ladybird/Kernel/API/FileSystem/FATStructures.h

/*
 * Copyright (c) 2022, Undefine <undefine@undefine.pl>
 *
 * SPDX-License-Identifier: BSD-2-Clause
 */

#pragma once

#include <AK/Types.h>

namespace Kernel {

// This structure represents the DOS 3.31 BIOS Partition Block.
// While DOS 3.31 predates FAT verions 12/16/32 (the versions supported by this driver),
// the fields in this block are common with the DOS 4 and DOS 7 BIOS Parameter blocks.
// This structure will be followed by an "Extended BIOS Partition Block" (EBPB).
//
// The DOS 4 EBPB is *typically* used by FAT 12/16 file systems, while the DOS 7 EBPB
// is *typically* used by FAT 32. _However_, any combination is possible, as the FAT
// version is only determined by the number of clusters.
//
// Note that the DOS 4 and DOS 7 EBPB extensions are incompatible with each other
// (contain fields in different orders and of different lenghts) and do not contain
// an explicit indication to differentiate them.
// This driver uses heuristics to identify the EBPB version (based on the signature bytes
// and sector counts).
// FIXME: Consider also using the MBR parition type field in the future.
struct [[gnu::packed]] DOS3BIOSParameterBlock {
    u8 boot_jump[3];
    char oem_identifier[8];
    u16 bytes_per_sector; // Offset 0x0B -- beginning of DOS 3.31 BPB.
    u8 sectors_per_cluster;
    u16 reserved_sector_count;
    u8 fat_count;
    u16 root_directory_entry_count;
    u16 sector_count_16bit;
    u8 media_descriptor_type;
    u16 sectors_per_fat_16bit;
    u16 sectors_per_track;
    u16 head_count;
    u32 hidden_sector_count;
    u32 sector_count_32bit; // 0x020 -- end of DOS 3.31 BPB.
};
// 11 is the boot jump/OEM identifier prefix prior to the official BPB.
static_assert(AssertSize<DOS3BIOSParameterBlock, 11 + 25>());

struct [[gnu::packed]] DOS4BIOSParameterBlock {
    // Begins at sector offset 0x024.
    u8 drive_number; // 0x024
    u8 flags;
    u8 signature;
    u32 volume_id;
    char volume_label_string[11];
    char file_system_type[8];
};
static_assert(AssertSize<DOS4BIOSParameterBlock, 26>());

struct [[gnu::packed]] DOS7BIOSParameterBlock {
    // Begins at sector offset 0x024.
    u32 sectors_per_fat_32bit; // 0x024
    u16 flags;
    u16 fat_version; // Expected value 0x2b2a.
    u32 root_directory_cluster;
    u16 fs_info_sector;
    u16 backup_boot_sector;
    u8 unused3[12];
    u8 drive_number;
    u8 unused4;
    u8 signature;
    u32 volume_id;
    char volume_label_string[11];
    char file_system_type[8];
};
static_assert(AssertSize<DOS7BIOSParameterBlock, 54>());

struct [[gnu::packed]] FAT32FSInfo {
    u32 lead_signature;
    u8 unused1[480];
    u32 struct_signature;
    u32 last_known_free_cluster_count;
    u32 next_free_cluster_hint;
    u8 unused2[12];
    u32 trailing_signature;
};
static_assert(AssertSize<FAT32FSInfo, 512>());

}