Shareware Overload/ShartewareOverload.cdr/graf/iconmakr.zip/DOTGRAPH.TXT

Text File | 1985-10-17 | 6KB | 129 lines

                  SIXTEEN COLOR DOT GRAPHICS FOR THE PC

    The  PC junior is the only member of the IBM PC family  that  can
    produce  the 16 color all points addressable 320 by 200  graphics
    without  the  recently  announced but  scarce  enhanced  graphics
    boards.   It seems at best that the PC will produce only a choice
    of two sets of 4 colors in an all points addressable mode.  I was
    working on a program that required graphics and wanted it to work
    on  all IBM PC machines and found a compromise that I would  like
    to share with you in this short note.

    I  found  that the Junior has a mode that displays all 16  colors
    in  160  by 200 resolution.  It requires 16000 bytes  of  display
    memory  since each byte contains two 4 bit nibbles of color  data
    to  display two such elongated dots.   In order to make  anything
    look  well  with  that mode however,  the dots must  actually  be
    displayed  two at a time  to produce square pixels thus  reducing
    to  a  160  by 100 display mode of  16  colors.   With  a  little
    experimenting  I was able to do this on the PC as well and in  so
    doing,  utilized the same code on both without using the ROM BIOS
    support provided on the Junior.

    It  seems  that  the only way to get 16  colors  with  the  color
    display adapter is to be in one of the color text modes.  This is
    the  only clue you need to proceed.   Notice that there are  some
    special  non ASCII characters displayed with codes from 80 to  FF
    hex.   One  of  those  codes is DE hex or 222  decimal  and  will
    display  an  80 column character with the left half that  of  the
    foreground  color  and  the  right  half  the  background  color.
    Fortunately both colors are defined in the attribute byte so that
    the  left and right halves can be  controlled  separately.   This
    produces  a  160  by 25 display that is dot addressable  with  16
    colors.   Notice  that we have used two bytes for  each  adjacent
    pair so that only one fourth of the display memory is used.

    Now for the tricky part.  By getting into debug and experimenting
    with  the 6845 video controller registers I was able to get  some
    rather interesting effects.   I have been told that you can cause
    damage to the video system by doing the wrong thing here so I was
    cautious.   What I wanted was a squish in the vertical  direction
    by a factor of four.  Therefore, after setting up for the 80 X 25
    color graphics text mode,  I knew that something might be done to
    the  6845 registers that controlled the vertical  characteristics
    of  the  display  if it were to be possible at  all.   These  are
    R4,R5,R6, and R7.  Then too there is R9 which has something to do
    with the scan line count per character.  In the 80 character mode
    register  4  contains 31 meaning that there is provision  for  31
    rows  not the 25 that we might expect.   This means  simply  that
    there  are  6  rows  to provide for the  vertical  retrace  time.
    Register  6 contains the expected 25 and register 7  contains  28
    which  is the vertical sync position.   This set up allows for  3
    undisplayed rows,  25 display rows and finally 3 undisplayed rows
    at  the bottom.   Meanwhile register 9 contains 7 which  actually
    allocates  scans  zero to 7 for a total of 8 scan lines per  row.
    The  total count on the scan lines is 248 with only 200  of  them
    actually displayed.




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    Thus assign 124 to register 4, 100 to register 6, 112 to register
    7,  and 1 to register 9.   There will be 124 rows with 12 at  the
    top  and bottom.   Each row has two scan lines for a total of 248
    scan  lines with only 200 of them actually displayed.   While  in
    debug I tried this and found that all of my 80 column text showed
    only  the top two scan lines but that I could display about  four
    times as many of them.   So that is the  solution.   Actually,  I
    coded  it in assembly language but to communicate  here,  I  will
    include a short segment of basic which does the same thing.   The
    assembly  language program is much much faster than this routine.

    10 WIDTH "SCRN:" 80                 ' 80 COLUMN MODE
    20 SCREEN 0,1,0,0                   ' COLOR MODE
    30 OUT &H3D8,9                      ' DISABLES BLINK BIT
    40 DEF SEG = &HB800                 ' COLOR DISPLAY AREA
    50 FOR I = 0 TO 80*100*2 STEP 2     ' FILL "SCREEN"
    60 POKE I,222                       ' HALF AND HALF CHAR
    70 POKE I + 1,0                     ' BLACK ON BLACK
    80 NEXT I
    90 OUT &H3D4,4                      ' R4  SELECTED
    100 OUT &H3D5,124                   ' R4  VERTICAL TOTAL
    110 OUT &H3D4,6                     ' R6  SELECTED
    120 OUT &H3D5,100                   ' R6  VERTICAL DISPLAYED
    130 OUT &H3D4,7                     ' R7  SELECTED
    140 OUT &H3D5,112                   ' R7  VERTICAL SYNC
    150 OUT &H3D4,9                     ' R9  SELECTED
    160 OUT &H3D5,1                     ' R9  SCAN LINES (2)


    Statement  number 40 is necessary to disable the blink else color
    codes larger than 7 produce a blinking display.  Now you have all
    of  the screen filled with black foreground and black  background
    "dots".   To  draw  a line of yellow dots in  column  25  without
    changing the other dot color in column 24, write:

    1000 FOR I = 0 TO 100
    1010 A = I * 160 + (25\2) * 2 + 1    ' LOCATION OF ATTRIBUTE
    1020 N = PEEK(A)                     ' GET ATTRIBUTE BYTE
    1030 POKE A,(N AND &HF0) OR &H0E     ' CHANGE DOT TO YELLOW
    1040 NEXT I

    When  you  exit  this program you will not be able  to  read  the
    display  messages because of being in this strange graphics mode.
    Just set the width to 40 to see what you are doing.















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