Atari ST series Technical Reference Manual page 71

Graphics
To understand the XBIOS graphics functions, you must first
understand how the ST display is arranged in memory. The
ST uses a bitmapped display. The bits in memory control the
dots (picture elements or pixels) on the screen. On a bit­
mapped screen, everything, including text, is drawn using a
number of dots.
The system used for the monochrome display on the ST
is very simple. Each dot on the screen is represented by a
single binary digit (bit) of memory. Screen memory is orga­
nized in such a way that the first byte represents the eight
dots in the top left comer of the screen, and each succeeding
byte represents the next eight dots to the right. Since each
line contains 640 dots across, the first 80 bytes fill up the top
line and the eighty-first byte is used to represent the first
eight dots on the second line. There are 400 lines of 80 bytes
each on the monochrome screen which means that 32,000
bytes of screen memory are used to represent the 256,000
dots on screen.
Figure 4-1. Monochrome Screen Memory
MSB=*LSB
0 1 1 1 0 1 0 1 = 117
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