Editing GEM (desktop environment) (section)

==Description==
The "full" GEM system consisted of three main parts:

#GEM VDI (Virtual Device Interface)
#GEM AES (Application Environment Services)
#GEM Desktop (an application providing drag-and-drop file management)

GEM '''VDI''' was the core graphics system of the overall GEM engine. It was responsible for "low level" drawing in the form of "draw line from here to here". VDI included a resolution and coordinate independent set of vector drawing instructions which were called from applications through a fairly simple interface. VDI also included environment information (state, or context), current color, line thickness, output device, etc.

These commands were then examined by GDOS, whose task it was to send the commands to the proper driver for actual rendering. For instance, if a particular GEM VDI environment was connected to the screen, the VDI instructions were then routed to the screen driver for drawing. Simply changing the environment to point to the printer was all that was needed (in theory) to print, dramatically reducing the developer workload (they formerly had to do printing "by hand" in all applications). GDOS was also responsible for loading up the drivers and any requested fonts when GEM was first loaded.

One major advantage VDI provided over the Macintosh was the way multiple devices and contexts were handled. In the Mac such information was stored in memory inside the application. This resulted in serious problems when attempting to make the Mac handle pre-emptive multitasking, as the drawing layer ([[QuickDraw]]) needed to have direct memory access into all programs. In GEM VDI however, such information was stored in the device itself, with GDOS creating "virtual devices" for every context &ndash; each window for instance.

GEM '''AES''' provided the window system, window manager, UI style and other GUI elements (widgets). For performance reasons, many of the GUI widgets were actually drawn using character graphics. Compared to the Macintosh, AES provided a rather spartan look and the system shipped with a single monospaced font.

AES performs its operations by calling the VDI, but in a more general sense the two parts of GEM were often completely separated in applications. Applications typically called AES commands to set up a new window, with the rest of the application using VDI calls to actually draw into that window.

GEM '''Desktop''' was an application program that used AES to provide a file manager and launcher, the traditional "desktop" environment that users had come to expect from the Macintosh. Unlike the Macintosh, the GEM Desktop ran on top of DOS ([[MS-DOS]], [[DOS Plus]] or [[DR DOS]] on the PC, GEMDOS on the Atari), and as a result the actual display was cluttered with computer-like items, including path names and wildcards. In general, GEM was much more "geeky" than the Mac, but simply running a usable shell on DOS was a huge achievement on its own. Otherwise, GEM has its own advantages over Mac OS such as proportional sliders.

Native PC GEM applications use the file extension .APP for executables, whereas GEM desktop accessories use the file extension .ACC instead.<ref name="Paul_1997_DRDOS6UN"/><ref name="Paul_1997_NWDOS7UN"/> All desktop accessories (and also a few simple applications) can be run under ViewMAX without modification.<ref name="Paul_1997_DRDOS6UN"/><ref name="Paul_1997_NWDOS7UN"/>