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Object File System

OFS is a lightweight flat file system that uses a single hybrid B-tree to store all files. It supports both 32bit and 64bit addressing and the B-tree can scale from 256 up to 2^64 maximum files. The 32bit OFS can access drives with a capacity of 2 terabytes while the 64bit OFS can access drives greater than 8 billion terabytes, using 512 byte sectors. See the design concepts page for more information on OFS.

Boot sector header
The root block pointer is accessed by BIOS code during setup and must be located in the first 2 terabytes on the boot device.

• 3 byte jump
• 9 byte version = "BRiX  0.2"
• 8bit sectors per track
• 8bit number of heads
• 8bit attributes
• 32bit root block pointer
• 32bit free block pointer
• 64bit bad block pointer
• 16byte volume ID

Attributes

• 7: 64bit OFS if set, else 32bit OFS
• 3-6: reserved
• 0-2: number of primary block levels in root tree

Bad sectors block
The bad block pointer in the boot sector will be NULL if the drive does not contain any bad sectors. A single sector block will be allocated when bad sectors are found during a scan. The block contains 63 64bit pointers to bad sectors and the last 64bits holds a pointer to the next bad sectors block.

• 127 32bit or 63 64bit pointers to bad sectors
• 32bit or 64bit next bad block

Free sectors block
The free block pointer must point to a single sector block containing 31 free sector entries, 1 64bit reserved value and 1 64bit pointer to the next free block.

• 63 or 31 entries (32bit or 64bit base and 32bit or 64bit size)
• 32bit or 64bit reserved
• 32bit or 64bit next free block

Root block
The root tree holds all files and an 8 to 64bit index is used to access each file. The number of primary levels for the root tree is stored in the boot sector attributes.

Primary block

• 256 32bit (1k) or 64bit (2k) pointers to another primary block or the final object block
•  
• There can be 0-7 levels of primary blocks.

Object block

• 256 64bit (2k) or 128bit (4k) object entries
  • 32bit or 64bit sector pointer to SLT or unfragmented data
  • 8bit attribute
    • 1-7: reserved
    • 0: uses SLT if set, else unfragmented
  • 24bit or 56bit size in sectors (objects can be 8 gigabytes or 32 million terabytes in size)
•  
• The object block is the last level in the root tree.

Sector Link Tables
If the 'SLT' attribute is not set then the sector pointer points to the first sector of the object. Otherwise it points to a Sector Link Table (SLT). When an object is created it will be stuck in continuous sectors. If there aren't enough continuous sectors then an SLT is created. Each entry in the SLT points to a run of sectors for the object. If the object grows beyond what a single SLT can provide then another SLT is created and linked to the end of the last SLT.

• 63 or 31 entries (32bit or 64bit base and 32bit or 64bit size)
• 32bit or 64bit reserved
• 32bit or 64bit next SLT pointer

The first object (ID=0) is reserved for the setup code and can't use a sector link table because the boot code doesn't support them.