Commodore 128 Programmer's Reference Manual page 252

242 COMMODORE 128
The bit map tells the computer which pixels in the foreground to enable on the
screen. Like a road map, it spells out exactly which pixels to turn on (in the foreground)
and off (in the background color) in order to display a picture on the screen. For
example, if the bit map started at location 8192 (the C128 BASIC default) the first
byte of the bit map corresponds to the bit map pixel coordinates 0,0 through 0,7. The
second byte of the bit map, location 8193, corresponds to coordinates 1,0 through 1,7
and so on. See Figure 8-25 to see how the bit map data in locations 8192-16191
correspond to the pixels on the visual screen:
X Coordinate
319,0
Y Coordinate
0
u
u
0
0
u
u
Figure 8-25. Relationship of Bit Map Data to Screen Pixels
Now you know how bit map mode operates internally within your C128. However,
you need an easy way to turn on and off pixels in the bit map in order to display
graphics on the screen. The new, high level BASIC 7.0 commands such as DRAW,
CIRCLE, BOX and PAINT allow you to control the turning on and off of bits and their
corresponding screen pixels. The use of the X and Y coordinates on the bit map
coordinate plane easily orient you to displaying graphics. You can display high-
resolution graphics in other ways, outside of the BASIC 7.0 graphics commands.
This includes using commercial software packages that employ graphics tablets or
joysticks to draw on the screen, writing your own draw routines using a joystick or
paddle, or physically entering data into the bit map (which is painfully tedious and not
recommended).