Electrical computers and digital processing systems: memory – Address formation – Slip control – misaligning – boundary alignment
Reexamination Certificate
1996-12-30
2001-10-30
Lim, Krisna (Department: 2153)
Electrical computers and digital processing systems: memory
Address formation
Slip control, misaligning, boundary alignment
C711S100000, C711S152000, C711S154000, C711S211000, C710S052000, C710S065000
Reexamination Certificate
active
06311259
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to magnetic storage media, and more specifically to a system and method for allowing unidirectional data to be read in either direction.
2. Related Art
Data recorded onto media such as magnetic tapes are typically recorded using a data format standard. For example, one common format for recording 18-track magnetic tapes is the American National Standards Institute (ANSI) standard ASC X3. This standard, for parallel, group coded recording provides for re-blocking and data compression. The inherent nature of this format, however, makes it uni-directional. In other words, tapes written using this format can only be read in the forward direction, thus precluding the processor from reading a block of data in reverse.
To accommodate this standard, tapes recorded using this format are only read in the forward direction. The data read in the forward direction are sent to a buffer and then sent across a channel to the host site for processing. However, certain situations may arise in which it is necessary and/or beneficial to read the data from the tape in the reverse direction. In one such situation, errors may occur when trying to read a block of data in the forward direction. From a “data read” perspective, these errors may be avoided by reading the block in the reverse direction. However, if the data are read in the reverse direction, the host cannot process the data due to the unidirectional nature of the format.
In another situation, the host may request that the data blocks be sent in the reverse order. For example, a portion of the tape may consist of N blocks, in the order 1, 2, . . . , N. The host may request that the blocks be sent across the channel in the reverse order: N, N−1, N−2, . . . 1. To do this the tape must be positioned to the beginning of the last block, block N, and this block must be read in the forward direction. The tape must then be rewound to the beginning of the next-to-last block, block N−1, and this block read in the forward direction. This process of reading a block and rewinding the tape to the beginning of the previous block continues until all blocks of interest are read in the forward direction and sent to the host.
When the second situation arises, a significant amount of time is spent repositioning the tape to read each block in the forward direction. This problem is especially prevalent where tape drives having slow acceleration characteristics (such as streaming tape drives) are used.
SUMMARY OF THE INVENTION
The present invention provides a system and method for reading uni-directional data from a storage media in either the forward or reverse direction and presenting those data to a requesting host in the proper (e.g., forward) direction.
According to the invention, data read in the reverse direction are stored in a data buffer. Pointers or other marking techniques are used to indicate the boundaries of the data blocks in the data buffer. The data are then read out of the data buffer in the opposite direction from which they are stored and sent to the host via a channel. Preferably, the read is performed at the block level where each block is read in the opposite direction and sent to the host.
Data buffer control circuitry used to control the data buffer control data reads and writes and keeps track of pointers used to denote block boundaries. The control circuitry can be implemented to determine the level at which the reversal is to be performed. For example, if data are written to and read from the data buffer at the word level, the bytes that make up each word must be reversed as the data are read from the data buffer.
Because the data can be read from the buffer in the opposite direction from which it was stored, data can be read from the media in reverse, stored in the buffer, and sent to the host in the forward direction.
A CRC (cyclic redundancy check) is performed on the data as it is read from the media. This CRC is later used to verify the integrity of the data as they are read from the data buffer in the opposite direction. The CRC generation function uses a symmetric polynomial such that an expected result will exist in CRC generation after the data have been read from the buffer in the opposite direction.
If data compression techniques are used to store the data on the media, the data may need to be read in the forward direction to enable decompression. In a situation where the host is incapable of performing the data decompression, it is desirable to use the read reversal technique of the invention to provide the data in the forward direction out of the buffer and to decompress the data prior to transmission across the channel to the host.
Also, where blocking techniques are used to process host data records into blocks before writing the data to the media, the block, may, in itself be uni-directional. In other words, where there are multiple host data records in a single block, the data are usually unidirectional. In this situation, it is desirable to use the read reversal technique of the invention to provide the data in the forward direction out of the buffer for transmission across the channel to the host.
Further features and advantages of the present invention, as well as the structure and operation of various embodiments of the present invention, are described in detail below with reference to the accompanying drawings.
REFERENCES:
patent: 3579195 (1971-05-01), Hallmark et al.
patent: 3766529 (1973-10-01), Mc Laughlin
patent: 5050018 (1991-09-01), Georgis et al.
patent: 5287478 (1994-02-01), Johnston et al.
patent: 5349481 (1994-09-01), Kauffman et al.
Boyer Keith G.
Gill Richard A.
Gohl Thomas H.
Brooks & Kushman P.C.
Lim Krisna
Storage Technology Corporation
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