Compatibility: 3.0+
Expects: AX    4310H
Returns: ES:BX far address of the XMS function dispatcher
Info: Obtains the address of the XMS function dispatcher, though which
all eXtended Memory Specification services are accessed.

Before using this fn, use INT 2fH 4300H to check to see if the
XMS provider has been installed.

See Extended Memory Specification (XMS) for information on how to
access XMS services and a description of the available fns.

Example: ;--------------- check for and access an EMM fn
dd  pfEmmFns     ;will hold 32-bit segment:offset addr
mov  ax,4300H
int  2fH
cmp  al,80H      ;is support present?
jne  no_ems      ; no, go

mov  ax,4310H    ;get entry address
int  2fH
mov  word ptr [pfEmmFns],bx
mov  word ptr [pfEmmFns+2],es
;--------------- now call a service
mov  ah,00H      ;get XMS version
call [pfEmmFns]  ; this is a far call
;--------------- on most calls, check for AX=0001 (error)

- -

INT 2fH 4310H: Get HIMEM.SYS (XMM) Entry Address