Commodore 128 Programmer's Reference Manual page 122

112 COMMODORE 128
background color register two. The same holds true for the third set of sixty-four ^
characters and background color register three, and the fourth set of sixty-four characters
and background color register four. The character color is controlled by color memory. j [
For example, in extended color mode, you can display a purple character with a yellow U
background on a black screen.
Each of the character display modes receives character information from one of }
two places in the Commodore 128 memory. Normally, character information is taken ! f
from character memory stored in a separate chip called ROM (Read Only Memory).
However, the Commodore 128 gives you the option of designing your own characters
and replacing the original characters with your own. Your own programmable characters j
are stored in RAM. —^
BIT MAP DISPLAY
Bit map mode allows you to display highly detailed graphics, pictures and intricate \j
drawings. This type of display mode includes standard bit map mode and multi-color bit
map mode. Bit map modes allow you to control each individual screen dot or pixel - - , -
(picture element) which provides for considerable detail in drawing pictures and other j J
computer art. These graphic displays are only supported in BASIC by the VIC chip. w
The 80-column chip is designed primarily for character display, but you can bit
map it through your own programs. See Chapter 10, Programming the 80-Column (8563) j !
Chip, for detailed information. U
The difference between text and bit map modes lies in the way in which each
screen addresses and stores information. The text screen can only manipulate entire j j
characters, each of which covers an area of 8 by 8 pixels on the screen. The more jj
powerful bit map mode exercises control over each pixel on your screen.
Standard bit map mode allows you to assign each screen dot one of two colors. Bit r
mapping is a technique that stores a bit in memory for each dot on the screen. In !
standard bit map mode, if the bit in memory is turned off, the corresponding dot on the ^
screen becomes the color of the background. If the bit in memory is turned on, the
corresponding dot on the screen becomes the color of the foreground image. The series j j
of 64,000 dots on the screen and 64,000 corresponding bits in memory control the ^
image you see on the screen. Most of the finely detailed computer graphics you see in
demonstrations and video games are bit mapped high-resolution graphics. ) (
Multi-color bit map mode is a combination of standard bit map mode and J
multi-color character mode. You can display each screen dot in one of four colors within
an 8 X 8 character grid. Again, as in multi-color character mode, there is a tradeoff [ /
between the horizontal resolution and color control. j I
SPLIT SCREEN DISPLAY
The third type of screen display, split screen, is a combination of the first two types. j (
The split-screen display outputs part of the screen as text and part in bit map mode imJ
(either standard or multi-color). The C128 is capable of this since it depends on two
parts of its memory to store the two screens: one part for text, and the other for
graphics.