| 词条 | Block availability map | |||||||||||||||||||||||||||||||||||||||||||
| 释义 |
In computer file systems, a block availability map (BAM)[1][2][3][4][5] is a data structure used to track disk blocks that are considered free (available for new data). It is used along with a directory to manage files on a disk (originally only a floppy disk, and later also a hard disk). In terms of Commodore DOS (CBM DOS) compatible disk drives, the BAM was a data structure stored in a reserved area of the disk (its size and location varied based on the physical characteristics of the disk). For each track, the BAM consisted of a bitmap of available blocks and (usually) a count of the available blocks. The count was held in a single byte, as all formats had 256 or fewer blocks per track. The count byte was simply the sum of all 1-bits in the bitmap of bytes for the current track. The following table illustrates the layout of Commodore 1541 BAM. The table would be larger for higher-capacity disks (described below).
The bitmap was contained in 3 bytes for Commodore 1541 format (single-sided) disks because it had 17 to 20 sectors per track (note 3 bytes can hold at least 20 bits).[6] Similarly, the Commodore 1571 used 3 bytes for the bitmap of each track, but the BAM was twice the size because there were twice as many tracks when formatted as double-sided.[7] In contrast, the Commodore 1581 disk drive used 5 bytes for the bitmap because the disk format had 40 blocks per track (note 5 bytes can hold 40 bits).[8] In the bitmap of any format, a 1 bit indicated the block was available (free), while a 0 bit indicated the block was not available (used), and the bitmap data was stored low-byte first. So the first byte held a map for blocks 0 to 7, the second byte held a map for blocks 8 to 15, and so on. Within a byte, the bitmap was ordered low-bit first. For example, the first byte would represent block 0 with the least significant bit and block 7 with the most significant bit. Storage devices by Creative Micro Designs, intended for use with CBM computers, also used a Block Availability Map which served the same purpose. However, these devices (FD-2000, FD-4000, and CMD-HD) did not include a count byte, and the bits in each byte were reversed (high-bit first). Although the bits were reversed (compared to CBM formats), the bytes were still stored in the same order (low-byte first).[9]
See also
References1. ^{{cite book|last=Englisch|first=Lothar|title=The Anatomy of the 1541 Disk Drive|year=1984|publisher=Abacus Software|location=Grand Rapids, MI|isbn=0-916439-01-1|page=89}} {{DEFAULTSORT:Block Availability Map}}2. ^{{cite book|title=1541 User's Guide|year=1982|publisher=Commodore Business Machines|page=9}} 3. ^{{cite book|title=1571 User's Guide|year=1985|publisher=Commodore Business Machines|page=23}} 4. ^{{cite book|title=1581 User's Guide|year=1987|publisher=Commodore Business Machines|page=34}} 5. ^{{cite book|title=FD Series User's Manual|year=1992|publisher=Creative Micro Designs|page=108}} 6. ^{{cite book|title=1541 User's Guide|year=1982|publisher=Commodore Business Machines|page=65}} 7. ^{{cite book|title=1571 User's Guide|year=1985|publisher=Commodore Business Machines|pages=108–109}} 8. ^{{cite book|title=1581 User's Guide|year=1987|publisher=Commodore Business Machines|pages=119–120}} 9. ^{{cite book|title=FD Series User's Manual|year=1992|publisher=Creative Micro Designs|pages=112–114}} 1 : Computer file systems |
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