Extra half-bright mode

The Extra-Halfbright-Modus (EHB for short) is a graphics mode of the Amiga computers from Commodore with the exception of the first Amiga 1000 . In addition to the normal 32 colors of the Amiga that can be displayed at the same time, it provides another 32 that have half the brightness of the first 32. This means that 64 colors can be displayed with just 32 color registers.

Amiga color palette

index	Red part (binary)	Green part (binary)	Blue component (binary)	colour
0	0000	0000	0000	black
1	1111	1111	0000	yellow
...	...	...	...	...
31	0111	1111	0111	green

The Amiga's color palette includes 32 entries, which are stored in the so-called color registers of the video chip. Each color is defined by a red, green and blue component (see RGB color space ), with four bits being available for each color component. Each stored color can therefore be addressed via an index from 0-31. The example table on the right illustrates the logical scheme of the so-called color palette .

Pixel representation

In order to display a pixel on the screen in color, the program must specify the index of the desired color, for example the value 31for "green". In binary representation , 5 bits are required for addressing 32 values ​​(2 ⁵ = 32).

colour	Address (binary)	Address (decimal)	Red part (binary)	Green part (binary)	Blue component (binary)
green	11111	31	0111	1111	0111

In the extra half-bright mode, one bit more is used for indexing the color palette, i.e. a total of 6 bits. This would theoretically make it possible to display 64 colors (2 ⁶ = 64). However, since the color palette only contains 32 colors, only indices from 0 to 31 can be addressed. The sixth, most significant (i.e. leftmost) bit is therefore not used for the index, but acts as a so-called flag . If this bit is not set - i.e. has the value 0 - the remaining five bits form the normal index again. According to the example above, (0) 11111 would become the index 11111 for the color green. If the sixth bit is set (i.e. has the value 1), the same palette entry with the index 11111 is used again, but here the bit values ​​of the individual color components are shifted one place to the right (due to the hardware in the Amiga chipset, corresponds in the decimal system of division by two), which changes the displayed color. Since the resulting color has about half the brightness of the base color, one speaks of the Extra-Halfbright mode (from English half half and brightness brightness). According to the example, the index (1) 11111 would refer to the palette entry 11111 for the color green, the individual color components would be shifted one place to the right so that a new color is created.

colour	Index (binary)	Index (decimal)	Red part (binary)	Green part (binary)	Blue component (binary)
Dark green	(1) 11111	31	0011	0111	0011

Individual evidence

1. ^ The Museum . old-computers.com