Delete me: FileCore Disc Map V1 (Rev #2, changes)
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FileCore
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» Technical Details
» Disc Map Type 1
Type 1 Information
Definitions used
| Term | Meaning |
|---|---|
| new map / Type 1 | Both terms are interchangeable and is used to define a specific map type suppored by FileCore |
| zone | Discs are dvidided into a number of zones, each of which is contiguous section of the disc |
| nzones | The number of zones on a disc |
| map block | Each sector of the map is known as a map block, and controls the allocation of a zone of the disc |
| ZoneCheck | Check byte for a zone’s map block. Is used to check that this zone’s map block is valid |
| FreeLink | Link to first free fragment in a specific zone. Is a fragment block giving the offset to the first free space fragment block in the allocation bytes |
| CrossCheck | Cross check byte for complete map. They are combined to check that the whole map is self-consistent |
Overview of New Map (Type 1) 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.
| Map | Information Stored | Compaction Required | Recovery Story |
|---|---|---|---|
| New (Type 1) | Space allocation | No | Two copies stored |
The type of map used is dependant 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.
| Zone Number | Detail |
|---|---|
| 0 | Contains the lowest numbered sectors on the disc |
| 1 | |
| 2 | |
| … | |
| nzones – 2 | |
| nzones – 1 | Contains the highest numbered sectors on the disc |
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:
| Contents | Notes |
|---|---|
| Header | |
| Disc Record | The Disc Record is only stored in Zone 0 |
| Allocation Bytes | |
| Unused |
Header Contents
| Offset | Name | Meaning |
|---|---|---|
| 0 | ZoneCheck | Check byte for this zone’s map block |
| 1 | FreeLink | Link to first free fragment in this zone |
| 3 | CrossCheck | Cross check byte for complete map |
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