driver.storage.vol.mbr¶

Master Boot Record (MBR) data structures and partition table helpers.

Overview¶

This unit defines the on-disk MBR layout and provides helpers for reading and writing partition table entries. It handles LBA-to-CHS address conversion, partition bootability flags, and population of TPartition_table fields in both LBA and CHS addressing formats.

The volume manager and storage manager use these types directly when reading and writing sector 0 of a disk.

Dependencies¶

debug.tracer

Types¶

TPartition_table / PPartition_table¶

One of the four 16-byte MBR partition entries (bitpacked to match on-disk layout).

Field	Type	Description
`attributes`	`uint8`	Bit 7 set = bootable partition
`CHS_start`	`array[0..2] of uint8`	CHS address of first sector
`system_id`	`uint8`	Partition type byte (e.g., `$0B` = FAT32, `$EE` = GPT protective)
`CHS_end`	`array[0..2] of uint8`	CHS address of last sector
`LBA_start`	`uint32`	LBA of first sector
`sector_count`	`uint32`	Number of sectors in partition

TMaster_Boot_Record / PMaster_Boot_Record¶

512-byte MBR sector (bitpacked).

Field	Description
`bootstrap`	440 bytes of bootstrap code
`signature`	4-byte disk signature
`rsv`	2 reserved bytes
`partition`	Array of 4 `TPartition_table` entries
`boot_sector`	Boot signature (should be `$AA55`)

T24bit¶

array[0..2] of uint8 — Helper type for packed 3-byte CHS values.

Functions and Procedures¶

get_bootable¶

function get_bootable(partition_table : PPartition_table) : boolean;

Returns true if bit 7 of attributes is set (partition is marked bootable).

set_bootable¶

procedure set_bootable(partition_table : PPartition_table);

Sets bit 7 of attributes to mark the partition as bootable.

setup_partition¶

procedure setup_partition(partition_table : PPartition_table; address : uint32; sectorSize : uint32);

Populates LBA_start, sector_count, and both CHS fields of a partition entry. address is the starting LBA; sectorSize is the partition length in sectors. CHS values are computed by the internal LBA_2_CHS function using standard 63 sectors/track and 255 heads/cylinder geometry.

Notes¶

The internal LBA_2_CHS function uses fixed CHS geometry (63 sectors/track, 255 heads/cylinder). On real hardware, CHS values are largely ignored by modern BIOSes in favour of LBA addressing, but they must be present for compatibility with legacy boot loaders.
There is a known bug in setup_partition: CHS_start is assigned twice (the second assignment overwrites the first with the end address). The CHS_end field is never set. This does not affect LBA-only boot scenarios.