Method and apparatus for randomizing data in a direct access storage

Abstract

Data byte fields and corresponding ECC byte fields to be magnetically recorded as multi-byte records is randomized prior to run-length encoding. The randomized/encoded data is then written. When the randomized/encoded data is read back, the data is first decoded, and then derandomized in a complementary fashion. The randomizing avoids the writing of byte patterns that tend to stress the ability of a head/disk interface to record the data transitions at proper linear track positions. When reading, the complementary derandomizing scheme avoids presenting a repeating pattern of read-errors to the ECC network. Thus, read-errors are not propagated through the read channel. Data is randomized, and subsequently derandomized, in a repeating or pseudo random, complementary sequence for each byte. The complementary randomizing/derandomizing sequence is determined by the count contained in a counter. This counter is initialized at the beginning of a record, and is progressively incremented to the predetermined next count as each byte is processed. Data randomizing the first four bits of each byte are multiplied by the first four bits of an 8-bit counter, and the last four bits are multiplied by the last four counter bits. During derandomizing, division by way of multiplication-of-the-inverse is used, as the first four bits of each byte are multiplied by the inverse order of the last counter four bits, and the last four bits are multiplied by the inverse order of the first counter bits.