Commodore 128 Programmer's Reference Manual page 311

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PROGRAMMING THE 80-COLUMN (8563) CHIP 301
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In general, add an offset of 1 for each character position to the right of the
_ upper-left corner of the frame. To reach the same column in the row below the current
character position, add an offset of 80. Although the screen (frame) appears as a
two-dimensional matrix of characters, the address references in memory are accessed
linearly in sequence. For example, although the character in the top-left corner of the
M screen is only 24 character rows away from the character in the lower-left corner of the
( ' screen, in memory, the locations of those characters are really 1920 (24 x 80) memory
locations apart. So a frame, the size of the default screen (80 x 25) requires 2000
P[ memory locations to store the contents of the 2000 character positions on the
! I frame. The contents of each display RAM location (acting as an indexed pointer into the
character definitions) tells the 8563 which character, out of the 256 available characters
j-j in either character set, to place at any particular position on the 80 by 25 character
! frame.
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CHARACTER ATTRIBUTES
The character attributes are a set of 8563 RAM locations that assign video enhance
ment qualities to the characters in the frame. Each character position in the frame
is assigned its own attribute byte, so a frame (screen) of 80 columns by 25 lines
requires 2000 attribute bytes. The default location of the attribute bytes for the 80
by 25 frame is stored in the range $0800 through $0FCF in the 16K of independent
8563 RAM.
The attributes (and the corresponding bits) that enhance the video qualities of the
characters in the frame are as follows:
BIT
"! 7
6
5
-^ 4
3
2
1
r
«
ATTRIBUTE
Alternate Character Set
Reverse Video Attribute
Underline Attribute
Blink Attribute
Red
Green
Blue
Intensity
The ALTERNATE CHARACTER SET (bit 7), when high (1), enables an
additional 256 characters to be displayed within the frame.