Error Recovery Scheme for the IBM 3850 Mass Storage System

The IBM 3850 Mass Storage System (MSS) stores digital data on flexible magnetic tape media; however, it is different in many respects from the conventional multitrack tape machines. In particular, the use of a single-element rotary read-write head imposes new demands in the areas of data encoding and error recovery. This paper presents a comprehensive scheme for error recovery for the 3850 MSS which features a new error-correction code in a serial, single-stripe data format. The recovery procedure is designed around resynchronizable sections of data which are rendered independent of each other in error modes through the use of zero-modulation encoding and self-contained error-detection pointers. These error-detection pointers and the resynchronization signals are utilized in conjunction with interleaved codewords of the error-correction code. The code is designed with a generating polynomial in which the roots are chosen from the set of elements of a 16-element subfield of the Galois field GF(2
⁸
). This choice provides the necessary code structure for desired code capabilities and facilitates fast decoding of errors with an economical implementation of the decoder. The scheme provides correction capabilities for various combinations of mixed-mode short and long errors common to magnetic tape recording of digital data.