Tech Tips

XY COORDINATES TO SCREEN MEMORY

Tech Tips regular Greg Vozzo of Brightwaters, New York shows how to convert XY coordinates (such as those used in PLOT, DRAWTO, etc.) into screen memory locations used by PEEK and POKE. Type RUN to start the program, which will fill screen memory (and thus the screen) with 0s.

To RUN the second part of the program, type GOTO 50. This POKES a 0 into screen memory, converts the location into XY coordinates, and then prints an A there. The third section of the program is run by typing GOTO 200. This will print HELLO at the designated XY coordinates and then overwrite the word by POKEing 0s into the appropriate screen memory locations.

 

FIVE-COLOR CURSOR TEXT

ANTIC modes 4 and 5 (same as Graphics 12 and 13) are special in that they allow four colors in a single character, and five on the whole screen. However, there is no cursor.

This program, also by Greg Vozzo of Brightwaters, New York, sets up a GRAPHICS 0 screen and changes it to a mixed screen of ANTIC modes 2, 4 and 5. The character set is altered and the screen colors are changed. Finally text is printed - and a cursor is present.

How did this happen? The redefined character set altered the SPACE character, which occupies the whole screen, into a solid block. This way, the cursor will show up in the color assigned to location 712 (background) when it overlays the solid block. In addition, the now-solid playfield is its own color, separate from the background, so a screen border is possible.

 

LINE WIPER

This short program by Mark Perez, of Fremont, California produces an impressive "line wipe" effect for erasing text from the screen. Its secret lies in the clever use of the ATASCII character set in Wipe1$ and Wipe2$. Use this routine to dress up your own programs

 

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