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Int 2Fh / 4300h         Get HIMEM.SYS Installed State                DOS 5.0+

    Determines whether an extended-memory manager (HIMEM.SYS) has been
    loaded.

        Entry   AX = 4300h

        Return  AL = 80h | 00h   XMS-driver loaded | not loaded


    The HIMEM.SYS driver provides a set of functions that programs use
    to independently manage extended memory. Although programs can
    also use these functions to manage the high memory area (HMA) and
    upper memory blocks (UMBs), programs should not do so if DOS
    already manages these areas.

    This function returns the installed state of any extended-memory
    manager as long as the manager conforms to the Lotus/Intel/
    Microsoft/AST eXtended Memory Specification (XMS) version 2.0.