Windows accelerators – XGA

Windows is so popular  an environment that graphics  card manufacturers find it  worthwhile to design  hardware that  provides Super VGA  resolution  at higher speeds. These cards  are usually not quite  full co-processor  designs but achieve  there speed by  implementing some of the more common windows graphics  operations  hardware. For example, Windows spends  most of its  time during screen  updates in  BitBlt operations – Bit Block transfers – which involve  moving a rectangular  section of the  image to another  location.  Is standard  video cards this  involves the use of the  processor  to transfer the data. An obvious  way of speeding  Windows   up is to implement  a BitBlt operation is hardware. Coupled with a suitable  Windows driver this can increase  the screen update  speed of windows by  10 to 20 times.  The amount of speed  increase  realized depends on the  speed of the processor  with which the  accelerator  is being  used.  Obviously , if you add an  accelerator  card to a machine  that is already  powerful enough  to perform  BitBlt  operations very fast, then the addition  will make little difference. On the other hand , On the other hand, if the processor is too slow to  perform  reasonably  fast BitBlt  operations  then adding an  accelerator  card may make  Windows usable  without having to upgrade the processor.
Most Windows accelerators are ad-hoc designs supported by  Windows drivers supplied by the manufactures. The  notable  exception  is IBM’s XGA standard. This adds the  8514/A  resolutions to the  existing  VGA resolutions in a  single card. The XGA  hardware  isn’t compatible  with VGA or  the 8514/A  standard but  its applications interface (AI) is a superset of the  8514/A  AI. This means the applications that can use the  8514/A can also use XGA  as long as  they don’t  try to use the  hardware directly – but of course most do!
XGA is optimized for Windows by supporting hardware  BitBlt  operations and   a suitable Windows driver is available  from IBM. Of course , once you are working within Windows, standards or   lack of them   matter much less. The big problem  with XGA  is that is very m