LCD Control
FF40 — LCDC: LCD control
LCDC is the main LCD Control register. Its bits toggle what elements are displayed on the screen, and how.
| 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
|---|---|---|---|---|---|---|---|
| LCD & PPU enable | Window tile map | Window enable | BG & Window tiles | BG tile map | OBJ size | OBJ enable | BG & Window enable / priority |
- LCD & PPU enable:
0= Off;1= On - Window tile map area:
0= 9800–9BFF;1= 9C00–9FFF - Window enable:
0= Off;1= On - BG & Window tile data area:
0= 8800–97FF;1= 8000–8FFF - BG tile map area:
0= 9800–9BFF;1= 9C00–9FFF - OBJ size:
0= 8×8;1= 8×16 - OBJ enable:
0= Off;1= On - BG & Window enable / priority [Different meaning in CGB Mode]:
0= Off;1= On
LCDC.7 — LCD enable
This bit controls whether the LCD is on and the PPU is active. Setting it to 0 turns both off, which grants immediate and full access to VRAM, OAM, etc.
CAUTION
Stopping LCD operation (Bit 7 from 1 to 0) may be performed during VBlank ONLY, disabling the display outside of the VBlank period may damage the hardware by burning in a black horizontal line similar to that which appears when the GB is turned off. This appears to be a serious issue. Nintendo is reported to reject any games not following this rule.
When the display is disabled the screen is blank, which on DMG is displayed as a white “whiter” than color #0.
On SGB, the screen doesn’t turn white, it appears that the previous picture sticks to the screen. (TODO: research this more.)
When re-enabling the LCD, the PPU will immediately start drawing again, but the screen will stay blank during the first frame.
LCDC.6 — Window tile map area
This bit controls which background map the Window uses for rendering. When it’s clear (0), the $9800 tilemap is used, otherwise it’s the $9C00 one.
LCDC.5 — Window enable
This bit controls whether the window shall be displayed or not. This bit is overridden on DMG by bit 0 if that bit is clear.
Changing the value of this register mid-frame triggers a more complex behaviour: see further below.
Note that on CGB models, setting this bit to 0 then back to 1 mid-frame may cause the second write to be ignored. (TODO: test this.)
LCDC.4 — BG and Window tile data area
This bit controls which addressing mode the BG and Window use to pick tiles.
Objects (sprites) aren’t affected by this, and will always use the $8000 addressing mode.
LCDC.3 — BG tile map area
This bit works similarly to LCDC bit 6: if the bit is clear (0), the BG uses tilemap $9800, otherwise tilemap $9C00.
LCDC.2 — OBJ size
This bit controls the size of all objects (1 tile or 2 stacked vertically).
Be cautious when changing object size mid-frame. Changing from 8×8 to 8×16 pixels mid-frame within 8 scanlines of the bottom of an object causes the object’s second tile to be visible for the rest of those 8 lines. If the size is changed during mode 2 or 3, remnants of objects in range could “leak” into the other tile and cause artifacts.
LCDC.1 — OBJ enable
This bit toggles whether objects are displayed or not.
This can be toggled mid-frame, for example to avoid objects being displayed on top of a status bar or text box.
(Note: toggling mid-scanline might have funky results on DMG? Investigation needed.)
LCDC.0 — BG and Window enable/priority
LCDC.0 has different meanings depending on Game Boy type and Mode:
Non-CGB Mode (DMG, SGB and CGB in compatibility mode): BG and Window display
When Bit 0 is cleared, both background and window become blank (white), and the Window Display Bit is ignored in that case. Only objects may still be displayed (if enabled in Bit 1).
CGB Mode: BG and Window master priority
When Bit 0 is cleared, the background and window lose their priority - the objects will be always displayed on top of background and window, independently of the priority flags in OAM and BG Map attributes.
When Bit 0 is set, pixel priority is resolved as described here.
Using LCDC
LCDC is a powerful tool: each bit controls a lot of behavior, and can be modified at any time during the frame.
One of the important aspects of LCDC is that unlike VRAM, the PPU never locks it. It’s thus possible to modify it mid-scanline!
Faux-layer textbox/status bar
A problem often seen in 8-bit games is objects rendering on top of the textbox/status bar. It’s possible to prevent this using LCDC if the textbox/status bar is “alone” on its scanlines:
- Set LCDC.1 to 1 for gameplay scanlines
- Set LCDC.1 to 0 for textbox/status bar scanlines
Usually, these bars are either at the top or bottom of the screen, so the bit can be set by the VBlank and/or STAT handlers. Hiding objects behind a right-side window is more challenging.