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.
XMS provider has been installed.
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
cmp al,80H ;is support present?
mov ax,4310H ;get entry address
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)