Begin adding scroll-down support

src/main.s

@@ -153,21 +153,9 @@ start:
 .vbl:
     halt
 
-    ; BC = *PENDING_ROW_PTR
-    ld a, [PENDING_ROW_PTR]
-    ld c, a
-    ld a, [PENDING_ROW_PTR + 1]
-    ld b, a
-
-    ; Write pending row if enqueued
-    or c
-    jr z, .dma_oam
-    ld hl, PENDING_ROW_DATA
-    ld d, SCRN_VX_B
-    MEMCPY bc, hl, d
+    call Map_Update
 
     ; ~160 cycles
-.dma_oam:
     ld a, HIGH(_OAM)
     call DMA_Start
 

src/map.s

@@ -3,26 +3,26 @@ INCLUDE "util.inc"
 
 SECTION "Map Data", WRAM0
 
-PAGEX:: DB
-PAGEY:: DB
+PAGEX:: DB ; X coordinate to enqueue map column at
+PAGEY:: DB ; Y coordinate to enqueue map row at
 
-LAST_SCX:: DB
-LAST_SCY:: DB
+LAST_SCX:: DB ; Value of SCX last frame
+LAST_SCY:: DB ; Value of SCY last frame
 
-PENDING_ROW_PTR:: DW
-PENDING_ROW_DATA:: DS SCRN_VX_B
+PENDING_ROW_PTR:: DW ; Where to write pending row data (0 = no write)
+PENDING_ROW_DATA:: DS SCRN_VX_B ; Row to be written
 
 CURRENT_DATA_START::
-CURRENT_TILE_PTR:: DW
-CURRENT_TILE_SIZE:: DB
-CURRENT_MAP_PTR:: DW
-CURRENT_MAP_COLLISION:: DW
-CURRENT_MAP_WIDTH:: DB
-CURRENT_MAP_HEIGHT:: DB
-CURRENT_SPAWN_X:: DB
-CURRENT_SPAWN_Y:: DB
-CURRENT_CAMERA_X:: DB
-CURRENT_CAMERA_Y:: DB
+CURRENT_TILE_PTR:: DW ; Location of tile data
+CURRENT_TILE_SIZE:: DB ; Length of tile data (num_tiles * 8)
+CURRENT_MAP_PTR:: DW ; Location of map data
+CURRENT_MAP_COLLISION:: DW ; Location of map collision data
+CURRENT_MAP_WIDTH:: DB ; Width of map in tiles
+CURRENT_MAP_HEIGHT:: DB ; Height of map in tiles
+CURRENT_SPAWN_X:: DB ; X coordinate to spawn player at
+CURRENT_SPAWN_Y:: DB ; Y coordinate to spawn player at
+CURRENT_CAMERA_X:: DB ; X coordinate of camera (top left of viewport)
+CURRENT_CAMERA_Y:: DB ; Y coordinate of camera (top left of viewport)
 CURRENT_DATA_END::
 
 SECTION "Map Code", ROM0
@@ -118,14 +118,14 @@ Map_Load::
 ; Update map state based on SCX/SCY
 Map_Scroll::
     ; If SCY = PAGEY, write map row
-    ; map coords = CURRENT_CAMERA_X, CURRENT_CAMERA_Y - 2
-    ; HL = _SCRN0 + 32 * (32 - page_y/8 - 2)
-    ; TODO: WTF does this shit even do...
     ld hl, PAGEY
     ld a, [rSCY]
     cp [hl]
     jr nz, .scroll_down_check
 
+    ; map coords = CURRENT_CAMERA_X, CURRENT_CAMERA_Y - 2
+    ; HL = _SCRN0 + 32 * (32 - page_y/8 - 2)
+    ; TODO: WTF does this shit even do...
     ld a, [rSCX]
     ld b, a
     ld a, [CURRENT_CAMERA_X]
@@ -180,10 +180,28 @@ Map_Scroll::
     jr .done
 
 .scroll_down_check:
-    ; If SCY = SCRN_Y - PAGEY, write map row
-    ; map coords = CURRENT_CAMERA_X, CURRENT_CAMERA_Y + 21
-    ; HL = _SCRN0 + 32 * ((32 - page_y) - 1)
-    ; TODO: Check math on these
+    ; Check SCY + SCRN_Y = PAGEY
+    ld a, [PAGEY]
+    sub SCRN_Y
+    ld hl, rSCY
+    cp [hl]
+    jr nz, .done
+
+    ld a, [CURRENT_CAMERA_X]
+    ld b, a
+
+    ld a, [CURRENT_CAMERA_Y]
+    add SCRN_X_B + 1
+    ld c, a
+
+    call compute_vram_ptr
+    call enqueue_row_write
+
+    ld a, [PAGEY]
+    add 8
+    ld [PAGEY], a
+
+    jr .done
 
     ; If SCX = PAGEX, write map col
     ; If SCX = SCRN_X - PAGEX, write map col
@@ -195,10 +213,45 @@ Map_Scroll::
 
     ret
 
+Map_Update::
+    ; Skip update if PENDING_ROW_PTR is 0
+    ld a, [PENDING_ROW_PTR]
+    ld c, a
+    ld a, [PENDING_ROW_PTR + 1]
+    ld b, a
+    or c
+    ret z
+
+    ld hl, PENDING_ROW_DATA
+    ld d, SCRN_VX_B
+    MEMCPY bc, hl, d
+
+    ret
+
+; Computes the offset into map RAM
+; @param e The map RAM y-coordinate
+; @return hl Pointer into map RAM
+compute_vram_ptr:
+    push bc
+    ld hl, _SCRN0
+    ld b, 0
+    ld c, SCRN_VX_B
+    ld a, e
+.loop:
+    or a
+    jr z, .done
+    add hl, bc
+    dec a
+    jr .loop
+.done:
+    pop bc
+    ret
+
 ; Write a row of map data into row buffer
 ; @param b Map X coordinate (signed)
 ; @param c Map Y coordinate (signed)
 ; @param hl Where to write the row in map VRAM
+; @destroy All registers
 enqueue_row_write:
     ; PENDING_ROW_PTR = HL
     ld a, l
@@ -306,7 +359,8 @@ enqueue_row_write:
     ret
 
 ; TODO: Deduplicate with enqueue_row_write? This version is needed when
-; loading a map, which happens synchronously while LCD is disabled
+; loading a map, which happens synchronously while LCD is disabled. One trick to
+; achieve this would be to call enqueue_row_write then Map_Update
 ;
 ; Write a row of map data into map RAM
 ; @param b Map X coordinate (signed)