TECH Help! describes all services of the Extended Memory Specification
version 2.0.  Version 2.0 has been standard since October, 1992 and is
usually available on 286+ computers running DOS 4.0+.

There are two parts to the XMS API.  The first uses INT 2fH 43xxH as a way
to test for installation and to obtain the entry address for the other
services.  After obtaining that address, you call it directly.  See
INT 2fH 4310H for an example.

See: INT 2fH 4300H (is HIMEM.SYS installed?)
INT 2fH 4310H (get XMM entry address)
                                                        General Functions
00H  Get XMS version number
                                                       Standard Functions
01H  Request high memory area
02H  Release high memory area
03H  Global enable A20
04H  Global disable A20
05H  Local enable A20
06H  Local disable A20
07H  Query A20 state
                                                     XMS Memory Functions
08H  Query free extended memory
09H  Allocate extended memory block
0aH  Free extended memory block
0bH  Move extended memory block
0cH  Lock extended memory block
0dH  Unlock extended memory block
0eH  Get handle information
0fH  Resize extended memory block
                                                            UMB Functions
10H  Request upper memory block
11H  Release upper memory block

See Also: XMS
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XMS Functions