Update::
     inc hl
     ld [hl], LOW(INIT_VY)
 
-.update_jump:
+; todo: deduplicate
+.jump_update_check:
     ld hl, PLAYER_JUMPING
     ld a, [hl]
     or a
     jr z, .right
 
+.jump_update:
     ; load y velocity into bc
     ld hl, PLAYER_VY
     ld b, [hl]
@@ -101,12 +103,6 @@ Player_Update::
     ld hl, PLAYER_VY
     STORE16 bc
 
-    ; old_y = y
-    ld hl, PLAYER_Y
-    ld a, [hl]
-    ld hl, PLAYER_OLD_Y
-    ld [hl], a
-
     ; y -= floor(vy)
     ld hl, PLAYER_Y
     ld a, [hl]
@@ -115,24 +111,39 @@ Player_Update::
 
     ; roll back jump if there was a collision
     call player_bg_collides
-    jr nz, .update_jump_oam
+    jr z, .jump_update_oam
+
+.jump_rollback:
+    ld hl, PLAYER_VY
+    bit 7, [hl]
 
-    ld hl, PLAYER_OLD_Y
-    ld a, [hl]
     ld hl, PLAYER_Y
-    ld b, [hl]
+    jr z, .jump_rollback_down
+    dec [hl]
+    jr .jump_rollback_check
+
+.jump_rollback_down:
+    inc [hl]
+
+.jump_rollback_check:
+    call player_bg_collides
+    jr nz, .jump_rollback
 
+    ; set PLAYER_JUMPING = 0 if VY < 0 (fell into ground)
     ld hl, PLAYER_VY
+    bit 7, [hl]
+
+    ; either way, set VY = 0
+    ld [hl], 0
+    ld hl, PLAYER_VY + 1
     ld [hl], 0
 
-    ; if colliding below, stop jump state
-    cp b
-    jr nc, .right
+    jr z, .jump_update_oam
 
     ld hl, PLAYER_JUMPING
     ld [hl], 0
 
-.update_jump_oam:
+.jump_update_oam:
     call update_oam
 
     ; check for move right
@@ -153,7 +164,7 @@ Player_Update::
     inc [hl]
 
     call player_bg_collides
-    jr z, .right_rollback
+    jr nz, .right_rollback
 
     ld hl, PLAYER_DIR
     ld [hl], 0
@@ -180,11 +191,11 @@ Player_Update::
     or a
     ret z
 
-    dec [hl]
-    dec [hl]
+    sub PLAYER_SPEED
+    ld [hl], a
 
     call player_bg_collides
-    jr z, .left_rollback
+    jr nz, .left_rollback
 
     ld hl, PLAYER_DIR
     ld [hl], $ff
@@ -194,7 +205,7 @@ Player_Update::
 
 .left_rollback:
     ld hl, PLAYER_X
-    dec [hl]
+    inc [hl]
     inc [hl]
     ret