Program CircleDemo;
Const NoRows = 200;
      NoCols = 320;
      XMax = NoCols shr 1;
      XMin = -XMax + 1;
      YMax = NoRows shr 1;
      YMin = -YMax + 1;

Type ScreenType = Record
         Width:Word;
     end;

Var Screen:ScreenType;
    ScreenY:Array[0..199] of Word;

Procedure CalcScreenY(Width:Word);
Var I:Integer;
begin
     for I:=0 to 199 do ScreenY[I]:=I * Width;
end;

Procedure SetPixel(X, Y:Word; Color:Word);
begin
     Mem[$A000:Y shl 8 + Y shl 6 + X]:=Color;
end;

Procedure Circle(X0, Y0, R:Integer);
Var X, Y, S:Integer;
    Xx, Yy:Array[1..8] of Integer;
    K:Word;
begin
     if (X0 + R > XMax) or (X0 - R < XMin) or
        (Y0 + R > YMax) or (Y0 - R < YMin) then Exit;
     SetPixel(X0 + R, Y0, 15);
     SetPixel(X0 - R, Y0, 15);
     SetPixel(X0, Y0 + R, 15);
     SetPixel(X0, Y0 - R, 15);
     X:=R;
     Y:=0;
     S:=3 - 2 * R;
     for K:=1 to 4 do
     begin
          Yy[K]:=Y0;
          Xx[4 + K]:=X0;
     end;
     Xx[1]:=X0 + R;
     Xx[2]:=Xx[1];
     Xx[3]:=X0 - R;
     Xx[4]:=Xx[3];
     Yy[5]:=Y0 + R;
     Yy[6]:=Y0 - R;
     Yy[7]:=Yy[5];
     Yy[8]:=Yy[6];
     While Y < X - 1 do
     begin
          Inc(Yy[1]);
          Yy[3]:=Yy[1];
          Dec(Yy[2]);
          Yy[4]:=Yy[2];
          Inc(Xx[5]);
          Xx[6]:=Xx[5];
          Dec(Xx[7]);
          Xx[8]:=Xx[7];
          if S <= 0 then S:=S + Y shl 2 + 6
          else
          begin
               S:=S + (Y - x) shl 2 + 10;
               Dec(X);
               Dec(Xx[1]);
               Xx[2]:=Xx[1];
               Inc(Xx[3]);
               Xx[4]:=Xx[3];
               Dec(Yy[5]);
               Yy[7]:=Yy[5];
               Inc(Yy[6]);
               Yy[8]:=Yy[6];
          end;
          inc(Y);
          if X = Y then for K:=1 to 4 do SetPixel(Xx[K], Yy[K], 15)
          else for K:=1 to 8 do SetPixel(Xx[K], Yy[K], 15);
     end;
end;

begin
     asm
        mov ax, 0013h
        int 10h
        push 320
        call CalcScreenY
     end;
     Circle(140, 60, 20);
     ReadLn;
     asm
        mov ax, 0003h
        int 10h
     end;
end.