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Figure 29 Header for Bit-Map Block

common-block-header

Offset

(Decimal)

Length

Bytes

4 stib-eerf-fo-rebmun

block size - 18

bit-map

The fields in Figure 29 are defined as:

common-block-header

is the 14-byte common block header shown in Figure 24.

number-of-free-bits

indicates how many bits in this bit-map identify blocks that are free (empty) in a key-sequenced

or queue file. For relative files, it indicates how many bits identify blocks that are not full.

bit-map

is an array of bits describing availability of index or data blocks. For a key-sequenced or queue

file, each bit tells whether the corresponding block is free (0) or in use (1). For a relative file,

each bit tells whether there is room for at least one more record in the corresponding block.

An empty bit-map has (8 * (block-size - 18) ) free bits.

With a 1024-byte block, for example, the map has 8048 available bits.

Figure 30 Arrangement of Bit-Map Blocks

header

data or index block

header

data or index block

bits b(1) b(2) b(3) . . . b(n)

Block 0 is the

first

bit-map block.

Block 1

Block N

Block N+1

Block N+2

Block 2N+1

• • •

174 Block Formats of Structured Files