ableos_time/sysdata/programs/filesystem_fat32/src/bios_parameter_block.hb

stn := @use("../../../libraries/stn/src/lib.hb");
.{string, memory, buffer, log} := stn

VALID_JUMP_BYTES := [u8].(0xEB, 0x3C, 0x90)

OemIdent := struct {
	dos_version: [u8; 8],
	dos_version_name: [u8; 8],
}

new_oem_ident := fn(major: int, minor: int): OemIdent {
	ver := [u8].(0, 0, 0, 0, 0, 0, 0, 0)
	return OemIdent.(ver, ver)
}

BiosParameterBlock := struct {
	jump_bytes: [u8; 3],
	oem_ident: OemIdent,
	bytes_per_sector: u16,
	sectors_per_cluster: u8,
	reserved_sectors: u16,
	// The amount of FileAllocationTables on the disk. Often 2.
	fat_count: u8,
	root_directory_count: u16,
	// if 0 then refer to large_sector_count
	total_sectors: u16,
	media_type: u8,
	// if 0 refer to sectors_per_fat in the ExtendedBootRecord
	sectors_per_fat: u16,
	sectors_per_track: u16,
	head_count: u16,
	hidden_sectors: u32,
	large_sector_count: u32,
}

bpb_sanity_check := fn(bpb: BiosParameterBlock): int {
	return 0
}

new_bpb := fn(): BiosParameterBlock {
	oem := new_oem_ident(0, 0)

	return BiosParameterBlock.(VALID_JUMP_BYTES, oem, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0)
}

sector_count := fn(bpb: BiosParameterBlock): u32 {
	if bpb.total_sectors == 0 {
		return bpb.large_sector_count
	} else {
		return bpb.total_sectors
	}
}

FatVersionNumber := struct {
	major_version: u8,
	minor_version: u8,
}

FormatReservation := [u8; 12]

// Padded with spaces.
VolumeName := [u8; 11]

SystemIdentifierString := [u8; 8]
VALID_SYSTEM_IDENTIFIER_STRING := [u8].(46, 41, 54, 33, 32, 20, 20, 20)
BOOTABLE_PARTITION_SIGNATURE := 0xAA55

BootCode := [u8; 420]

ExtendedBootRecord := struct {
	sectors_per_fat: u32,
	flags: u16,
	fat_version_number: FatVersionNumber,
	// Typically set to 2.
	root_directory_cluster_number: u32,
	fsinfo_structure: u16,
	backup_boot_sector: u16,
	// When a volume is formated these bytes should be zero. As a sanity check I guess?
	format_reserved: FormatReservation,
	// 0x00 floppy or 0x80 hard disk
	drive_number: u8,
	nt_reserved: u8,
	// must be 0x28 or 0x29
	signature: u8,
	volume_id_serial: u32,
	volume_id_name: VolumeName,
	// The spec says this is always FAT32. Untrustworthy and another sanity check I guess.
	system_identifier_string: SystemIdentifierString,
	boot_code: BootCode,
	partition_signature: u16,
}

ebr_sanity_check := fn(ebr: ExtendedBootRecord): int {
	ret := 0
	if ebr.drive_number != 0x0 | ebr.drive_number != 0x80 {
		log.warn("EBR-Drive-Number sanity check failed\0")
	}

	if ebr.signature != 0x28 | ebr.signature != 0x29 {
		log.warn("EBR-Signature sanity check failed\0")
	}

	if ebr.system_identifier_string != VALID_SYSTEM_IDENTIFIER_STRING {
		log.warn("EBR-Signature-Identifier-String sanity check failed\0")
	}
	return 0
}

new_ebr := fn(): ExtendedBootRecord {
	version := FatVersionNumber.(0, 0)
	fmt_res := FormatReservation.(0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0)
	vol_name := @as([u8; 11], idk)
	boot_code := @as([u8; 420], idk)

	return ExtendedBootRecord.(
		0,
		0,
		version,
		0,
		0,
		0,
		fmt_res,
		0,
		0,
		0,
		0,
		vol_name,
		VALID_SYSTEM_IDENTIFIER_STRING,
		boot_code,
		0,
	)
}

VALID_LEAD_FS_INFO := 0x41615252
VALID_TRAIL_FS_INFO := 0xAA550000

FSInfo := struct {
	// Must be 0x41615252 to indicate a valid FSInfo structure
	lead_signature: u32,
	lead_reserved: [u8; 480],
	// If the value is 0xFFFFFFFF, then the free count is unknown and must be computed. However, this value might be incorrect and should at least be range checked (<= volume cluster count) 
	last_known_free_cluster_count: u32,
	last_known_avalible_cluster: u32,
	trail_reserved: [u8; 12],
	trail_signature: u32,
}

fs_info_sanity_check := fn(fs_info: FSInfo): int {
	ret := 0
	if fs_info.lead_signature != VALID_LEAD_FS_INFO {
		ret &= 1
		log.warn("Invalid leading signature in FSInfo.\0")
	}
	if fs_info.last_known_free_cluster_count == 0xFFFFFFFF {
		ret &= 2
		log.warn("Last known free cluster count unknown.\0")
	}
	if fs_info.last_known_avalible_cluster == 0xFFFFFFFF {
		ret &= 4
		log.warn("Last known avalible cluster count unknown.\0")
	}
	if fs_info.trail_signature != VALID_TRAIL_FS_INFO {
		ret &= 8
		log.warn("Invalid trailing signature in FSInfo.\0")
	}

	return ret
}

new_fs_info := fn(): FSInfo {
	lead_reserved := @as([u8; 480], idk)
	trail_reserved := @as([u8; 12], idk)
	return FSInfo.(
		VALID_LEAD_FS_INFO,
		lead_reserved,
		0,
		0,
		trail_reserved,
		VALID_TRAIL_FS_INFO,
	)
}