Atari ST series Technical Reference Manual page 72

Each bit of screen memory can hold either the number 0
or 1. On a monochrome system, only one bit is needed to
represent a screen dot or pixel (picture element), because
each dot on the screen is either white (off) or black (on). But
with a color ST system, things are somewhat different. In
medium-resolution mode, any dot can be one of four colors.
Two binary digits are used to yield four possible combina­
tions:
Bit
Pattern Value
00 0
01 1
10 2
11 3
Since each dot of color needs two bits of memory to de­
scribe it, each 16-pixel group on the screen is represented by
two consecutive words in memory. The first word supplies
the low bits of color information and the second supplies the
high-color bits. In low-resolution mode, any dot can be one
of 16 colors, so four bits are required to describe a single
pixel. Each 16-pixel group on the screen is represented by
four consecutive words in screen memory. The first word
contains all of the low-color bits for the group, and each suc­
cessive word holds the next higher bit (see Figure 4-2). In
low- and medium-resolution modes, each row of dots re­
quires 160 bytes of display memory, as opposed to the 80
bytes required for each line in the high-resolution mode. But
since these modes use only 200 rows of dots, they each re­
quire the same 32,000 bytes of display memory as used by
the high-resolution mode.
In monochrome mode, each bit can represent a specific
color because there are two bit states (1 and 0) and 2 colors
(black and white). But the ST color screen is capable of dis­
playing 512 different colors. Clearly, in color mode, each set
of bits can't represent a different color, using a code where 0
represents white, 1 stands for black, and so on. Instead, the
number stored in the memory location that corresponds to a
screen dot location refers to a hardware color register.
The color registers may be thought of as a set of 16
pens, each of which may be filled with ink that is colored in
any of the 512 shades supported by the ST. Register 0 always
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CHAPTER 4