boot/x86/fat32.s

   1 ; FAT32 driver
   2 ; BOOT_DRIVE, si and bp must be set up by the code using this driver.
   3 ; (Specifically, si must be a pointer to the partition's partition
   4 ; table entry and bp must be a pointer to the BPB.)
   5 ; Before reading any cluster chains, callers must call get_1st_data_sec
   6 ; and must be careful not to trash ecx after calling it.
   7
   8 ; eax - start cluster number
   9 ; es:di - where to load the cluster chain
  10 ; NOTE: Cluster chain might be the root directory or a file.
  11 read_cluster_chain:
  12         push eax
  13         push ebx
  14         push ecx
  15         push edx
  16         push es
  17         push di
  18 .loop:
  19         ; get the first sector of the cluster to read
  20         push eax
  21         sub eax, 2
  22         xor ebx, ebx
  23         mov bl, BPB_SecPerClus
  24         mul ebx
  25         add eax, ecx
  26         mov ebx, eax
  27         pop eax
  28
  29         ; read the cluster
  30         push cx
  31         movzx cx, BPB_SecPerClus
  32         call read_sectors
  33         pop cx
  34
  35         ; increment the load offset (and segment if needed)
  36         push eax
  37         xor eax, eax
  38         movzx ax, BPB_SecPerClus
  39         mul word BPB_BytsPerSec
  40         add di, ax
  41         jno .get_next_cluster
  42         mov ax, es
  43         add ax, 0x1000
  44         mov es, ax
  45
  46 .get_next_cluster:
  47         pop eax
  48         ; get FAT sector number and offset containing the next cluster number
  49         xor ebx, ebx
  50         mov bl, 4 ; put 4 in EBX, doing this instead of "mov ebx, 4" saves us an entire byte
  51         mul ebx
  52         movzx ebx, word BPB_BytsPerSec
  53         div ebx
  54         movzx ebx, word BPB_RsvdSecCnt
  55         add eax, ebx
  56         add eax, [si]
  57         mov ebx, edx
  58         ; reminder for myself: EAX is FAT sector number, (E)BX is offset into FAT sector
  59
  60         ; load the FAT sector we're looking for
  61
  62         push cx
  63         push di
  64         push ebx ; offset
  65         push es ; we want to read at 0:1000, not STAGE3_SEGMENT:1000
  66         push eax ; desired LBA
  67         xor ax, ax
  68         mov es, ax
  69         pop ebx ; pop LBA into EBX
  70         mov di, 0x1000
  71         mov cx, 1
  72         call read_sectors
  73
  74         ; find the cluster we're looking for
  75         pop es ; restore STAGE3_SEGMENT
  76         pop ebx ; pop FAT offset back into EBX for cmp
  77         mov eax, [di+bx]
  78         pop di
  79         pop cx
  80         cmp eax, 0xffffff7
  81         jl .loop ; if cluster number is lower than the defective cluster value, we're not done
  82                  ; TODO: perhaps check is it equal defective and error out in that case?
  83 .done:
  84         ; cleanup and return
  85         pop di
  86         pop es
  87         pop edx
  88         pop ecx
  89         pop ebx
  90         pop eax
  91         ret
  92
  93 ; es:di - where to load the sector(s)
  94 ; ebx - start LBA address
  95 ; cx - how many sectors to read
  96 read_sectors:
  97         pusha
  98         mov ah, 0x42
  99         push dword 0
 100         push dword ebx
 101         push es
 102         push di
 103         push cx
 104         push word 0x10
 105         mov si, sp
 106         mov dl, [BOOT_DRIVE]
 107         int 0x13
 108         jc .error
 109         add sp, 16
 110         popa
 111         ret
 112         .error:
 113                 mov si, read_error
 114                 call print
 115                 hlt
 116                 jmp $-1
 117
 118 ; ds:si - pointer to partition table entry
 119 ; RETURN: ecx - first data sector
 120 get_1st_data_sec:
 121         push eax
 122         push ebx
 123         movzx ax, BPB_NumFats
 124         mul dword BPB_FatSz32 ; space occupied by all FATs
 125         movzx ebx, word BPB_RsvdSecCnt
 126         add eax, ebx ; space occupied by the reserved area
 127         add eax, [si] ; space before the partition
 128         mov ecx, eax
 129         pop ebx
 130         pop eax
 131         ret
 132
 133 read_error: db "Read error", 0
 134
 135 BOOT_DRIVE: db 0