BGI Graphics Model

When considering the functions listed here, keep in mind that BGI performs
most drawing operations using an implicit drawing or tracing color (COLOR),
fill color (FILLCOLOR), and pattern (FILLPATTERN).  For example, the
PIESLICE call accepts no pattern or color information, but instead uses the
previously set COLOR value to trace the edge of the slice, and the
previously set FILLCOLOR and FILLPATTERN values for the interior.

For efficiency, many operations take place at the position of the current
pointer, or CP. For example, the LINE routine accepts only a single (x,y)
coordinate pair, using the CP as the starting point of the line and the
passed coordinate pair as the ending point. Many functions (LINE, to name
one) affect CP, and the MOVE function can be used to explicitly adjust CP.
The BGI coordinate system places the origin (pixel 0,0) at the upper
left-hand corner of the screen.