Compatibility: 3.0+
Expects: AX    4300H
Returns: AL    status: 00H = is not installed
80H = is installed
Info: Checks to see if HIMEM.SYS or compatible XMS provider has been
loaded in CONFIG.SYS and is resident in memory.

To access XMS services, use INT 2fH 4310H (get XMM entry address)
and place FAR calls to the entry address returned by that fn.

See Extended Memory Specification for info on XMS services.

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INT 2fH 4300H: Is HIMEM.SYS (XMS Support) Installed?