code: function (offset, length) {
  
                  if (arguments.length === 1) {
                      length = arguments[0];
                      offset = this.offset;
                  }
  
                  if (length < 0 || offset + length > this.length) {
                      error.raise(10001, this.length, offset + length);
                  }
  
                  var value = '';
                  for (var i = 0; i < length; ++i) {
                      var c = this.readUint8(offset + i);
                      value += String.fromCharCode(c);
                  }
  
                  this.offset = offset + length;
  
                  return value;
              }
docs: 读取一个string
  
  @param {number} offset 偏移
  @param {number} length 长度
  @return {string} 字符串