CIRCLE.DOC

Documentation for CIRCLE.CO

 Machine Language Circle routine
   v2.0 12/10/87
   c1987 Jon Diercks [73327,2353]
 based on an algorithm published
   by Mike Higgins
 as modified for M100
   by Jay Holovacs [74756,413]
   and Neil Wick   [71056,613]

  CIRCLE.CO is a very small (228 bytes), very fast (clocked at 30 times faster 
than its BASIC counterpart) machine language routine to allow BASIC programmers
to draw circles on the M100's LCD, in either "black" or "white" mode.
  I developed CIRCLE.CO from the BASIC subroutine CIRCLE.BAS by Jay Holovacs 
and Neil Wick, which they developed from an algorithm by Mike Higgins.  Many 
thanks to these three for their contributions to this project.
  CIRCLE.CO resides comfortably in the ALT LCD buffer at 64704 and must be 
LOADMed there by any BASIC program that uses it.  It should be noted that 
CIRCLE.CO is M100 SPECIFIC!  It has not been tested on the the 102 but it 
should work.  With a few minor modifications, it should also work on the T200 (
see CIRCLE.SRC for CS format assembler code).
  Getting it going:
  Download and run CIRCLE.LDR.  CIRCLE.CO should now appear on your menu.  The 
LDR can be KILLed as it will no longer be used.  For a demonstration of how the
routine is used from within a BASIC program, download CIRCLE.DEM and run it 
with CIRCLE.CO in memory.  This demo consists of three little ditties I made up
to test out the circle routine.
  A quick look at a listing of CIRCLE.DEM should show you how to use CIRCLE.CO 
from BASIC.  As long as you're not using any other routines that run in or 
affect the ALT LCD buffer, you can LOADM"CIRCLE" at the start of the program 
and leave it there.  Now, whenever you want to draw a circle, get the 
coordinates of the center (X,Y), and the radius (R), and do this:
  CALL64704,R,256*X+Y
  Rules:  X, Y and R must each be >= 0 and <= 255.  Note that this means you 
can specify centers which are not on the screen, as well as radii that will put
part or all of the circle off of the screen.  This is OK, because CIRCLE.CO 
will "clip" points that are beyond the screen boundary.  You can NOT specify 
negative values for X, Y, or R;  this means that you can only get "off screen" 
centers to the right and down.  Another limitation:  you can not draw partial 
circles (arcs) with this routine.  (you couldn't do it with the BASIC routine 
either, though)
  Another benefit:  you CAN draw circles in "black" or "white" mode (which you 
couldn't do with the BASIC version).  The default mode on initial LOADM is 
black.  The toggle byte is at 64705.  To enable white or "erase" mode, 
POKE64705,0.  To re-enable black or "draw" mode, POKE64705,1.  Simple enough?
  Tip:  You will notice that the "paranoia" dittie in CIRCLE.DEM uses 
concentric circles.  In order to fill all the space using concentric circles, I
had to actually draw TWO circles for each radius - one with the center offset 
one pixel from the other.  Without this doubling, some pixels would be missed. 
Of course, sometimes it may be desirable to miss a few, as in the "pulsar" 
demo, which leaves those dots there for a kind of starry effect.
  ML programmers will want to see CIRCLE.SRC, the CS format source code, but it
is not necessary for operation.  If anyone successfully revises the SRC for the
T200, please share it with us.
  As always, send me all your comments, problems, etc.  Most of all, enjoy!
  Doing it the ML way...(flying CIRCLES around the rest...)
  Jon Diercks [73327,2353]