INT 21 - Phar Lap 386/DOS-Extender - CALL REAL-MODE PROCEDURE, REGISTERS
        AX = 2510h
        EBX = CS:IP of real-mode procedure to call
        ECX = number of two-byte words to copy to protected-mode stack to
              real-mode stack
        DS:EDX -> pointer to parameter block (see below)
Return: CF clear if successful
            all segment registers unchanged,
            EDX unchanged
            all other general registers contain values set by real-mode proc
            all other flags are set as they were left by real-mode procedure
            real-mode register values are returned in the parameter block
        CF set on error
            EAX = error code
                01h not enough real-mode stack space
Note:   unlike most of the preceding 25xxh functions, this one is not
          supported by FlashTek X-32VM
SeeAlso: AX=250Fh

Format of parameter block:
Offset  Size    Description
 00h    WORD    real-mode DS value
 02h    WORD    real-mode ES value
 04h    WORD    real-mode FS value
 06h    WORD    real-mode GS value
 08h    DWORD   real-mode EAX value
 0Ch    DWORD   real-mode EBX value
 10h    DWORD   real-mode ECX value
 14h    DWORD   real-mode EDX value