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

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■ 4.3 cell

The cell is a picture pattern of 8 dots wide x 8 dots high 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 16.77 million colors (16,777,216 colors). The size of each dot color data and the VRAM capacity required for one cell of data vary depending on the number of colors.

● Number of character colors

There are two types of color formats for displaying characters: palette format and RGB format. In the palette format, the data of the color RAM address specified by the palette number in the pattern name data and the dot color code in the cell data is used as the display color data. In the RGB format, the cell data is used as the display color data as it is. Table 4.1 shows the number of character colors and the number of bits per dot at that time in 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
Palette format	16 colors	4 bits
	256 colors	8 bits
	2048 colors	16 bits (use only the lower 11 bits)
RGB format	32768 colors	16 bit
RGB format	16.77 million colors	32 bits (use only MSB and lower 24 bits)

[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 per 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 bytes	→
8 bits / dot	64 bytes / cell	20H bytes	→
16 bits / dot	128 bytes / cell	20H bytes	→
32 bits / dot	256 bytes / cell	20H bytes	→

Reference: Click the details column in "Table 4.2" for the data structure of cells by the number of character colors.

● Transparent dots

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

The data values for transparent dots are shown in Table 4.3.

Table 4.3 Transparent dot data values

Color format	Number of character colors	Number of bits per dot	Transparent code
Palette format	16 colors	4 bits / dot	0H (4 bits)
	256 colors	8 bits / dot	00H (8 bits)
	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 32768 colors and 16.77 million colors. For 16.77 million colors, specify 8 bits for each of RGB, but for 32768 colors, specify the upper 5 bits of each of the 8 bits for RGB, and the lower 3 bits are fixed at 0. The most significant bit is a transparent bit that specifies whether it is a transparent dot, and when it is 0, it becomes a transparent dot.
If you specify to disable the transparency code in the screen display enable register, the transparency bit is ignored.
Figure 4.6 shows the 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	Ten	09 09	08 08	07 07	06 06	05 05	04	03 03	02 02	01 01	00

● For RGB 5 bits each

Color data

Transparent	ignore							BLUE data 8 bits
31	30	29	28	27	26	twenty five	twenty four	twenty three	twenty two	twenty one	20	19	18	17	16

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

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★ HARDWARE Manual VDP2 User's Manual ★ Chapter 4 Scroll screen