INT 15 - SYSTEM - GET EXTENDED MEMORY SIZE (286+)
AH = 88h
Return: CF clear if successful
AX = number of contiguous KB starting at absolute address 100000h
CF set on error
AH = status
80h invalid command (PC,PCjr)
86h unsupported function (XT,PS30)
Note:   TSRs which wish to allocate extended memory to themselves often hook
this call, and return a reduced memory size.  They are then free to
use the memory between the new and old sizes at will.

See also: 1587

1588 - SYSTEM - GET EXTENDED MEMORY SIZE (286+)