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function SomaHex(Valor1, Valor2: String): String;
begin
  Result := Format('%x', [StrToInt(HexDisplayPrefix + Valor1) + StrToInt(HexDisplayPrefix + Valor2)]);
end;

function SubtrairHex(Valor1, Valor2: String): String;
begin
  Result := Format('%x', [StrToInt(HexDisplayPrefix + Valor1) - StrToInt(HexDisplayPrefix + Valor2)]);
end;

function CheckSum(linha: String): String;
var
  i: Integer;
begin
  Result := '00';
  i := 1;
  while i < length(linha) do
  begin
    Result := SomaHex(Result, linha[i]+linha[i+1]);
    Inc(i, 2);
  end;
  if Length(Result) > 2 then
    Result := SubtrairHex('100', Copy(Result, 2, 2))
  else
    Result := SubtrairHex('100', Result);
end;

ShowMessage(CheckSum('6a696d6d796a616d65737061747269636b76616e686f6c6c6768616e736b756c6c'));