FileCore Disc Map New (changes)

Showing changes from revision #1 to #2: Added | Removed | Changed

FileCore

» Technical Details

» Disc Map New

Disc Map New

Definitions used

Overview of New Disc Map

A disc has a section of information defined as a Map. It controls the allocation of the disc to the files and directories.

The type of map used is dependent on the format of the disc. These are detailed here.

The advantages of the New map discs are:

• Files need not be stored contiguously, so you don’t need to compact the disc. (However, FileCore does try to create new map files in one block, and will also try to merge file fragments back together again if it is compacting a zone of the disc.)
• The disc map has no limit on size or number of entries, so ‘Map full’ errors do not occur
• The map keeps a record of defects when the disc is formatted, so omits defective sectors
• Defects are kept as objects on the disc, so they don’t need to be taken into account when calculating disc addresses, and can be mapped out without reformatting

Zone Numbering

The numbering of zones start at 0, and successively contain higher numbered sectors on a disc. The map’s length is nzones × sectors long.

Map Location

The location of the map is stored in the middle zone, or as close to the middle as possible, based on whether the number of zones are odd or even. The map’s location is: nzones/2 (rounded down).

So a disc where nzones = 99, the map’s location is at the start of zone 49
A disc where nzones = 100, the map’s location is at the start of zone 50

Map Block Format

The map block controls the allocation of a zone of the disc. The first map block controls zone 0, the second controls zone 1, and so on.

The general format of a map block is:

Header Contents

Disc Record

Multiple Disc Record Types are supported by FileCore. The Current default used by FileCore is the Large Disc Record Type.

Allocation Bytes

The Allocation Byte is the part of the map block that controls the allocation of a zone. Together, the allocation bytes from all the map block control the allocation of the whole disc. Each bit corresponds to an allocation unit on the disc.

The size of an allocation unit is defined in the disc record by log2bpmb, and so must be a power of two bytes. An allocation unit is not necessarily one sector – it may be smaller or larger.

Not only must space be logically mapped in whole allocation units; it must also be physically allocated in whole sectors. Consequently, the smallest unit by which allocation may be changed is the larger of the sector size and the allocation unit. This unit is known as the granularity.

A disc is split into a number of disc objects, each of which consists of one or more fragments spread over the surface of the disc. Fragments need not be held in the same zone, and their size can vary by whole units of granularity. Fragments have a minimum size, which is explained below:

Three disc objects are special, and contain:

• Bad sectors (for a perfect disc, this disc object will not be present)
• Boot block, map and root directory
• Free space

All other disc objects contain either a directory (optionally with small files held within that directory), or one or more files that are held in a common disc object.

Fragments

The allocation bytes are treated as an array of bits, with the lsb of a byte coming before the msb in the array.

The array is split into a series of fragment blocks, each representing a fragment.

Since each bit in the array corresponds with an allocation unit on the disc, the length of the fragment block (in bits) must be the same as the size of the fragment (in allocation units). The stream of 0 bits are used to pad the fragment block to the correct length, and the 1 bit to terminate the fragment block.

There are two fragment IDs with special meanings:

Other fragment IDs represent either free space fragments, or allocated fragments:

• A fragment id for a free space fragment is the unsigned offset, in bits, from the beginning of its fragment block to the beginning of the next free space fragment block in the same map block (or 0 if there are no more).

The chain hence always runs from the beginning of the map block to the end. The offset to the first free space fragment block is given by the FreeLink fragment block in the map block’s header. Because that fragment block is 2 bytes long, and must have a terminating 1 bit, idlen cannot be greater than 15.

• A fragment id for an allocated fragment is a unique identifier for the disc object to which that space is allocated. Any other fragments allocated to the same disc object will have the same fragment id.

Therefore, the following deductions can be made:

This An is object because may have a number of fragments allocated to it in several zones. These fragments must be logically joined together in some way to make the object appear as a contiguous sequence of bytes. The naïve approach would be to have the first fragment block on cannot the disc be smaller the that first fragment of the object. New map discs do not do this. The first fragment in an object is the first fragment on the disc searching from zone (idlenfragment id + / 1 bits (thefragment idids per zone , ) and upwards, wrapping round from the terminating disc’s 1 end bit) to its start. Any subsequent fragments belonging to the same disc object are joined in the order they are found by this search.

See also

• FileCore Overview