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.