amd64/bareMetalOS-0.5.2/pure64.boot0/src/fat16.asm

   1 ; =============================================================================
   2 ; Pure64 -- a 64-bit OS loader written in Assembly for x86-64 systems
   3 ; Copyright (C) 2008-2011 Return Infinity -- see LICENSE.TXT
   4 ;
   5 ; FAT16 functions
   6 ; =============================================================================
   7
   8
   9 ; -----------------------------------------------------------------------------
  10 ; readcluster -- Read a cluster from the FAT16 partition
  11 ; IN:   AX - (cluster)
  12 ;       RDI - (memory location to store at least 32KB)
  13 ; OUT:  AX - (next cluster)
  14 ;       RDI - points one byte after the last byte read
  15 readcluster:
  16         push rsi
  17         push rdx
  18         push rcx
  19         push rbx
  20
  21         and rax, 0x000000000000FFFF             ; Clear the top 48 bits
  22         mov rbx, rax                            ; Save the cluster number to be used later
  23
  24         cmp ax, 2                               ; If less than 2 then bail out...
  25         jl near readcluster_bailout             ; as clusters start at 2
  26
  27 ; Calculate the LBA address --- startingsector = (cluster-2) * clustersize + data_start
  28         xor rcx, rcx
  29         mov cl, byte [fat16_SectorsPerCluster]
  30         push rcx                                ; Save the number of sectors per cluster
  31         sub ax, 2
  32         imul cx                                 ; EAX now holds starting sector
  33         add eax, dword [fat16_DataStart]        ; EAX now holds the sector where our cluster starts
  34         add eax, [fat16_PartitionOffset]        ; Add the offset to the partition
  35
  36         pop rcx                                 ; Restore the number of sectors per cluster
  37         call readsectors                        ; Read one cluster of sectors
  38
  39 ; Calculate the next cluster
  40 ; Psuedo-code
  41 ; tint1 = Cluster / 256  <- Dump the remainder
  42 ; sector_to_read = tint1 + ReservedSectors
  43 ; tint2 = (Cluster - (tint1 * 256)) * 2
  44         push rdi
  45         mov rdi, hdbuffer1                      ; Read to this temporary buffer
  46         mov rsi, rdi                            ; Copy buffer address to RSI
  47         push rbx                                ; Save the original cluster value
  48         shr rbx, 8                              ; Divide the cluster value by 256. Keep no remainder
  49         movzx ax, [fat16_ReservedSectors]       ; First sector of the first FAT
  50         add eax, [fat16_PartitionOffset]        ; Add the offset to the partition
  51         add rax, rbx                            ; Add the sector offset
  52         mov rcx, 1
  53         call readsectors
  54         pop rax                                 ; Get our original cluster value back
  55         shl rbx, 8                              ; Quick multiply by 256 (RBX was the sector offset in the FAT)
  56         sub rax, rbx                            ; RAX is now pointed to the offset within the sector
  57         shl rax, 1                              ; Quickly multiply by 2 (since entries are 16-bit)
  58         add rsi, rax
  59         lodsw                                   ; AX now holds the next cluster
  60         pop rdi
  61
  62         jmp readcluster_end
  63
  64 readcluster_bailout:
  65         xor ax, ax
  66
  67 readcluster_end:
  68         pop rbx
  69         pop rcx
  70         pop rdx
  71         pop rsi
  72 ret
  73 ; -----------------------------------------------------------------------------
  74
  75
  76 ; -----------------------------------------------------------------------------
  77 ; findfile --
  78 ; IN:   RSI(Pointer to file name, must be in 'FILENAMEEXT" format)
  79 ; OUT:  AX(Staring cluster), 0x0 if not found
  80 ; Notes: Only searches the root sector.. not the following sectors.
  81 findfile:
  82         push rsi
  83         push rdi
  84         push rdx
  85         push rcx
  86         push rbx
  87
  88         clc                             ; Clear carry
  89         xor rax, rax
  90         mov eax, [fat16_RootStart]      ; eax points to the first sector of the root
  91         add eax, [fat16_PartitionOffset]        ; Add the offset to the partition
  92         mov rdx, rax                    ; Save the sector value
  93
  94 os_fat16_find_file_read_sector:
  95         mov rdi, hdbuffer1
  96         push rdi
  97         mov rcx, 1
  98         call readsectors
  99         pop rdi
 100         mov rbx, 16                     ; Each record is 32 bytes. 512 (bytes per sector) / 32 = 16
 101
 102 os_fat16_find_file_next_entry:
 103         cmp byte [rdi], 0x00            ; end of records
 104         je os_fat16_find_file_notfound
 105
 106         mov rcx, 11
 107         push rsi
 108         repe cmpsb
 109         pop rsi
 110         mov ax, [rdi+15]                ; AX now holds the starting cluster # of the file we just looked at
 111         jz os_fat16_find_file_done      ; The file was found. Note that rdi now is at dirent+11
 112
 113         add rdi, byte 0x20
 114         and rdi, byte -0x20
 115         dec rbx
 116         cmp rbx, 0
 117         jne os_fat16_find_file_next_entry
 118
 119 ; At this point we have read though one sector of file names. We have not found the file we are looking for and have not reached the end of the table. Load the next sector.
 120
 121         add rdx, 1
 122         mov rax, rdx
 123         jmp os_fat16_find_file_read_sector
 124
 125 os_fat16_find_file_notfound:
 126         stc                             ; Set carry
 127         xor rax, rax
 128
 129 os_fat16_find_file_done:
 130         cmp ax, 0x0000                  ; BUG HERE
 131         jne wut                         ; Carry is not being set properly in this function
 132         stc
 133 wut:
 134         pop rbx
 135         pop rcx
 136         pop rdx
 137         pop rdi
 138         pop rsi
 139 ret
 140 ; -----------------------------------------------------------------------------
 141
 142
 143 ; =============================================================================
 144 ; EOF