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Circular Scan of VTOC Usage Map

IP.com Disclosure Number: IPCOM000047222D
Original Publication Date: 1983-Oct-01
Included in the Prior Art Database: 2005-Feb-07
Document File: 3 page(s) / 50K

Publishing Venue

IBM

Related People

Chou, JK: AUTHOR [+4]

Abstract

In a diskette drive, data is stored on the floppy disk in storage locations of a predetermined size referred to as sectors. A Volume Table Of Contents (VTOC) stored at a predefined location in a predetermined number of sectors on the diskette contains records that point to the sector storage location of files stored on the diskette. When the diskette is first loaded into a using system, its VTOC is read into the processor. The processor also generates a usage map of the VTOC to keep track of the allocation of VTOC sectors. For each new allocation, the VTOC usage map is scanned in a circular or wrap-around manner from the last allocation rather than being scanned from the beginning of the map. In the example described herein, each sector has a capacity of 256 bytes. Each VTOC sector can contain two 128-byte record labels.

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Circular Scan of VTOC Usage Map

In a diskette drive, data is stored on the floppy disk in storage locations of a predetermined size referred to as sectors. A Volume Table Of Contents (VTOC) stored at a predefined location in a predetermined number of sectors on the diskette contains records that point to the sector storage location of files stored on the diskette.

When the diskette is first loaded into a using system, its VTOC is read into the processor. The processor also generates a usage map of the VTOC to keep track of the allocation of VTOC sectors. For each new allocation, the VTOC usage map is scanned in a circular or wrap-around manner from the last allocation rather than being scanned from the beginning of the map. In the example described herein, each sector has a capacity of 256 bytes. Each VTOC sector can contain two 128-byte record labels. Therefore, each sector within the VTOC has two separate areas which can be allocated for record labels. These can be thought of as the first and second halves within a VTOC sector. Such a VTOC is illustrated in Fig. 1. This VTOC contains 100 record labels and thus requires 50 sectors or physical blocks.

The Physical Block Numbers (PBN), as shown, happen to be PBN17 to PBN66. The VTOC usage map in Fig. 2 is a binary string where each bit represents a particular VTOC sector half. If there are 50 VTOC sectors or PBNs, then 100 bits in the usage map are required to represent every sector half within the VTOC. The drawing below shows the relationship of each bit in the VTOC usage map to the actual VTOC sector and its allocation. Each eight bits of the usage map are stored as a byte, and each such byte is identified by an INDEX number. Since there are 100 bits, 121/4 bytes are required. The usage map is stored in random-access memory by the processor. At initial program load of the using system, a variable is set to start the search of the VTOC usage map at the bit which represents the first half of sector 17. The processor scans the usage map looking for a binary zero indicating that the associated VTOC sector has not been allocated and is available. When the first available sector is located from the usage map, the bit at that usage map position is changed to a binary one. In addition, the processor stores that locat...