Atari 800

Memory Scan Counter
Addresses each byte
One line worth of memory is
loaded into the shift register
Memory
Shift register data is displayed for four TV scan lines in this example.
In Instruction Register (IR) display modes 8 through F, one or two
bits of memory are used to specify what is to be displayed on each pixel
of the screen. Pixel sizes range from 1/2 clock by 1 TV line to 4 clocks
by 8 TV lines. The OS and BASIC support most of these graphics modes
(BASIC
GRAPHICS command). Two modes, C and E, are not supported by the
OS. These modes have rectangular pixels, which are approximately twice as
wide as they are high.
In IR mode F, only one color (COLPF2) can be displayed. Two different
luminances are available. If a bit is a zero, then the luminance of the
corresponding pixel comes from COLPF2. If the bit is a one, them the
luminanc e is determined by the contents of COLPFl (abbreviated to PF1).
In IR modes 9,B, and C, two different colors can be displayed. A
zero indicates background color and a one indicates PFO color. The
difference between the various modes is in the size of the pixels.
In IR modes 8,A,D, and E, two bits are used to specify the color
of each pixel. This allows four different colors to be displayed.
However, only four pixels can be packed into each byte, instead of eight
as in the previous modes. The bit assignments are shown below.
SHIFT REGISTER
7 6[5 4 [3 2 1 0
II.ll
1 6 I 5 4 3 2 I 1 o
2 bits form
one pixel