Subject: Reading more than one key

82. *****
 Q: How do I detect if more than one standard key is pressed down?

 A: The example code below relies very heavily on a Usenet posting
by Lou Duchez ljduchez@en.com who wishes to acknowledge Bill Seiler
for the handling of ports. The KeyNrDown and TEST routines are by
myself. Besides being a demonstration the TEST procedure can be used
to get the scan codes of the different keys instead of relying on
external documentation.
  Uses Dos;
  {}
  var keydown: array[0..127] of boolean;   { status array }
      oldkbdint: procedure;       { points to the "normal" keyboard handler }
      port60h, port61h: byte;     { used within the interrupt for storage }
  {}
  { The replacement keyboard handler }
  procedure newkbdint; interrupt;
  begin
    port60h := port[$60];
    keydown[port60h and $7f] := (port60h <= $7f);
    port61h := port[$61];
    port[$61] := port61h or $80;
    port[$61] := port61h;
    port[$20] := $20;
  end;
  {}
  { Get the scancode of the key pressed down, 128 for none }
  function KeyNrDown : byte;
  var i : byte;
  begin
    KeyNrDown := 128;
    for i := 0 to 127 do if KeyDown[i] then KeyNrDown := i;
  end;
  {}
  { Test by displaying the scan codes of the keys pressed }
  procedure TEST;
  var k, k1 : byte;
  begin
    k1 := 128;
    repeat
      k := KeyNrDown;
      if k <> k1 then begin
        write (k, ' ');
        if (k1 = 30) and (k = 31) then writeln ('Pressed A and S ');
        k1 := k;
      end;
    until k = $01; {escape}
  end; {test}
  {}
  begin
    { turn on the replacement keyboard handler }
    fillchar(keydown, 128, #0);  { sets array to all "false" }
    getintvec($09, @oldkbdint);  { record location of old keyboard int }
    setintvec($09, @newkbdint);  { this line installs the new interrupt }
    {}
    TEST;
    {}
    { turn off the replacement keyboard handler }
    setintvec($09, @oldkbdint);
  end.
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