One of the reasons I bought a 2068 was to use the advanced video modes described in appendix C of the BASIC manual. I was disappointed when I found it impossible to access these modes using only BASIC.
After learning machine code, I was able to use the Dual Screen Mode in my BASIC programs. The following listing uses the 2 screens to do simple straight line animation to demonstrate this mode. You will have to do a little programming to change the pictures but that should present no problem for most readers.
Once you type in the program, save it before you run it. The program uses a short machine code routine and if you make a typing error, the program might crash. There is an error checking routine in the program, but it will not detect transposed digits.
When you run this program don’t use the BREAK key; if you do, it will cause the computer to “hang.”
Most of the time the computer is displaying the 2nd screen and if there is an error message it will be printed on the first screen and you will not see it. If you can’t resist hitting the BREAK key, be prepare to enter OUT 255,0 “blind” in order to switch to screen 1.
Changing the Program
The only programming change you will have to make is in the three DATA statements. The first number is how many “frames” the program will draw.
The second digit is the number of DRAW statements it takes to make the picture.
The second DATA statement contains the PLOT and DRAW coordinates to create the first frame.
The third DATA statement contains the information to draw the last frame. The computer works out all the intermediate frames.
Please note that you must start your drawing with a PLOT and only use DRAW statements thereafter. You also must have the same number of lines in the first and last drawings.