Int 21 Fn 2510  P - Phar Lap 386/dos-extender - Call Real-mode Procedure,  [E]

   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

See Also: AX=250Eh,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