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New Direct Access Storage Device Format and Attachment

IP.com Disclosure Number: IPCOM000041388D
Original Publication Date: 1984-Jan-01
Included in the Prior Art Database: 2005-Feb-02
Document File: 8 page(s) / 63K

Publishing Venue

IBM

Related People

Hoffman, RL: AUTHOR [+2]

Abstract

A computer system such as the IBM System/38 has a non-standard sector size of X+2n bytes, as shown in Fig. 1, where the sector consists of 8 bytes of header data and 512 bytes of data. This contrasts to a normal sector size of 2n bytes. In the past, for System/38, the headers and data were stored contiguously on the direct-access storage device (DASD), as shown in Fig. 2a. Each DASD file controller attachment uses a pair of address registers to direct the header to one address and to direct the data to a second address in main storage. In order to eliminate the non-standard sector size, it would be possible to store headers on a separate DASD device as in Fig. 2b or as in the preferred arrangement, the headers are blocked together to fill the standard fixed block architecture (FBA) sector size, i.

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New Direct Access Storage Device Format and Attachment

A computer system such as the IBM System/38 has a non-standard sector size of X+2n bytes, as shown in Fig. 1, where the sector consists of 8 bytes of header data and 512 bytes of data. This contrasts to a normal sector size of 2n bytes. In the past, for System/38, the headers and data were stored contiguously on the direct-access storage device (DASD), as shown in Fig. 2a. Each DASD file controller attachment uses a pair of address registers to direct the header to one address and to direct the data to a second address in main storage. In order to eliminate the non-standard sector size, it would be possible to store headers on a separate DASD device as in Fig. 2b or as in the preferred arrangement, the headers are blocked together to fill the standard fixed block architecture (FBA) sector size, i.e, 64 8-byte headers would fit in a 512 byte sector (Fig. 2c). Performance degradation due to reads and writes of the header sector is alleviated by a header lookaside buffer which maintains copies of recently used header sectors. Read and write operations are performed by assembling the header from the lookaside buffer with the data from the DASD device. If the requested header is not found in the lookaside buffer, then the appropriate header sector is read from the DASD device and stored in the lookaside buffer. The format of a DASD track where sectors are dedicated to data headers and alternates is shown in Fig. 3. It should be noted that the reformatted track provides header sectors on each track in a manner similar to the provision of alternate sectors. Header sectors are not in the relative block address allocation of data sector addressing. One header sector is provided for every 32 or fewer data sectors. Within the header sector, 16 bytes are for each data page covered by the header sector. These values apply to 512 byte sectors and other values would be used for other sector sizes. For the example in Fig. 3, there are 50 sectors per track. There are two header sectors where the first covers 32 sectors and the second covers 14 sectors. There are two alternate sectors. The header sector for a given data sector is always on the same track except if alternate data sectors are assigned to alternate tracks. The header sector cannot be reassigned to another track if defective. At least one alternate sector for each header sector is required on each track, but this may be an alternate data sector if the availability to swap header sectors with data sectors exists. A push-down reassignment procedure must be used to locate the header sector immediately following the data sector it covers. The format of a header sector is set forth in Fig. 4. Information of the data sectors for a maximum of 32 data sectors can be contained in one track. The header in this instance includes the 8-byte S/38 header, a 6-byte unused area and a 2-byte cyclic redundancy check (CRC) field. When a read takes pl...