*******PC/PCjr COMPATIBILITY...HOW FAR CAN WE TAKE IT?***********
  
     Gerry Gutowski - Lilly Computer Club
     Indianapolis, Indiana
  
           PCjr owners seem to share a common dream, i.e. to be able to
     use any piece of software designed for the IBM PC on its little
     brother. Most of us who have had their jr's for a while have been
     both pleased to find that many PC programs will run on the jr, and
     frustrated by some that won't. There are of course multiple
     reasons why achieving total compatibility is difficult. On the
     plus side, it is encouraging to see that a number of third party
     manufacturers are addressing the issue. Over the next few months
     I'll be attempting to keep you advised of progress toward that
     goal.
  
          Let's first take a look at some of the major factors
     affecting compatibility between the two machines. Perhaps the
     easiest to overcome is the memory limitation initially imposed
     upon the jr by IBM. Third party suppliers such as Tecmar allow the
     jr owner to expand total RAM to 640K. With as "little" as 256K,
     memory hungry software such as the original version of Lotus 123
     will run on the jr...that is until one attempts to address the
     graphics functions of 123. This brings us to the second and
     perhaps most severe blockade to compatibility.  This has to do
     with the way the two units provide for the allocation of their
     video display. While the PC addresses separate chips outside of
     its RAM capacity to do the job, the jr ordinarily uses the top two
     pages (16K) of RAM for this task. Many of us with Tecmar's
     expansion boards have found that reconfiguration of the video
     display parameters via the CONPCJR -P2 program clears up some, but
     not all compatibility problems. Some of the remaining difficulties
     stem from the superior color/graphics board found on the jr. It
     seems that this is a case of overkill when programs are attempting
     to address the less sophisticated graphics board of the PC.  Both
     Rapport and Falcon second drive addon units for the jr claim to
     provide a switch which temporarily defeats jr's graphics system
     and replaces it with one more like that found on the PC. Under
     these conditions the graphics capabilities of Lotus 123 are
     useable. With this problem now solved, one major obstacle to
     compatibility remains.
  
          Some programs designed for the PC utilize DMA (Direct Memory
     Access). This is a process wherein programs call upon files on
     either of the disks in a two drive system to be accessed without
     ever being handled by the CPU. None of the second drive systems
     offered to date provide this capability. The task seems to be
     addressable via an appropriate change on the drive controller
     board. Yet hope springs eternal, one of the major players in the
     game has mysteriously delayed introduction of its second drive
     system. Lets hope that Tecmar has the foresight to include this
     feature in its package which is due to be announced soon. Even
     with DMA and a "graphics switch" all compatibility problems may
     not be resolved but the majority should be at this stage.
  
          Lastly I'd like to deal with what I will call "nearly
     compatible" programs. I'll define these as programs which seem
     fully functional on a jr with the exception of one or more
     features. For example, boot up "Hitchhikers Guide To The Universe"
     on a PC and you'll note that that this text adventure comes up in
     an 80 column display and you are offered the option of viewing the
     text in color or mono. On the jr booting provides a 40 column text
     with no option for color. Normally this would not be a problem
     since one could precede program loading with a MODE CO 80 command
     setting from DOS. Unfortunately, since "Hitchhikers..." is self-
     booting it resets the system wiping out the color option. Since
     the program is copy protected, one is prevented from inserting a
     MODE CO 80 command in the AUTOEXEC.BAT file. Otherwise the program
     run fine, but one can't help feeling like he got a little less
     than he paid for just because its being run on a jr. In a few
     instances some of us have been able to reintroduce such missing
     features. If you are interested in helping solve such issues or
     have further information on solving compatibility problems I'd
     like to hear from you.
  
     If you wish to contribute any of your own experiences, please for-
     them to Bob Hutchinson on this bulletin board.