INT 21 - Phar Lap 386/DOS-Extender - EXECUTE PROGRAM
        AX = 25C3h
        ES:EBX -> pointer to parameter block (see below)
        DS:EDX -> pointer to ASCIIZ program filename
Return: CF clear if successful
            all registers unchanged
        CF set on error
            EAX = 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