source: mainline/uspace/dist/src/bithenge/fat.bh@ f1bed857

#!/usr/bin/perl -w
#
# SPDX-FileCopyrightText: 2012 Sean Bartell
#
# SPDX-License-Identifier: BSD-3-Clause
#

# FAT filesystem script.
# Largely based on https://en.wikipedia.org/wiki/File_Allocation_Table
# Currently only FAT12 and FAT16 are supported.

transform u8 = uint8;
transform u16 = uint16le;
transform u32 = uint32le;

transform fat_attributes = struct {
        .read_only <- bit;
        .hidden <- bit;
        .system <- bit;
        .volume_label <- bit;
        .subdirectory <- bit;
        .archive <- bit;
        .device <- bit;
        .reserved <- bit;
} <- bits_le <- known_length(1);

transform file_data(data, bits, fat, cluster_size, start) = (in.data) <- struct {
        .cluster <- (data[(start-2)*cluster_size, cluster_size]);
        .last_cluster_number <- switch (bits) {
                12: (488);       # 0x00000ff8
                16: (65528);     # 0x0000fff8
                32: (268435448); # 0x0ffffff8
        };
        .next <- (fat[start]);
        if (.next == 0 || .next >= .last_cluster_number) {
                .data <- (.cluster);
        } else {
                .rest <- file_data(data, bits, fat, cluster_size, .next) <- known_length(0);
                .data <- (.cluster ++ .rest);
        }
};

transform fat_dir_entry(data, bits, fat, cluster_size, self_start, parent) = struct {
        .filename <- known_length(8);
        .extension <- known_length(3);
        .attrs <- fat_attributes;
        .flags <- u8;
        .ctime_fine <- u8;
        .ctime <- u16;
        .cdate <- u16;
        .adate <- u16;
        .permissions <- u16;
        .mtime <- u16;
        .mdate <- u16;
        .start <- u16;
        .size <- u32;
        .size_shown <- if (.size > 32) { (32) } else { (.size) };

        if (.start != 0 && .start != self_start && .start != parent && .filename[0] != 229) {
                .data
                    <- if (.attrs.subdirectory) {
                        repeat { fat_dir_entry(data, bits, fat, cluster_size, .start, self_start) }
                    } else {
                        (in[0,.size_shown])
                    }
                    <- if (.size != 0) { (in[0,.size]) } else { (in) }
                    <- file_data(data, bits, fat, cluster_size, .start);
        }
};

transform fat_table(bits, num_clusters) = switch (bits) {
        12: partial {repeat(num_clusters) { uint_le(12) }} <- bits_le;
        16: partial {repeat(num_clusters) { u16 }};
        32: partial {repeat(num_clusters) { u32 }};
};

transform fat_super(disk) = struct {
        .jump_instruction <- known_length(3);
        .oem_name <- ascii <- known_length(8);

        # DOS 2.0 BPB
        .bytes_per_sector <- u16; # must be power of two, at least 32
        .sectors_per_cluster <- u8; # must be power of two
        .num_reserved_sectors <- u16;
        .num_fats <- u8; # at least 1
        .num_root_entries <- u16; # 0 for FAT32
        .num_sectors_16 <- u16;
        .media_descriptor <- u8;
        .sectors_per_fat <- u16; # 0 for FAT32

        # DOS 3.0/3.2/3.31 BPB
        .sectors_per_track <- u16;
        .num_heads <- u16;

        # DOS 3.31 BPB
        .bpb331 <- struct {
                .ignore <- nonzero_boolean <- (.num_sectors_16);
                .num_hidden_sectors <- u32;
                .num_sectors_32 <- u32;
        };

        .drive_number <- u8;
        .chkdsk_flags <- u8;
        .extended_boot_signature <- u8;
        if (.extended_boot_signature == 41) {
                .volume_id <- u32;
                .volume_label <- ascii <- known_length(11);
                .type <- ascii <- known_length(8);
        }

        .boot_signature <- (disk[510,2]); # b"\x55\xaa"; TODO: what if .bytes_per_sector < 512?
};

transform fat_filesystem_tree(disk) = struct {
        .super <- partial{fat_super(disk)} <- (disk);

        .num_sectors <- if (.super.bpb331.ignore) {
                (.super.num_sectors_16)
        } else {
                (.super.bpb331.num_sectors_32)
        };

        .cluster_size <- (.super.sectors_per_cluster * .super.bytes_per_sector);
        .first_root_sector <- (.super.num_reserved_sectors + .super.num_fats * .super.sectors_per_fat);
        .first_data_sector <- (.first_root_sector +
            (.super.num_root_entries * 32 + .super.bytes_per_sector - 1) //
            .super.bytes_per_sector);
        .num_clusters <- (2 + (.num_sectors - .first_data_sector) // .super.sectors_per_cluster);
        .bits <- if (.num_clusters < 4085) { (12) }
            else { if (.num_clusters < 65525) { (16) } else { (32) } };

        .fats <- partial(.super.num_reserved_sectors * .super.bytes_per_sector) {
                repeat(.super.num_fats) {
                        fat_table(.bits, .num_clusters) <-
                                known_length(.super.sectors_per_fat * .super.bytes_per_sector)
                }
        } <- (disk);

        .root <- partial(.first_root_sector * .super.bytes_per_sector) {
                repeat(.super.num_root_entries) {
                        fat_dir_entry(disk[.first_data_sector * .super.bytes_per_sector:],
                            .bits, .fats[0], .cluster_size, 0, 0)
                }
        } <- (disk);
};

transform fat_filesystem = partial {fat_filesystem_tree(in)};

transform main = fat_filesystem;