Initial, partially unsuccessful version

graphicsoverride.asm

@@ -0,0 +1,62 @@
+macro OverwriteVramTile(firstTileIndex, count)
+    LDA.w #<firstTileIndex>+512*8*8*4/16
+    LDX.b #<count>
+    JSR UploadTileToVram
+endmacro
+
+GraphicsOverrideHook:
+    JSL .resumeOriginalCode
+
+    LDA $0A ; Floor tileset loaded
+    CMP #$0A ; Floor tileset index used by Ice and Mire
+    BEQ .loadedIceOrMireFloor
+    RTL
+
+.loadedIceOrMireFloor
+    LDA $040C ; Load dungeon number
+    CMP #$12 ; Ice Palace
+    BEQ .isInIcePalace
+    RTL
+
+.isInIcePalace
+    ; We've just loading the floor tiles in Ice Palace.
+    ; Replace the bridge tiles with the bombos medallion
+
+    LDA.b #IcePalaceMedallionGfx>>16
+    STA $02
+    REP #$20
+    LDA.w #IcePalaceMedallionGfx
+    STA $00
+
+    %OverwriteVramTile($CA, 2)
+    %OverwriteVramTile($DA, 2)
+
+    SEP #$20
+    RTL
+
+.resumeOriginalCode
+    ; Overwritten instructions:
+    PHB
+    LDA.b #$00
+    PHA
+    PLB
+    LDA.b #$80
+    JML GraphicsOverrideResume
+
+; A = VRAM word address to upload to (Actual VRAM address / 2)
+; X = Tiles to upload
+; $00-$02 = Pointer to graphics data
+UploadTileToVram:
+    ; TODO: This should probably be optimized with a DMA
+    STA $2116
+    TXA : ASL #4 : TAX ; X = Amount of words to upload (Tiles to upload * 16)
+-   LDA [$00]
+    STA $2118
+    INC $00
+    INC $00
+    DEX
+    BNE -
+    RTS
+
+IcePalaceMedallionGfx:
+    incbin ice_bombos_floor.bin