Atari ST series Technical Reference Manual page 143

All three
mapped, rather than character-oriented. This means that
everything the computer displays, including text, is com­
posed of a series of dots or pixels (picture elements). To help
the programmer cope with the ST's heavy emphasis on
graphics, the Operating System offers the programmer a
wide range of graphics functions. High level support for
graphics operations on the ST is provided by the GEM VDI,
as described in COMPUTEl's Technical Reference Guide, Atari
ST Volume One: The VDI. TOS, however, includes a set of
graphics primitives known as the line A routines. These are
the low-level graphics routines called by the VDI.
The line A routines and the GEM VDI have advantages
and disadvantages. Using the VDI results in a more portable
code than using the line A routines, since the VDI routines
may be accessed from C language programs that can be com­
piled and executed on either the ST or MS-DOS computers
running under GEM. The VDI is also a little easier to use be­
cause C compilers for the ST include GEM bindings that al­
low programmers to use VDI functions as if they were stan­
dard C functions. This ease of use has a performance cost,
however, since the computer wastes time translating the
commands from a format that is convenient for the user into
a format more convenient for the Operating System.
While the line A routines require a little more effort to
use than the VDI, they offer slightly faster performance in re­
turn. They also require less overhead, since it isn't necessary
to perform GEM initialization routines or open a VDI work­
station in order to use them. They also offer a few options
not found in the VDI routines, such as a choice of 16 logic
operations for text printing. Though the line A routines are
specific to the ST, they provide full compatibility within the
ST family. This means that the same routines will work on
screen display modes on the ST are bit­
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