<21/86>VDP2 User's Manual/4.3 Cell

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■4.3 Cell

A cell is a picture pattern of 8 dots horizontally x 8 dots vertically and is stored in VRAM. The number of character colors (number of colors per cell) can be selected from 16 colors, 256 colors, 2048 colors, 32768 colors, and 16,770,000 colors (16,777,216 colors). The size of each dot color data and the VRAM capacity required for one cell's worth of data vary depending on the number of colors.

●Number of character colors

Color formats for displaying characters include palette format and RGB format. In the palette format, the display color data is the color RAM address data specified by the palette number in the pattern name data and the dot color code in the cell data. In RGB format, cell data is used as display color data as is. Table 4.1 shows the number of character colors and the number of bits per dot for each color format.

Table 4.1 Number of character colors and dot data size

color format	Number of character colors	Number of bits per dot
pallet format	16 colors	4 bits
	256 colors	8 bit
	2048 colors	16 bits (only lower 11 bits are used)
RGB format	32768 colors	16 bit
RGB format	16.77 million colors	32 bits (only MSB and lower 24 bits are used)

[Note] In color RAM modes 0 and 2, 2048 colors become 1024 colors.

●Cell data structure

The data structure of each cell stored in the character pattern table changes depending on the number of bits in 1 dot. The boundary when storing in VRAM is 20H, regardless of the number of bits per dot.
The structure of cell data is shown in Table 4.2, "Details".

Table 4.2 Cell data structure

Number of bits per dot	cell data	boundary	detail
4 bits/dot	32 bytes/cell	20H byte	→
8 bits/dot	64 bytes/cell	20H byte	→
16 bits/dot	128 bytes/cell	20H byte	→
32 bits/dot	256 bytes/cell	20H byte	→

Reference: For the cell data structure based on the number of character colors, click on the details column of "Table 4.2"

●Transparent dots

The dot color code (transparent code) for transparent dots changes depending on the color format. If the color format is palette format, a dot becomes a transparent dot when all bits per dot are 0. In the case of RGB format, when the most significant bit of the dot data is 0, it becomes a transparent dot.
In the case of palette format, the color data at the beginning of the palette corresponds to the transparency code, so it is not normally used. By disabling the transparency code, this color data can also be used, and is controlled by the screen display enable register.

Table 4.3 shows the data values for transparent dots.

Table 4.3 Transparent dot data values

color format	Number of character colors	Number of bits per dot	transparent code
pallet format	16 colors	4 bits/dot	0H (4 bits)
	256 colors	8 bits/dot	00H (8 bit)
	2048 colors	16 bits/dot	000H (lower 11 bits)
RGB format	32768 colors	16 bits/dot	MSB (bit 15) is 0
RGB format	16.77 million colors	32 bits/dot	MSB (bit 31) is 0

●RGB format dot data

When the color format is RGB format, the number of character colors can be selected from 32,768 colors and 16,770,000 colors. For 16.77 million colors, specify 8 bits each for RGB, but for 32768 colors, specify the upper 5 bits of each 8 bit RGB, and the lower 3 bits are fixed to 0. Also, the most significant bit is a transparency bit that specifies whether the dot is transparent or not, and when it is 0, it becomes a transparent dot.
If you specify to disable the transparent code in the screen display enable register, the transparent bit will be ignored.
Figure 4.6 shows dot data in RGB format.

Figure 4.6 Dot data in RGB format

●For 32768 colors

color data

transparent	BLUE data 5 bits					GREEN data 5 bits					RED data 5 bits
15	14	13	12	11	10	09	08	07	06	05	04	03	02	01	00

●For RGB each 5 bits

color data

transparent	ignore							BLUE data 8 bits
31	30	29	28	27	26	25	24	23	22	21	20	19	18	17	16

GREEN data 8 bits								RED data 8 bits
15	14	13	12	11	10	09	08	07	06	05	04	03	02	01	00

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