INT 21 - Phar Lap 386/DOS-Extender - CALL REAL-MODE PROCEDURE
        AX = 250Eh
        EBX = CS:IP of real-mode procedure to call
        ECX = number of two-byte words to copy from protected-mode stack
              to real-mode stack
Return: CF clear if successful
            all segment registers unchanged
            all general registers contain values set by real-mode procedure
            all other flags set as they were left by real-mode procedure
            stack unchanged
        CF set on error
            EAX = error code
                01h not enough real-mode stack space
Note:   this function is also supported by FlashTek X-32VM; under X-32VM, the
          call will fail if ECX > 0000003Fh
SeeAlso: AX=2510h,AH=E1h"OS/286",INT 31/AX=0301h