Seagate STT8000A manual Track numbering, Track format, Frames

Chapter 6 Tape format

Track numbering

All even numbered tracks, and the Directory Track, are recorded in the forward direction (the direction from the BOT marker to the EOT marker). All odd numbered tracks are recorded in the reverse direction (the direction from the EOT marker to the BOT marker). All even numbered tracks are located below the Directory Track; all odd numbered tracks are above it.

Track format

Tracks are recorded sequentially beginning with track 0, then track 1, and so on. Before recording, data are grouped into blocks, and blocks are grouped into frames with 64 blocks per frame.

Two numbering methods are used for blocks: physical numbering and logical numbering. Both numbers start from 0 at the beginning of each partition.

Physical numbering is related directly to the recorded block on the tape. Each

￿new block is given a unique physical number, regardless of its contents.

Logical numbering does not relate to the blocks physically recorded on the tape; rather, this type of numbering is the block numbering system used by the host computer. Often the host system operates with logical blocks that are a different size from the 512-byte blocks that are physically recorded on the tape.

The host blocks can be larger or smaller than 512-bytes and can also be fixed or variable. Fixed host blocks contain the same number of data bytes in each block. Variable host blocks may contain a different number of data bytes in each block.

The format provides both a physical block number for each block recorded on the tape and a logical block number that can span more than one physical block. These two numbers are recorded in the control field of every block.

Frames

Every track on the tape is recorded in blocks that contain 512 data bytes. The data bytes are 8-bit bytes, which are numbered b0 to b7 with b7 being the most significant bit.

A frame is made up of 64 blocks—52 data blocks plus 12 error-correction code (ECC) blocks. Figure 6-1 illustrates the general track layout of sequentially recorded frames.

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