94. Blanking the screen
    ~~~~~~~~~~~~~~~~~~~
It might sometimes be useful if you could blank the screen with a command,
but turn it back on again, revealing all the original text/graphics
unchanged. You can do this with the command VDU23,0,R,V,0;0;0;  This is a
general command to write value V into register R of the 6845 CRT controller
chip. If you make R=0, then V=0 will blank the screen, and V=127 will turn it
back on again in modes 0 to 3, and V=63 should be used for modes 4 to 7. It
would be interesting to write an Event-driven routine which blanked the
screen if none of the keys have been touched for more than a minute or so,
and restored the screen as soon as a key is pressed, checking to see which
mode is in use.