Commodore Plus 4 - Page 221

Graphics Programming in BASIC 809
Line-by-Line Explanation
10 Clear the screen.
20 Turn on extended color mode.
30 For each background color register:
40 Get a color and luminance.
50 Put the selection into the register and continue.
60 For each possible character:
70 Put up a set with background color 1.
80 Put up a set with background color 2.
90 Put up a set with background color 3.
100 Put up a set with background color 4.
110 Next character.
120 I points to a spot in color memory.
130 For each color and luminance:
140 Put color and luminance into color memory.
150 Put color and luminance into color memory.
160 Increment pointer.
170 Next luminance and color.
180 Wait for a key press, then turn off the extended color mode.
190 Return the background color to white.
Smooth Scrolling
When the last line of the Plus/4 screen is filled with text, the screen is automati
cally scrolled. That is, the top line is discarded, the rest of the lines are moved up
one line, and the last line is cleared. Thus, the text moves up a whole line at a time.
In some applications it is desirable to scroll the text (or bit map) smoothly, that is,
one pixel at a time, either vertically or horizontally.
Vertical scrolling is handled by memory location 65286 ($FF06). Normally, the
first step is to shrink the screen to 24 lines. The effect of this is to expand the
border to cover one character line. The information to be added at the top or