ddddoooouuuubbbblllleeee sets the display mode to double buffer mode. It does not take
effect until ggggccccoooonnnnffffiiii is called. In double buffer mode, the bitplanes are
partitioned into two groups, the front bitplanes and the back bitplanes.
Double buffer mode displays only the front bitplanes. Drawing routines
normally update only the back bitplanes; ffffrrrroooonnnnttttbbbb and bbbbaaaacccckkkkbbbbuuuu can override
the default.
In double buffer mode, ggggccccoooonnnnffffiiii calls ffffrrrroooonnnnttttbbbb((((....FFFFAAAALLLLSSSSEEEE....)))) and bbbbaaaacccckkkkbbbbuuuu((((....TTTTRRRRUUUUEEEE....))))....
SSSSEEEEEEEE AAAALLLLSSSSOOOO
backbu, frontb, gconfi, getbuf, getdis, RGBmod, single, swapbu