ISSOtm: Create detailed page about LCDC to de-clobber Video Display

2019-02-02T23:29:40Z

Create detailed page about LCDC to de-clobber Video Display

New page

'''LCDC''' is the main '''LCD C'''ontrol register. Its bits toggle what elements are displayed on the screen, and how.

= Description =
== LCDC.7 - LCD Display 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 V-Blank 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 that do not follow 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 Display Select ==
This bit controls which background map the Window uses for rendering. When it's reset, the $9800 tilemap is used, otherwise it's the $9C00 one.

== LCDC.5 - Window Display Enable ==
This bit controls whether the window shall be displayed or not. (TODO : what happens when toggling this mid-scanline ?) This bit is overridden on DMG by [[#LCDC.0_-_BG.2FWindow_Display.2FPriority|bit 0]] if that bit is reset.

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 & Window Tile Data Select ==
This bit controls which [[Video_Display#VRAM_Tile_Data|addressing mode]] the BG and Window use to pick tiles.

Sprites aren't affected by this, and will always use $8000 addressing mode.

== LCDC.3 - BG Tile Map Display Select ==
This bit works similarly to [[#LCDC.6 - Window Tile Map Display Select|bit 6]]: if the bit is reset, the BG uses tilemap $9800, otherwise tilemap $9C00.

== LCDC.2 - OBJ Size ==
This bit controls the sprite size (1 tile or 2 stacked vertically).

Be cautious when changing this mid-frame from 8x8 to 8x16 : "remnants" of the sprites intended for 8x8 could "leak" into the 8x16 zone and cause artifacts.

== LCDC.1 - OBJ Display Enable ==
This bit toggles whether sprites are displayed or not.

This can be toggled mid-frame, for example to avoid sprites 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/Window Display/Priority ==
LCDC.0 has different meanings depending on Gameboy type and Mode:

=== Monochrome Gameboy, SGB and CGB in Non-CGB Mode: BG Display ===
When Bit 0 is cleared, both background and window become blank (white), and the [[#LCDC.5 - Window Display Enable|Window Display Bit]] is ignored in that case. Only Sprites may still be displayed (if enabled in Bit 1).

=== CGB in CGB Mode: BG and Window Master Priority ===
When Bit 0 is cleared, the background and window lose their priority - the sprites will be always displayed on top of background and window, independently of the priority flags in OAM and BG Map attributes.

= 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 especially in NES games is sprites 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 handler]]

LCDC - Revision history

ISSOtm: Create detailed page about LCDC to de-clobber Video Display