0040h:0013h           Memory Size in Kilobytes (0-640)

    Length: 2 bytes

    Contains the number of contiguous 1K memory blocks in the system
    (up to 640K).

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    Use Int 12h to access the memory size using the ROM BIOS
    interface.

    This memory value is determined at power-on time by either
    examining the DIP switches on the system board or using the value
    stored in the CMOS battery, whichever way is appropriate according
    to the type of machine.

    This is the amount of memory available to the entire system. This
    is not the amount of memory available to the user's program. Use
    Int 21h, Function 48h, to determine the amount of memory available
    to a user's program.

    To determine the amount of memory above the 1024K address range,
    use Int 15h, Service 88h.

    If the CMOS battery memory value or the DIP switch value is
    greater than the actual amount of memory, then the actual amount
    of memory is returned by this interrupt.