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Int 13h, 0Bh            Write Long                                diagnostics

    Writes one or more long sectors to a fixed disk from memory. A
    long sector is a sector of information plus 4 bytes representing
    the ECC code for the sector. This service can be performed on
    fixed disks only.

        Entry   AH    = 0Bh
                AL    = Number of sectors to write (1-127)
                CH    = Cylinder number (10-bit value; upper 2 bits in CL)
                CL    = Starting sector number
                DH    = Head number
                DL    = Drive number (Bit 7 must be set)
                ES:BX = Address of memory buffer

        Return  AH = Status of operation (See Service 01h)
                AL = Number of sectors written
                CF   Set if error, else cleared

    Writes the information at a memory buffer to the sectors starting
    at the specified location (head, cylinder, and track)


    Note
    This service is for diagnostics only. For a more generalized
    version, see Service 03h, which writes sectors for both diskettes
    and fixed disks. Also see Int 26h (DOS), which  allows the writing
    of absolute sectors from any type of block device.

    The cylinder number is a ten-bit quantity (0 through 1023). Its
    most significant two bits are in bits 7 and 6 of CL; the remaining
    eight bits are in CH. The starting sector number fits in the
    low-order portion (lower 6 bits) of CL.

    The value returned in AL (number of sectors written) may not give
    the correct number of sectors, even though no writing error has
    occurred. Use the results of the Carry flag and AH (status flag)
    to determine the status of the operation.

    The number of sectors to write (AL) can be 127 sectors at most.

    If an error is encountered while writing a sector, use Service 0h
    to reset the drive and retry the operation. It is recommended that
    at least 3 retries be attempted before an error is signalled,
    since the error may have resulted from the diskette motor not
    being up to speed.

    Because of the architecture of the DMA channel, an error will
    occur if the buffer in memory for the sectors overlaps a 64K page
    boundary. A 64K page boundary is a memory location which is one of
    the following (10000h, 20000h, 30000h, etc.). Ensure that no part
    of your buffer falls on such a boundary. If it does, then create a
    new buffer or start the buffer just after the boundary.

    For the AT, XT-286, and PC Convertible, the BIOS executes Int 15h,
    Service 90h (Device Busy), for the diskette (Type = 01h) and the
    fixed disk (Type = 00h) prior to waiting for the interrupt. Int
    15h, Service 91h (Interrupt Complete), is executed upon
    completion.