Commodore Plus 4 - Page 232

ZZO Programming Graphics
Characters
In addition to being used for most applications involving text, characters are
frequently the most efficient means of displaying many other types of graphic
information on the screen. The character set can be expanded to include 256
completely different characters. The entire screen is controlled through the use of
only 1000 memory locations for screen codes (and 1000 for color information).
This makes access to the screen extremely fast.
Locating a Character Set inMemory The standard built-in upper case/ graphics
character set is located in ROM at $D000. The upper/ lower case set is located in
ROM at $D400. If you are using a custom character set, it is usually located in
RAM. To tell the graphics chip to look at RAM, you must clear bit 2 of $FF12.
The character set is located by setting the high 6 bits of $FF 13 to the high 6 bits of
the character set location. Care should be taken to preserve the 2 low bits of
$FF13 because they are used for other purposes.
Normally, a character set consists of 128 eight-byte patterns (IK). Hence a
character set location must be on a IK boundary. Any screen code greater than
127 will produce a reversed screen image in high-resolution mode. To expand the
character set to 256 separate patterns, turn off the hardware reverse by setting bit
7 of $FF07 to 1. When this is done, your character set must be located on a 2K
boundary.
Locating Screen and Color Memory A character is placed on the screen by
placing its screen code into a location of screen memory. The screen code
(multiplied by 8) is used by the graphics chip to look up the character's pattern in
the character set. The color and luminance associated with a particular screen
location are stored in the corresponding location in color memory.
These two sections of memory are each IK (1000 locations for the 25 rows of 40
columns and 24 unused locations). They are treated as a single 2K block. Color
memory is located by setting the high 5 bits of $FF14 to the high 5 bits of the
desired color memory location. Screen memory is always located in the IK
immediately following color memory. Color memory must be located on a 2K
boundary. Normally, color memory is located at $0800 (2048) and screen memory
at S0C00 (3072).
High-Resolution Characters In high-resolution character mode, each dot in an
8><8 character cell is controlled by a single bit in the character definition. The
screen code stored in screen memory (multiplied by 8 and added to the character
set base address) points at the character definition. The character definition
consists of 8 bytes. The first byte corresponds to the top row of the character and