关于 图像显示的问题

procedure TForm1.LoadYCbCr2(FileName: String);
var Fifo:array[0..3]of BYTE; F:TFileStream; b,Y,Cb,Cr:byte;  st,i,j1,j2:integer;
  FastBmp:T24bitBmp;
begin
  F:= TFileStream.Create(FileName,fmOpenRead);
  ZeroMemory(@Fifo,4);
  st := 0;
  j1:=1;
  j2:=2;
  i:=0;
  FastBmp:=T24bitBmp.Create ;
  FastBmp.LoadFromGraphic(Bitmap);
  while (F.Position < F.Size ) do
  begin
    F.Read(b,1);
    Fifo[0] := Fifo[1];
    Fifo[1] := Fifo[2];
    Fifo[2] := Fifo[3];
    Fifo[3] := b;

    case st of
 0: if (Fifo[0] = $FF) and (Fifo[1] = $00) and (Fifo[2]= $00)  then
          begin
            if (Fifo[3]=$80) then st := 1
            else
              if (Fifo[3] = $C7) then st := 2;
            //Find First FF0000XY
          end;   
请问：这个程序里面的：              
if (Fifo[0] = $FF) and (Fifo[1] = $00) and (Fifo[2]= $00),         Fifo[3]=$80,           Fifo[3] = $C7   ，  
 是起什么作用？ 在网上查了两天都没查出来。    谢谢指教。