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Parallel Scan for a Disk-Storage Device

IP.com Disclosure Number: IPCOM000042819D
Original Publication Date: 1984-Jun-01
Included in the Prior Art Database: 2005-Feb-04
Document File: 3 page(s) / 52K

Publishing Venue

IBM

Related People

Burns, CS: AUTHOR [+4]

Abstract

A scan command is a software tool implemented in disk adapters to assist in the location of information on the disk device. It is a high-speed comparison function whose speed is set by the disk data rate. The scan command is performed by comparing data, as it is read from the disk, to a predetermined data pattern (scan compare field). Each byte of the data is checked to see if it is higher than, lower than, or equal to the compare field. Areas that are not to be checked are marked with a delimiter field in the compare buffer. The scan comparison has traditionally been done at a serial bit level as the data is read from the disk. This requires that the scan logic be imbedded directly into the disk device hardware and must be duplicated for each device attached.

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Parallel Scan for a Disk-Storage Device

A scan command is a software tool implemented in disk adapters to assist in the location of information on the disk device. It is a high-speed comparison function whose speed is set by the disk data rate. The scan command is performed by comparing data, as it is read from the disk, to a predetermined data pattern (scan compare field). Each byte of the data is checked to see if it is higher than, lower than, or equal to the compare field. Areas that are not to be checked are marked with a delimiter field in the compare buffer. The scan comparison has traditionally been done at a serial bit level as the data is read from the disk. This requires that the scan logic be imbedded directly into the disk device hardware and must be duplicated for each device attached. This article describes a mechanism by which the scan comparison can be done on parallel data as it is loaded into the disk adapter buffers. This method has the advantage of being a common implementation for all disk devices and of being shared by multiple files. The figure diagrams the parallel-scan circuitry. Scan compare logic A consists of a network of exclusive-OR (XOR) blocks that can selectively compare the scan compare field to the delimiter register C or to the disk data. The outputs of the XORs are combined to form two output signals: LOW, indicating that the disk data is less than the compare field, and EQUAL, indicating that the compare field is equal to the disk data or to the delimiter. Scan mode control B is a set of three latches set by microcode that indicate the type of comparison to be performed. The three valid comparisons are: EQUAL (disk data and compare data are equal), HIGH/EQUAL (disk data is greater than or equal to the compare data), or LOW/EQUAL (disk data is lower than or equal to the compare data). Delimiter register C holds the 8-bit value that is used to indicate which areas are to be excluded from the compare function. It is loaded by microcode. Equal latch D is set when the disk data and compare field are equal. Low latch E is set when the disk data is less than the compare field. Mask latch F is set to indicate that the current byte of disk data matches the delimiter value. Hit latch G indicates, when set, that a scan hit has occurred. Further scanning is terminated at this point. Gate scan hit latch H is used to gate the hit latch G at the proper time. Scanning is broken into the three time periods (each being 200 nsec in duration): (P1) Fetch Compare Field, (P2) Delimiter Check, and (P3) File Data Che...