2.2.2.2 BID (Block ID)
Every block allocated in the PST file is identified using the BID structure. This structure varies in size according the format of the file. In the case of ANSI files, the structure is a 32-bit unsigned value, while in Unicode files it is a 64-bit unsigned long. In addition, there are two types of BIDs:
BIDs used in the context of Pages (section 2.2.2.7) use all of the bits of the structure (below) and are incremented by 1.
Block BIDs (section 2.2.2.8) reserve the two least significant bits for flags (see below). As a result these increment by 4 each time a new one is assigned.
Shown as used by Blocks (section 2.2.2.8):
Unicode:
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A - r (1 bit): Reserved bit. Readers MUST ignore this bit and treat it as zero before looking up the BID from the BBT. Writers MUST<4> set this bit to zero.
B - i (1 bit): MUST set to 1 when the block is "Internal", or zero when the block is not "Internal". An internal block is an intermediate block that, instead of containing actual data, contains metadata about how to locate other data blocks that contain the desired information. For more details about technical details regarding blocks, see section 2.2.2.8.
bidIndex (Unicode: 62 bits; ANSI: 30 bits): A monotonically increasing value that uniquely identifies the BID within the PST file. bidIndex values are assigned based on the bidNextB value in the HEADER structure (see section 2.2.2.6). The bidIndex increments by one each time a new BID is assigned.