INT 25 - DOS 1+ - ABSOLUTE DISK READ (except partitions > 32M)
        AL = drive number (00h = A:, 01h = B:, etc)
        CX = number of sectors to read
        DX = starting logical sector number (0000h - highest sector on drive)
        DS:BX -> buffer for data
Return: CF clear if successful
        CF set on error
            AH = status
                 80h device failed to respond (timeout)
                 40h seek operation failed
                 20h controller failed
                 10h data error (bad CRC)
                 08h DMA failure
                 04h requested sector not found
                 03h write-protected disk (INT 26 only)
                 02h bad address mark
                 01h bad command
            AL = error code (same as passed to INT 24 in DI)
        may destroy all other registers except segment registers
Notes:  original flags are left on stack, and must be popped by caller
        this call bypasses the DOS filesystem
BUG:    DOS 3.1 through 3.3 set the word at ES:[BP+1Eh] to FFFFh if AL is an
        invalid drive number
SeeAlso: INT 26