INT 25h (37)             Absolute Disk Read

    Reads one or more sectors on a specified logical disk.

       On entry:      AL         Drive number (0=A, 1=B)
                      CX         Number of sectors to read
                      DX         Starting sector number
                      DS:DX      Buffer to store sector read

       Returns:       AX         Error code (if CF is set; see below)
                      Flags      DOS leaves the flags on the stack

  --------------------------------------------------------------------------

    This DOS service (and its write counterpart, Interrupt 26h) is
    comparable to service provided by the ROM-BIOS in Interrupt 13h,
    except for these two differences:

     1.  DOS numbers disk sectors sequentially, starting at cylinder 0,
         head 0, sector 1. The BIOS service identifies sectors by three
         separate coordinates--cylinder, head, and sector). The following
         formula converts BIOS-numbered sectors to the DOS format:

      DOS.Sector.Number = (BIOS.Sector-1) + BIOS.Head * Sectors.per.Head +
                          BIOS.Cylinder * Sectors.per.Head * Heads.per.Disk

     2.  DOS works with logical drives, while the BIOS works with physical
         drives only. That means that the DOS interrupt can be used to
         read a phantom drive B, a RAM drive, or a logical drive that has
         been mapped to a nondefault physical drive via the ASSIGN
         command.

    Error information is reported in AX as follows:

              Error code in AH:

                01h       Invalid command
                02h       Address mark not found
                03h       Attempt to write on write-protected diskette
                04h       Sector not found
                05h       Reset failure
                07h       Drive parameter activity failure
                08h       DMA overrun
                09h       DMA boundary error
                10h       CRC or ECC data error

                11h       Possible error corrected by ECC (AL contains
                          burst length)
                20h       Controller failure
                40h       Bad seek
                80h       Drive timeout
                BBh       Undefined error
                FFh       Sense operation failure

              Error code in AL:

                00h       Write-protect error
                01h       Invalid drive number
                02h       Drive not ready
                03h       Invalid command
                04h       CRC error
                05h       Bad request structure length
                06h       Seek error
                07h       Unknown medium; disk format not recognized
                08h       Sector not found
                09h       Printer out of paper
                0Ah       Write error
                0Bh       Read error
                0Ch       General, nonspecific error

    Note that the AH error information is the same as that returned in AH
    by Interrupt 13h, Service 2, and the AL error information is the same
    returned in DL in response to a critical error (Interrupt 24h).

    Following a call to Interrupt 25h, DOS leaves one word (the flag
    contents at the time the interrupt was invoked) on the stack. You
    should POP this word to prevent stack growth.