Difference between revisions of "Video Display"

Revision as of 15:17, 8 May 2017

LCD Control Register

FF40 - LCDC - LCD Control (R/W)

 Bit 7 - LCD Display Enable             (0=Off, 1=On)
 Bit 6 - Window Tile Map Display Select (0=9800-9BFF, 1=9C00-9FFF)
 Bit 5 - Window Display Enable          (0=Off, 1=On)
 Bit 4 - BG & Window Tile Data Select   (0=8800-97FF, 1=8000-8FFF)
 Bit 3 - BG Tile Map Display Select     (0=9800-9BFF, 1=9C00-9FFF)
 Bit 2 - OBJ (Sprite) Size              (0=8x8, 1=8x16)
 Bit 1 - OBJ (Sprite) Display Enable    (0=Off, 1=On)
 Bit 0 - BG Display (for CGB see below) (0=Off, 1=On)

LCDC.7 - LCD Display Enable

CAUTION: Stopping LCD operation (Bit 7 from 1 to 0) may be performed during V-Blank ONLY, disabling the display outside of the V-Blank period may damage the hardware. This appears to be a serious issue, Nintendo is reported to reject any games that do not follow this rule. V-blank can be confirmed when the value of LY is greater than or equal to 144. When the display is disabled the screen is blank (white), and VRAM and OAM can be accessed freely.

--- LCDC.0 has different Meanings depending on Gameboy Type ---

LCDC.0 - 1) Monochrome Gameboy and SGB: BG Display

When Bit 0 is cleared, the background becomes blank (white). Window and Sprites may still be displayed (if enabled in Bit 1 and/or Bit 5).

LCDC.0 - 2) 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.

LCDC.0 - 3) CGB in Non CGB Mode: BG and Window Display

When Bit 0 is cleared, both background and window become blank (white), ie. the Window Display Bit (Bit 5) is ignored in that case. Only Sprites may still be displayed (if enabled in Bit 1). This is a possible compatibility problem - any monochrome games (if any) that disable the background, but still want to display the window wouldn't work properly on CGBs.

LCD Status Register

FF41 - STAT - LCDC Status (R/W)

 Bit 6 - LYC=LY Coincidence Interrupt (1=Enable) (Read/Write)
 Bit 5 - Mode 2 OAM Interrupt         (1=Enable) (Read/Write)
 Bit 4 - Mode 1 V-Blank Interrupt     (1=Enable) (Read/Write)
 Bit 3 - Mode 0 H-Blank Interrupt     (1=Enable) (Read/Write)
 Bit 2 - Coincidence Flag  (0:LYC<>LY, 1:LYC=LY) (Read Only)
 Bit 1-0 - Mode Flag       (Mode 0-3, see below) (Read Only)
           0: During H-Blank
           1: During V-Blank
           2: During Searching OAM
           3: During Transferring Data to LCD Driver

The two lower STAT bits show the current status of the LCD controller.

 Mode 0: The LCD controller is in the H-Blank period and
         the CPU can access both the display RAM (8000h-9FFFh)
         and OAM (FE00h-FE9Fh)
 
 Mode 1: The LCD controller is in the V-Blank period (or the
         display is disabled) and the CPU can access both the
         display RAM (8000h-9FFFh) and OAM (FE00h-FE9Fh)
 
 Mode 2: The LCD controller is reading from OAM memory.
         The CPU <cannot> access OAM memory (FE00h-FE9Fh)
         during this period.
 
 Mode 3: The LCD controller is reading from both OAM and VRAM,
         The CPU <cannot> access OAM and VRAM during this period.
         CGB Mode: Cannot access Palette Data (FF69,FF6B) either.

The following are typical when the display is enabled:

 Mode 2  2_____2_____2_____2_____2_____2___________________2____
 Mode 3  _33____33____33____33____33____33__________________3___
 Mode 0  ___000___000___000___000___000___000________________000
 Mode 1  ____________________________________11111111111111_____

The Mode Flag goes through the values 0, 2, and 3 at a cycle of about 109uS. 0 is present about 48.6uS, 2 about 19uS, and 3 about 41uS. This is interrupted every 16.6ms by the VBlank (1). The mode flag stays set at 1 for about 1.08 ms.

Mode 0 is present between 201-207 clks, 2 about 77-83 clks, and 3 about 169-175 clks. A complete cycle through these states takes 456 clks. VBlank lasts 4560 clks. A complete screen refresh occurs every 70224 clks.)

A hardware bug in the gray Game Boys makes the LCD interrupt sometimes trigger when writing to STAT (including writing $00) during a H-Blank or V-Blank period.

LCD Interrupts

INT 40 - V-Blank Interrupt

The V-Blank interrupt occurs ca. 59.7 times a second on a regular GB and ca. 61.1 times a second on a Super GB (SGB). This interrupt occurs at the beginning of the V-Blank period (LY=144). During this period video hardware is not using video ram so it may be freely accessed. This period lasts approximately 1.1 milliseconds.

INT 48 - LCDC Status Interrupt

There are various reasons for this interrupt to occur as described by the STAT register ($FF40). One very popular reason is to indicate to the user when the video hardware is about to redraw a given LCD line. This can be useful for dynamically controlling the SCX/SCY registers ($FF43/$FF42) to perform special video effects.

LCD Position and Scrolling

FF42 - SCY - Scroll Y (R/W), FF43 - SCX - Scroll X (R/W)

Specifies the position in the 256x256 pixels BG map (32x32 tiles) which is to be displayed at the upper/left LCD display position. Values in range from 0-255 may be used for X/Y each, the video controller automatically wraps back to the upper (left) position in BG map when drawing exceeds the lower (right) border of the BG map area.

FF44 - LY - LCDC Y-Coordinate (R)

The LY indicates the vertical line to which the present data is transferred to the LCD Driver. The LY can take on any value between 0 through 153. The values between 144 and 153 indicate the V-Blank period. Writing will reset the counter.

FF45 - LYC - LY Compare (R/W)

The Gameboy permanently compares the value of the LYC and LY registers. When both values are identical, the coincident bit in the STAT register becomes set, and (if enabled) a STAT interrupt is requested.

FF4A - WY - Window Y Position (R/W), FF4B - WX - Window X Position minus 7 (R/W)

Specifies the upper/left positions of the Window area. (The window is an alternate background area which can be displayed above of the normal background. OBJs (sprites) may be still displayed above or behind the window, just as for normal BG.)

The window becomes visible (if enabled) when positions are set in range WX=0..166, WY=0..143. A position of WX=7, WY=0 locates the window at upper left, it is then completely covering normal background.

If WX is set to 0, the window will "stutter" when SCX is changed.

LCD Monochrome Palettes

FF47 - BGP - BG Palette Data (R/W) - Non CGB Mode Only

This register assigns gray shades to the color numbers of the BG and Window tiles.

 Bit 7-6 - Shade for Color Number 3
 Bit 5-4 - Shade for Color Number 2
 Bit 3-2 - Shade for Color Number 1
 Bit 1-0 - Shade for Color Number 0

The four possible gray shades are:

 0  White
 1  Light gray
 2  Dark gray
 3  Black

In CGB Mode the Color Palettes are taken from CGB Palette Memory instead.

FF48 - OBP0 - Object Palette 0 Data (R/W) - Non CGB Mode Only

This register assigns gray shades for sprite palette 0. It works exactly as BGP (FF47), except that the lower two bits aren't used because sprite data 00 is transparent.

FF49 - OBP1 - Object Palette 1 Data (R/W) - Non CGB Mode Only

This register assigns gray shades for sprite palette 1. It works exactly as BGP (FF47), except that the lower two bits aren't used because sprite data 00 is transparent.

LCD Color Palettes (CGB only)

FF68 - BCPS/BGPI - CGB Mode Only - Background Palette Index

This register is used to address a byte in the CGBs Background Palette Memory. Each two byte in that memory define a color value. The first 8 bytes define Color 0-3 of Palette 0 (BGP0), and so on for BGP1-7.

 Bit 0-5   Index (00-3F)
 Bit 7     Auto Increment  (0=Disabled, 1=Increment after Writing)

Data can be read/written to/from the specified index address through Register FF69. When the Auto Increment Bit is set then the index is automatically incremented after each <write> to FF69. Auto Increment has no effect when <reading> from FF69, so the index must be manually incremented in that case.

FF69 - BCPD/BGPD - CGB Mode Only - Background Palette Data

This register allows to read/write data to the CGBs Background Palette Memory, addressed through Register FF68. Each color is defined by two bytes (Bit 0-7 in first byte).

 Bit 0-4   Red Intensity   (00-1F)
 Bit 5-9   Green Intensity (00-1F)
 Bit 10-14 Blue Intensity  (00-1F)

Much like VRAM, Data in Palette Memory cannot be read/written during the time when the LCD Controller is reading from it. (That is when the STAT register indicates Mode 3). Note: Initially all background colors are initialized as white.

FF6A - OCPS/OBPI - CGB Mode Only - Sprite Palette Index, FF6B - OCPD/OBPD - CGB Mode Only - Sprite Palette Data

These registers are used to initialize the Sprite Palettes OBP0-7, identically as described above for Background Palettes. Note that four colors may be defined for each OBP Palettes - but only Color 1-3 of each Sprite Palette can be displayed, Color 0 is always transparent, and can be initialized to a don't care value. Note: Initially all sprite colors are uninitialized.

RGB Translation by CGBs

When developing graphics on PCs, note that the RGB values will have different appearance on CGB displays as on VGA monitors: The highest intensity will produce Light Gray color rather than White. The intensities are not linear; the values 10h-1Fh will all appear very bright, while medium and darker colors are ranged at 00h-0Fh. The CGB display will mix colors quite oddly, increasing intensity of only one R,G,B color will also influence the other two R,G,B colors. For example, a color setting of 03EFh (Blue=0, Green=1Fh, Red=0Fh) will appear as Neon Green on VGA displays, but on the CGB it'll produce a decently washed out Yellow.

RGB Translation by GBAs

Even though GBA is described to be compatible to CGB games, most CGB games are completely unplayable on GBAs because most colors are invisible (black). Of course, colors such like Black and White will appear the same on both CGB and GBA, but medium intensities are arranged completely different. Intensities in range 00h..0Fh are invisible/black (unless eventually under best sunlight circumstances, and when gazing at the screen under obscure viewing angles), unfortunately, these intensities are regularly used by most existing CGB games for medium and darker colors. Newer CGB games may avoid this effect by changing palette data when detecting GBA hardware. A relative simple method would be using the formula GBA=CGB/2+10h for each R,G,B intensity, probably the result won't be perfect, and (once colors became visible) it may turn out that the color mixing is different also, anyways, it'd be still ways better than no conversion. Asides, this translation method should have been VERY easy to implement in GBA hardware directly, even though Nintendo obviously failed to do so. How did they say, This seal is your assurance for excellence in workmanship and so on?

LCD OAM DMA Transfers

FF46 - DMA - DMA Transfer and Start Address (R/W)

Writing to this register launches a DMA transfer from ROM or RAM to OAM memory (sprite attribute table). The written value specifies the transfer source address divided by 100h, ie. source & destination are:

 Source:      XX00-XX9F   ;XX in range from 00-F1h
 Destination: FE00-FE9F

It takes 160 microseconds until the transfer has completed (80 microseconds in CGB Double Speed Mode), during this time the CPU can access only HRAM (memory at FF80-FFFE). For this reason, the programmer must copy a short procedure into HRAM, and use this procedure to start the transfer from inside HRAM, and wait until the transfer has finished:

  ld  (0FF46h),a ;start DMA transfer, a=start address/100h
  ld  a,28h      ;delay...
 wait:           ;total 5x40 cycles, approx 200ms
  dec a          ;1 cycle
  jr  nz,wait    ;4 cycles

Most programs are executing this procedure from inside of their VBlank procedure, but it is possible to execute it during display redraw also, allowing to display more than 40 sprites on the screen (ie. for example 40 sprites in upper half, and other 40 sprites in lower half of the screen).

LCD VRAM DMA Transfers (CGB only)

FF51 - HDMA1 - CGB Mode Only - New DMA Source, High

FF52 - HDMA2 - CGB Mode Only - New DMA Source, Low

FF53 - HDMA3 - CGB Mode Only - New DMA Destination, High

FF54 - HDMA4 - CGB Mode Only - New DMA Destination, Low

FF55 - HDMA5 - CGB Mode Only - New DMA Length/Mode/Start

These registers are used to initiate a DMA transfer from ROM or RAM to VRAM. The Source Start Address may be located at 0000-7FF0 or A000-DFF0, the lower four bits of the address are ignored (treated as zero). The Destination Start Address may be located at 8000-9FF0, the lower four bits of the address are ignored (treated as zero), the upper 3 bits are ignored either (destination is always in VRAM).

Writing to FF55 starts the transfer, the lower 7 bits of FF55 specify the Transfer Length (divided by 10h, minus 1). Ie. lengths of 10h-800h bytes can be defined by the values 00h-7Fh. And the upper bit of FF55 indicates the Transfer Mode:

Bit7=0 - General Purpose DMA

When using this transfer method, all data is transferred at once. The execution of the program is halted until the transfer has completed. Note that the General Purpose DMA blindly attempts to copy the data, even if the LCD controller is currently accessing VRAM. So General Purpose DMA should be used only if the Display is disabled, or during V-Blank, or (for rather short blocks) during H-Blank. The execution of the program continues when the transfer has been completed, and FF55 then contains a value if FFh.

Bit7=1 - H-Blank DMA

The H-Blank DMA transfers 10h bytes of data during each H-Blank, ie. at LY=0-143, no data is transferred during V-Blank (LY=144-153), but the transfer will then continue at LY=00. The execution of the program is halted during the separate transfers, but the program execution continues during the 'spaces' between each data block. Note that the program may not change the Destination VRAM bank (FF4F), or the Source ROM/RAM bank (in case data is transferred from bankable memory) until the transfer has completed! Reading from Register FF55 returns the remaining length (divided by 10h, minus 1), a value of 0FFh indicates that the transfer has completed. It is also possible to terminate an active H-Blank transfer by writing zero to Bit 7 of FF55. In that case reading from FF55 may return any value for the lower 7 bits, but Bit 7 will be read as "1".

Confirming if the DMA Transfer is Active

Reading Bit 7 of FF55 can be used to confirm if the DMA transfer is active (1=Not Active, 0=Active). This works under any circumstances - after completion of General Purpose, or H-Blank Transfer, and after manually terminating a H-Blank Transfer.

Transfer Timings

In both Normal Speed and Double Speed Mode it takes about 8us to transfer a block of 10h bytes. That are 8 cycles in Normal Speed Mode, and 16 'fast' cycles in Double Speed Mode. Older MBC controllers (like MBC1-4) and slower ROMs are not guaranteed to support General Purpose or H-Blank DMA, that's because there are always 2 bytes transferred per microsecond (even if the itself program runs it Normal Speed Mode).

VRAM Tile Data

Tile Data is stored in VRAM at addresses 8000h-97FFh, this area defines the Bitmaps for 192 Tiles. In CGB Mode 384 Tiles can be defined, because memory at 0:8000h-97FFh and at 1:8000h-97FFh is used.

Each tile is sized 8x8 pixels and has a color depth of 4 colors/gray shades. Tiles can be displayed as part of the Background/Window map, and/or as OAM tiles (foreground sprites). Note that foreground sprites may have only 3 colors, because color 0 is transparent.

As it was said before, there are two Tile Pattern Tables at $8000-8FFF and at $8800-97FF. The first one can be used for sprites and the background. Its tiles are numbered from 0 to 255. The second table can be used for the background and the window display and its tiles are numbered from -128 to 127.

Each Tile occupies 16 bytes, where each 2 bytes represent a line:

 Byte 0-1  First Line (Upper 8 pixels)
 Byte 2-3  Next Line
 etc.

For each line, the first byte defines the least significant bits of the color numbers for each pixel, and the second byte defines the upper bits of the color numbers. In either case, Bit 7 is the leftmost pixel, and Bit 0 the rightmost.

So, each pixel is having a color number in range from 0-3. The color numbers are translated into real colors (or gray shades) depending on the current palettes. The palettes are defined through registers FF47-FF49 (Non CGB Mode), and FF68-FF6B (CGB Mode).

VRAM Background Maps

The Gameboy contains two 32x32 tile background maps in VRAM at addresses 9800h-9BFFh and 9C00h-9FFFh. Each can be used either to display "normal" background, or "window" background.

BG Map Tile Numbers

An area of VRAM known as Background Tile Map contains the numbers of tiles to be displayed. It is organized as 32 rows of 32 bytes each. Each byte contains a number of a tile to be displayed. Tile patterns are taken from the Tile Data Table located either at $8000-8FFF or $8800-97FF. In the first case, patterns are numbered with unsigned numbers from 0 to 255 (i.e. pattern #0 lies at address $8000). In the second case, patterns have signed numbers from -128 to 127 (i.e. pattern #0 lies at address $9000). The Tile Data Table address for the background can be selected via LCDC register.

BG Map Attributes (CGB Mode only)

In CGB Mode, an additional map of 32x32 bytes is stored in VRAM Bank 1 (each byte defines attributes for the corresponding tile-number map entry in VRAM Bank 0):

 Bit 0-2  Background Palette number  (BGP0-7)
 Bit 3    Tile VRAM Bank number      (0=Bank 0, 1=Bank 1)
 Bit 4    Not used
 Bit 5    Horizontal Flip            (0=Normal, 1=Mirror horizontally)
 Bit 6    Vertical Flip              (0=Normal, 1=Mirror vertically)
 Bit 7    BG-to-OAM Priority         (0=Use OAM priority bit, 1=BG Priority)

When Bit 7 is set, the corresponding BG tile will have priority above all OBJs (regardless of the priority bits in OAM memory). There's also an Master Priority flag in LCDC register Bit 0 which overrides all other priority bits when cleared.

As one background tile has a size of 8x8 pixels, the BG maps may hold a picture of 256x256 pixels, an area of 160x144 pixels of this picture can be displayed on the LCD screen.

Normal Background (BG)

The SCY and SCX registers can be used to scroll the background, allowing to select the origin of the visible 160x144 pixel area within the total 256x256 pixel background map. Background wraps around the screen (i.e. when part of it goes off the screen, it appears on the opposite side.)

The Window

Besides background, there is also a "window" overlaying the background. The window is not scrollable i.e. it is always displayed starting from its left upper corner. The location of a window on the screen can be adjusted via WX and WY registers. Screen coordinates of the top left corner of a window are WX-7,WY. The tiles for the window are stored in the Tile Data Table. Both the Background and the window share the same Tile Data Table.

Both background and window can be disabled or enabled separately via bits in the LCDC register.

VRAM Sprite Attribute Table (OAM)

Gameboy video controller can display up to 40 sprites either in 8x8 or in 8x16 pixels. Because of a limitation of hardware, only ten sprites can be displayed per scan line. Sprite patterns have the same format as BG tiles, but they are taken from the Sprite Pattern Table located at $8000-8FFF and have unsigned numbering.

Sprite attributes reside in the Sprite Attribute Table (OAM - Object Attribute Memory) at $FE00-FE9F. Each of the 40 entries consists of four bytes with the following meanings:

Byte0 - Y Position

Specifies the sprites vertical position on the screen (minus 16). An off-screen value (for example, Y=0 or Y>=160) hides the sprite.

Byte1 - X Position

Specifies the sprites horizontal position on the screen (minus 8). An off-screen value (X=0 or X>=168) hides the sprite, but the sprite still affects the priority ordering - a better way to hide a sprite is to set its Y-coordinate off-screen.

Byte2 - Tile/Pattern Number

Specifies the sprites Tile Number (00-FF). This (unsigned) value selects a tile from memory at 8000h-8FFFh. In CGB Mode this could be either in VRAM Bank 0 or 1, depending on Bit 3 of the following byte. In 8x16 mode, the lower bit of the tile number is ignored. IE: the upper 8x8 tile is "NN AND FEh", and the lower 8x8 tile is "NN OR 01h".

Byte3 - Attributes/Flags:

 Bit7   OBJ-to-BG Priority (0=OBJ Above BG, 1=OBJ Behind BG color 1-3)
        (Used for both BG and Window. BG color 0 is always behind OBJ)
 Bit6   Y flip          (0=Normal, 1=Vertically mirrored)
 Bit5   X flip          (0=Normal, 1=Horizontally mirrored)
 Bit4   Palette number  **Non CGB Mode Only** (0=OBP0, 1=OBP1)
 Bit3   Tile VRAM-Bank  **CGB Mode Only**     (0=Bank 0, 1=Bank 1)
 Bit2-0 Palette number  **CGB Mode Only**     (OBP0-7)

Sprite Priorities and Conflicts

When sprites with different x coordinate values overlap, the one with the smaller x coordinate (closer to the left) will have priority and appear above any others. This applies in Non CGB Mode only. When sprites with the same x coordinate values overlap, they have priority according to table ordering. (i.e. $FE00 - highest, $FE04 - next highest, etc.) In CGB Mode priorities are always assigned like this.

Only 10 sprites can be displayed on any one line. When this limit is exceeded, the lower priority sprites (priorities listed above) won't be displayed. To keep unused sprites from affecting onscreen sprites set their Y coordinate to Y=0 or Y=>144+16. Just setting the X coordinate to X=0 or X=>160+8 on a sprite will hide it but it will still affect other sprites sharing the same lines.

Writing Data to OAM Memory

The recommended method is to write the data to normal RAM first, and to copy that RAM to OAM by using the DMA transfer function, initiated through DMA register (FF46). Beside for that, it is also possible to write data directly to the OAM area by using normal LD commands, this works only during the H-Blank and V-Blank periods. The current state of the LCD controller can be read out from the STAT register (FF41).

Accessing VRAM and OAM

CAUTION

When the LCD Controller is drawing the screen it is directly reading from Video Memory (VRAM) and from the Sprite Attribute Table (OAM). During these periods the Gameboy CPU may not access the VRAM and OAM. That means, any attempts to write to VRAM/OAM are ignored (the data remains unchanged). And any attempts to read from VRAM/OAM will return undefined data (typically a value of FFh).

For this reason the program should verify if VRAM/OAM is accessible before actually reading or writing to it. This is usually done by reading the Mode Bits from the STAT Register (FF41). When doing this (as described in the examples below) you should take care that no interrupts occur between the wait loops and the following memory access - the memory is guaranteed to be accessible only for a few cycles directly after the wait loops have completed.

VRAM (memory at 8000h-9FFFh) is accessible during Mode 0-2

 Mode 0 - H-Blank Period,
 Mode 1 - V-Blank Period, and
 Mode 2 - Searching OAM Period

A typical procedure that waits for accessibility of VRAM would be:

 ld   hl,0FF41h    ;-STAT Register
@@wait:            ;\
 bit  1,(hl)       ; Wait until Mode is 0 or 1
 jr   nz,@@wait    ;/

Even if the procedure gets executed at the <end> of Mode 0 or 1, it is still proof to assume that VRAM can be accessed for a few more cycles because in either case the following period is Mode 2 which allows access to VRAM either. In CGB Mode an alternate method to write data to VRAM is to use the HDMA Function (FF51-FF55).

OAM (memory at FE00h-FE9Fh) is accessible during Mode 0-1

 Mode 0 - H-Blank Period
 Mode 1 - V-Blank Period

Aside from that, OAM can be accessed at any time by using the DMA Function (FF46). When directly reading or writing to OAM, a typical procedure that waits for accessibility of OAM Memory would be:

 ld   hl,0FF41h    ;-STAT Register
@@wait1:           ;\
 bit  1,(hl)       ; Wait until Mode is -NOT- 0 or 1
 jr   z,@@wait1    ;/
@@wait2:           ;\
 bit  1,(hl)       ; Wait until Mode 0 or 1 -BEGINS-
 jr   nz,@@wait2   ;/

The two wait loops ensure that Mode 0 or 1 will last for a few clock cycles after completion of the procedure. In V-Blank period it might be recommended to skip the whole procedure - and in most cases using the above mentioned DMA function would be more recommended anyways.

Note

When the display is disabled, both VRAM and OAM are accessible at any time. The downside is that the screen is blank (white) during this period, so that disabling the display would be recommended only during initialization.