boot/x86/fat32/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 ; RETURN: CF set = error
  11 ; NOTE: Cluster chain might be the root directory or a file.
  12 read_cluster_chain:
  13         push eax
  14         push es
  15         push di
  16         .loop:
  17                 call read_cluster
  18                 jc .done
  19                 cmp eax, 0xffffff7
  20                 jl .loop
  21 .done:
  22         pop di
  23         pop es
  24         pop eax
  25         ret
  26
  27 ; eax - cluster number
  28 ; es:di - where to load the cluster (incremented automatically)
  29 ; RETURN: eax - next cluster number
  30 ;         CF set = error
  31 read_cluster:
  32         push ebx
  33         push ecx
  34         push edx
  35 .loop:
  36         ; get the first sector of the cluster to read
  37         push eax
  38         sub eax, 2
  39         xor ebx, ebx
  40         mov bl, BPB_SecPerClus
  41         mul ebx
  42         add eax, ecx
  43         mov ebx, eax
  44         pop eax
  45
  46         ; read the cluster
  47         push cx
  48         movzx cx, BPB_SecPerClus
  49         call read_sectors
  50         pop cx
  51         cmp dl, 0
  52         je .error
  53
  54         ; increment the load offset (and segment if needed)
  55         push eax
  56         xor eax, eax
  57         movzx ax, BPB_SecPerClus
  58         mul word BPB_BytsPerSec
  59         add di, ax
  60         jno .get_next_cluster
  61         mov ax, es
  62         add ax, 0x1000
  63         mov es, ax
  64
  65 .get_next_cluster:
  66         pop eax
  67         ; get FAT sector number and offset containing the next cluster number
  68         xor ebx, ebx
  69         mov bl, 4 ; put 4 in EBX, doing this instead of "mov ebx, 4" saves us an entire byte
  70         mul ebx
  71         movzx ebx, word BPB_BytsPerSec
  72         div ebx
  73         movzx ebx, word BPB_RsvdSecCnt
  74         add eax, ebx
  75         add eax, [si]
  76         mov ebx, edx
  77         ; reminder for myself: EAX is FAT sector number, (E)BX is offset into FAT sector
  78
  79         ; load the FAT sector we're looking for
  80         push di
  81         push ebx ; offset
  82         push es ; we want to read at 0:1000, not STAGE3_SEGMENT:1000
  83         push eax ; desired LBA
  84         xor ax, ax
  85         mov es, ax
  86         pop ebx ; pop LBA into EBX
  87         mov di, 0x6000
  88         mov cx, 1
  89         call read_sectors
  90         cmp dl, 0
  91         ; find the cluster we're looking for
  92         pop es ; restore STAGE3_SEGMENT
  93         pop ebx ; pop FAT offset back into EBX for cmp
  94         mov eax, [di+bx]
  95         pop di
  96         jne .end
  97         ; cleanup and return
  98 .error:
  99         stc
 100 .end:
 101         pop edx
 102         pop ecx
 103         pop ebx
 104         ret
 105
 106 ; es:di - where to load the sector(s)
 107 ; ebx - start LBA address
 108 ; cx - how many sectors to read
 109 ; RETURN: DL=0 means error
 110 read_sectors:
 111         clc
 112         push ax
 113         push si
 114         mov ah, 0x42
 115         push dword 0
 116         push dword ebx
 117         push es
 118         push di
 119         push cx
 120         push word 0x10
 121         mov si, sp
 122         mov dl, [BOOT_DRIVE]
 123         int 0x13
 124         mov dl, 1
 125         jnc .done
 126         mov dl, 0
 127 .done:
 128         add sp, 16
 129         pop si
 130         pop ax
 131         ret
 132
 133 ; ds:si - pointer to partition table entry
 134 ; RETURN: ecx - first data sector
 135 get_1st_data_sec:
 136         push eax
 137         push ebx
 138         movzx ax, BPB_NumFats
 139         mul dword BPB_FatSz32 ; space occupied by all FATs
 140         movzx ebx, word BPB_RsvdSecCnt
 141         add eax, ebx ; space occupied by the reserved area
 142         add eax, [si] ; space before the partition
 143         mov ecx, eax
 144         pop ebx
 145         pop eax
 146         ret
 147
 148 read_error: db "Read error", 0
 149
 150 BOOT_DRIVE: db 0