Due to the special scrolling engine, every graphic function have to know the width of the screen buffer in byte unit. So, every graphic function has in parameter a structure that defines the screen memory :
For exemple, the classical LCD screen is defined by :
The screen size is defined as a structure too :
Note that the width can be not equal to
. This is the case for the LCD buffer for the Ti 89.
And the position of the screen :
The position is used to defined a frame in the buffer of the screen. For exemple, if you want to define a new screen in the LCD screen with a border of 10 pixels : its x position will equal to , y to
, its width to
and its height to
(its memory is still
).
And then the global definition of the screen
With the previous example, it will be :