Module gba_addresses::vram

Video RAM (VRAM).

The exact usage of this memory depends on your video mode:

• Mode 0: Layers 0, 1, 2 and 3 are text mode.
• Mode 1: Layers 0 and 1 are textual, layer 2 is affine.
• Mode 2: Layers 2 and 3 are affine.
• Mode 3: Layer 2 is a 240x160 direct color bitmap.
• Mode 4: Layer 2 is two 240x160 indexed color bitmaps.
• Mode 5: Layer 2 is two 160x128 direct color bitmaps.

A tile is always 8x8, but tiles can be either 4bpp and 8bpp.

Many things related to VRAM work in "Charblocks":

• A charblock is 16kb.
• In terms of charblocks, VRAM has 4 blocks for background tiles and 2 blocks for object tiles.
• In video modes 0, 1, and 2 both tile data and screenblocks occupy the first 4 charblocks.
• In video mode 3, 4, and 5, the bitmap data occupies all background charblocks and also the first object charblock, so only the final charblock is available for object tile data.

A "Screenblock" gives the layout of how tiles should be arranged.

• A screenblock index always is in terms of 2kb, so indexes are always in the range 0..32. Note that the actual size of a full screenblock isn't necessarily 2kb.
• Screenblocks and tile image data both have to share space in VRAM.
• Text Mode:
  • 2 bytes per screen entry, 32x32 entries per screenblock (2kb).
  • A layer can be 1, 2, or 4 screenblocks:
    • 32x32: one screenblock (square)
    • 64x32: one screenblock, and the next screenblock on the right (horizontal).
    • 32x64: one screenblock, and the next screenblock beneath (vertical).
    • 64c64: the base screenblock, then down, then right, then down right (large square).
• Affine Mode:
  • 1 byte per entry, entry count per screenblock depends on the background size: 16x16 (256b), 32x32 (1kb), 64x64 (4kb), or 128x128 (16kb).
  • Each layer is a one screenblock.

You cannot validly write a single byte to VRAM. If you do so, the 8-bits are written to both the upper and lower 8-bits of the given 16-bit segment of memory. This can cause a problem when working with the video mode 4 bitmap, or with the video mode 1 or 2 affine screenblocks. To modify a single byte correctly you must read at least 16-bits, mask in the new 8-bit value, and then write back the larger value.

• Size: 96kb
• Wait states: 0
• Bus Size: 16-bit
• Read/Write: 16/32

Structs

CharblockAddress

This is just a usize, but it also allows indexing to a specific tile.

Constants

AFFINE_SCREENBLOCK_ENTRY_SIZE	The size of an affine mode screen entry.
AFFINE_SIZE0_SCREENBLOCK_ENTRY_COUNT	The number of entries in a size 0 affine screenblock.
AFFINE_SIZE0_SCREENBLOCK_SIZE	The size of a size 0 affine screenblock.
AFFINE_SIZE1_SCREENBLOCK_ENTRY_COUNT	The number of entries in a size 1 affine screenblock.
AFFINE_SIZE1_SCREENBLOCK_SIZE	The size of a size 1 affine screenblock.
AFFINE_SIZE2_SCREENBLOCK_ENTRY_COUNT	The number of entries in a size 2 affine screenblock.
AFFINE_SIZE2_SCREENBLOCK_SIZE	The size of a size 2 affine screenblock.
AFFINE_SIZE3_SCREENBLOCK_ENTRY_COUNT	The number of entries in a size 3 affine screenblock.
AFFINE_SIZE3_SCREENBLOCK_SIZE	The size of a size 3 affine screenblock.
CHARBLOCK_4BPP_COUNT	The number of 4bpp tiles in a Charblock.
CHARBLOCK_8BPP_COUNT	The number of 8bpp tiles in a Charblock.
CHARBLOCK_BG_BASE_ADDR	The base address of the background charblocks.
CHARBLOCK_BG_COUNT	There are 4 background charblocks.
CHARBLOCK_OBJ_BASE_ADDR	The base address of the object charblocks.
CHARBLOCK_OBJ_COUNT	There are 2 object charblocks.
CHARBLOCK_OBJ_TILE_COUNT	The number of object tile indexes available.
CHARBLOCK_SIZE	The size of a single Charblock.
TEXT_SCREENBLOCK_ENTRY_COUNT	The number of entries in a text screenblock.
TEXT_SCREENBLOCK_ENTRY_SIZE	The size of a text mode screen entry.
TEXT_SCREENBLOCK_SIZE	The size of a text mode screenblock.
TILE_4BPP_SIZE	The size of a 4bpp tile.
TILE_8BPP_SIZE	The size of an 8bpp tile.
VRAM_BASE_ADDR	The base address for VRAM, regardless of video mode.
VRAM_FRAME0_BASE_ADDR	Base address of the bitmap frame 0 (video modes 3, 4, and 5).
VRAM_MODE4_FRAME1_BASE_ADDR	Base address of the bitmap frame 1 (video modes 4 or 5).

Functions

index_bg_charblock	Index to a given background charblock address.
index_obj_tile	Index to a given object tile.
index_screenblock	Index to the start of a particular screenblock.