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Implementation of Last Tag Indicators Using Illegal Tag Table Entries

IP.com Disclosure Number: IPCOM000038042D
Original Publication Date: 1989-Oct-01
Included in the Prior Art Database: 2005-Jan-31
Document File: 1 page(s) / 12K

Publishing Venue

IBM

Related People

Bailey, RN: AUTHOR [+2]

Abstract

Disclosed is a circuit implemented within a DMA Controller to detect illegal tag table entries, and to use the occurrence of an illegal entry to signal the controller that all DMA transfers are complete.

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Implementation of Last Tag Indicators Using Illegal Tag Table Entries

Disclosed is a circuit implemented within a DMA Controller to detect illegal tag table entries, and to use the occurrence of an illegal entry to signal the controller that all DMA transfers are complete.

The circuit described is used within a serial link adapter. The DMA controller portion of the adapter controls the flow of data between the serial link and the system main storage. In order to access information from main storage at the correct address, a tag table is provided which is loaded by the system, and contains the addressses of data to be accessed, along with offset and line count information. The address information is simply the memory page where data will be accessed. The offset field contains the 6-bit value of the starting location within a memory page where data will be accessed. The line count field contains a 6-bit value identifying the number of cache line transfers to be performed at the page identified by this tag table entry. In this implementation, there exists 16 tag table entries.

When the DMA Controller is signalled to begin the transfer process, offset and line count information is loaded into counters and checked for validity. Validity is determined by comparing these two values. If, the line count is larger than the total number of lines available for transfer as defined by the offset, then the tag is not valid. In addition, if, when the tag is retrieved from the table, a parity error is detected, the tag is assumed to b...