Working With Graphics; Character Memory; Programmable Characters - Commodore VIC-20 Programmer's Reference Manual

WORKING WITH GRAPHICS
The graphics ability of the VIC 20 is more powerful and
sophisticated than many users realize. The following material is a
concept·by-concept guide to help you make better use of these
graphics features to enhance your games and other programs.
CHARACTER MEMORY
Each character is formed in an 8-by-8 grid of dots, where each
dot may be either "on" or "off," The character images are stored in
a special chip called the "Character Generafor ROM." The
characters are stored as a set of 8 bytes for each character, with
each byte representing the dot pattern of a row in the character, and
each bit representing a dot. A zero (0) bit means that dot is off, and
a one (1) bit means the dot is on.
The character memory in ROM begins at location 32768. The
first 8 bytes contain the pattern for the
@
sign, which has a
character code value of zero on the screen. The next 8 bytes, from
location 32776 to 32783, contain the information for forming the
letter A.
IMAGE BINARY PEEK
00011000
24
00100100 36
01000010
66
•
01111110
126
01000010
66
01000010 66
01000010 66
•
00000000 0
Each complete character settekes up 2Kof memory. 8 bytes per
character and 256 characters. Since there are two character sets,
one for upper case and graphics and the other with upper and lower
case, the character generator ROM takes up a total of 4K.
PROGRAMMABLE CHARACTERS
Since the characters are stored in ROM, it would seem like there
is no way to change them for customizing characters. However, the
memory location that tells the VIC where to find the characters is in
a RAM location in the VIC chip, which can be changed to point to
e2