pang-o-lin/src/mkfat.c

#include <stdint.h>
#include <unistd.h>
#include <string.h>

#include "fat12.h"
#include "fat12-internals.h"
#include "io.h"

#include <stdio.h>

// Create a FAT12 filesystem using the given file descriptor,
// with length _bytes_.
// It will define a root directory size and padding size so that the file sectors
// end up on 4096-byte boundaries.
int fat12_mkfs(int fd, uint32_t bytes) {
    uint8_t mbr[512];
    const uint32_t sector_size  = 512;
    uint32_t cluster_size = 4096 / sector_size; // 4 kiB cluster sizes match flash erase block size
    if (bytes > 16000000) {
        // We run out of clusters just below 16 megabytes.  Use bigger cluseters.
        cluster_size = 8192 / sector_size;
    }
    const uint32_t reserved_sectors = 1; // The MBR is the only reserved sector

    // XXX What happens if this is an even multiple of sectors?
    // Size of FAT (in bytes)
    uint32_t fat_size = 1 + ((((bytes * 3) / sector_size) / cluster_size) / 2);
    uint32_t fat_size_sectors = ((fat_size - 1) / sector_size) + 1;

    uint32_t root_directory_sector_count = cluster_size - (fat_size_sectors + reserved_sectors);
    uint32_t root_directory_entry_count = (sector_size / sizeof(struct fat_directory_entry)) * root_directory_sector_count;

    fprintf(stderr, "bytes: %d\n", bytes);
    fprintf(stderr, "sector_size: %d\n", sector_size);
    fprintf(stderr, "cluster_size: %d\n", cluster_size);
    fprintf(stderr, "reserved_sectors: %d\n", reserved_sectors);
    fprintf(stderr, "fat_size: %d\n", fat_size);
    fprintf(stderr, "fat_size_sectors: %d\n", fat_size_sectors);
    fprintf(stderr, "root_directory_sector_count: %d\n", root_directory_sector_count);
    fprintf(stderr, "root_directory_entry_count: %d\n", root_directory_entry_count);
    fprintf(stderr, "\n");

    memset(mbr, 0, sizeof(mbr));

    // jmp instruction
    mbr[0] = 0xeb;
    mbr[1] = 0x3c;
    mbr[2] = 0x90;

    // OEM name
    memcpy(mbr+3, "Fomu MBR", 8);

    // Bytes per sector (512)
    fat12_write_u16(mbr + 0x0b, sector_size);

    // Sectors per cluster (8), so cluster is 512 * 8
    fat12_write_u8(mbr + 0x0d, cluster_size);

    // Reserved sector count (8 sectors, to pad things to 4096-byte boundaries)
    fat12_write_u16(mbr + 0x0e, reserved_sectors);

    // Number of FATs (1)
    fat12_write_u8(mbr + 0x10, 1);

    // Maximum number of root directory entries (16)
    fat12_write_u16(mbr + 0x11, root_directory_entry_count);

    // Total number of 512-byte sectors
    fat12_write_u16(mbr + 0x13, bytes / sector_size);

    // Type of media (0xf8 == fixed disk)
    fat12_write_u8(mbr + 0x15, 0xf8);

    // Sectors per FAT
    fat12_write_u16(mbr + 0x16, fat_size_sectors);

    // Sectors per track
    fat12_write_u16(mbr + 0x18, 32);

    // Number of heads
    fat12_write_u16(mbr + 0x1a, 64);

    // Hidden sectors
    // mbr[0x1c] = 0x00;
    // mbr[0x1d] = 0x00;
    // mbr[0x1e] = 0x00;
    // mbr[0x1f] = 0x00;

    // Total number of sectors (unused, since we put it in 0x13)
    // mbr[0x20] = 0;
    // mbr[0x21] = 0;
    // mbr[0x22] = 0;
    // mbr[0x23] = 0;

    // BIOS drive
    fat12_write_u8(mbr + 0x24, 0x80);

    // Unused
    fat12_write_u8(mbr + 0x25, 0);

    // Boot signature
    fat12_write_u8(mbr + 0x26, 0x29);

    // Serial number
    fat12_write_u8(mbr + 0x27, 0xf0);
    fat12_write_u8(mbr + 0x28, 0xb0);
    fat12_write_u8(mbr + 0x29, 0x00);
    fat12_write_u8(mbr + 0x2a, 0x07);

    // Volume Name
    memcpy(mbr + 0x2b, "Fomu       ", 11);

    // Filesystem
    memcpy(mbr + 0x36, "FAT12   ", 8);

    // Signature
    fat12_write_u8(mbr + 0xfe, 0x55);
    fat12_write_u8(mbr + 0xff, 0xaa);

    // Write MBR to disk
    if (pang_write(fd, mbr, sizeof(mbr)) != sizeof(mbr)) {
        return -1;
    }

    memset(mbr, 0, sizeof(mbr));

    // The first two clusters in the FAT are reserved
    fat12_set_cluster(mbr, 0, 0xff8); // 0xff8 instead of 0xfff indicates "Hard Disk"
    fat12_set_cluster(mbr, 1, 0xfff); // 0xfff is reserved value

    // Root directory is only one cluster
    // fat12_set_cluster(mbr, 2, 0xfff);

    if (pang_write(fd, mbr, sizeof(mbr)) != sizeof(mbr)) {
        return -1;
    }

    // Fill in the rest of the FAT
    uint32_t fat_sector;
    memset(mbr, 0, sizeof(mbr));
    for (fat_sector = 1; fat_sector < fat_size_sectors; fat_sector++) {
        if (pang_write(fd, mbr, sizeof(mbr)) != sizeof(mbr)) {
            return -1;
        }
    }

    // Fill in the root directory
    uint32_t root_directory_sector;
    for (root_directory_sector = 0; root_directory_sector < cluster_size; root_directory_sector++) {
        if (pang_write(fd, mbr, sizeof(mbr)) != sizeof(mbr)) {
            return -1;
        }
    }

    return 0;
}