root/platform/g9/sub/FAT32_detect.S

/* [<][>][^][v][top][bottom][index][help] */
   1 
   2 "    MOV     R1, R4\n"              //  pointer to MBR in R1
   3 "    BL      mbr_read_dryos\n"      //  total sectors count in R0 before and after call
   4 
   5 // Start of DataGhost's FAT32 autodetection code
   6 // Policy: If there is a partition which has type W95 FAT32, use the first one of those for image storage
   7 // According to the code below, we can use R1, R2, R3 and R12.
   8 // LR wasn't really used anywhere but for storing a part of the partition signature. This is the only thing
   9 // that won't work with an offset, but since we can load from LR+offset into LR, we can use this to do that :)
  10 "    MOV     R12, R4\n"             // Copy the MBR start address so we have something to work with
  11 "    MOV     LR, R4\n"              // Save old offset for MBR signature
  12 "    MOV     R1, #1\n"              // Note the current partition number
  13 "    B       dg_sd_fat32_enter\n"   // We actually need to check the first partition as well, no increments yet!
  14 "dg_sd_fat32:\n"
  15 "    CMP     R1, #4\n"              // Did we already see the 4th partition?
  16 "    BEQ     dg_sd_fat32_end\n"     // Yes, break. We didn't find anything, so don't change anything.
  17 "    ADD     R12, R12, #0x10\n"     // Second partition
  18 "    ADD     R1, R1, #1\n"          // Second partition for the loop
  19 "dg_sd_fat32_enter:\n"
  20 "    LDRB    R2, [R12, #0x1BE]\n"   // Partition status
  21 "    LDRB    R3, [R12, #0x1C2]\n"   // Partition type (FAT32 = 0xB)
  22 "    CMP     R3, #0xB\n"            // Is this a FAT32 partition?
  23 "    CMPNE   R3, #0xC\n"            // Not 0xB, is it 0xC (FAT32 LBA) then?
  24 "    CMPNE   R3, #0x7\n"            // exFat?
  25 "    BNE     dg_sd_fat32\n"         // No, it isn't. Loop again.
  26 "    CMP     R2, #0x00\n"           // It is, check the validity of the partition type
  27 "    CMPNE   R2, #0x80\n"
  28 "    BNE     dg_sd_fat32\n"         // Invalid, go to next partition
  29                                     // This partition is valid, it's the first one, bingo!
  30 "    MOV     R4, R12\n"             // Move the new MBR offset for the partition detection.
  31 
  32 "dg_sd_fat32_end:\n"
  33 // End of DataGhost's FAT32 autodetection code
  34 

/* [<][>][^][v][top][bottom][index][help] */