Int 21 Fn 25C3  P - Phar Lap 386/dos-extender - Execute Program            [E]

   AX = 25C3h
   ES:EBX -> pointer to parameter block (see below)
   DS:EDX -> pointer to ASCIZ program filename

Return: CF clear if successful
       all registers unchanged
   CF set on error
       EAX = error code (see below)

Values for error code:
 01h function code in AL is invalid ???
 02h file not found or path invalid
 05h access denied
 08h insufficient memory to load program
 0Ah environment invalid
 0Bh invalid file format

Format of parameter block:
Offset Size    Description
 00h   DWORD   32-bit offset of environment string
 04h   WORD    segment selector of environment string
 06h   DWORD   32-bit offset of command-tail string
 0Ah   WORD    segment selector of command-tail string