Chapter 19: Images

Overview

OpenVMS programs are known as executable images or just images, and the process of starting program execution is called image activation. Under OpenVMS, image files have the filetype.EXE. The term image refers to the contents of the executable file— some sections of the file contain program contents exactly as they are to be laid out in memory. Hence, some sections of the executable file are exact images of their memory counterparts.

Image executions are requested directly by using the DCL RUN command or a foreign command, and most other DCL commands invoke an image to carry out the requested action.

When a user process invokes a normal image, it is mapped into the P0 region of the process. Note the use of the term mapped rather than loaded. OpenVMS does not directly load image pages into memory, but instead updates the process page tables to use pages in the image as backing store. Because of this, pages of the image are faulted in as needed. Pages in the image file that are never needed will not unnecessarily consume physical memory.

For most images, activation includes steps that allow the inclusion of a debugger, image dump facility, or traceback facility. The debugger is a special program that allows detailed control and examination of a running program. The image dump facility enables memory contents to be saved to disk in case of a severe error, allowing for examination at a later time. Lastly, the traceback facility displays debugging and stack information about some types of serious errors. For security reasons, support for these features is disabled for some types of images, including installed images, described later.

Some images are simple, self-contained programs and require no further preparation. However, a simple main image may be linked against (contain references to) shareable or installed images (described later), calling for additional preparation before being executed.

When the program has reached its end, has aborted due to a severe error, or has been manually aborted (not merely interrupted), image rundown occurs. This refers to the steps taken to return the process to the same state it was in before the image ran. This includes restoring process privileges and the working set size to their initial values and deleting P0 and portions of P1 space. After image rundown, control is returned to DCL, if present. If the process has no CLI (as is the case with some detached processes), the process is terminated.