Saved from: http://www.blog.cadnauseam.com/2008/09/01/what-is-loaded-at-autocad-startup-and-when/
Warning, CAD nerd stuff ahead. This is a long and technical post and if you’re using AutoCAD in a largely out-of-the-box state you probably won’t care about any of it.

If your modification of AutoCAD extends beyond the trivial, you may find it useful to know what AutoCAD loads, and in what order things are loaded. It is possible for LISP files in particular to tread on each other’s toes, so knowing what gets loaded when can be useful information for diagnosing such clashes. This post aims to provide that information. It uses AutoCAD 2009 as an example, but the same principles apply to all releases from AutoCAD 2006 onwards.

On startup, the first things AutoCAD loads are its CUI files. It first loads the Enterprise CUI file, then the Main CUI file, then any partial CUI files attached to the Main, then any partial CUI files attached to the Enterprise. I have no idea of the reasoning behind this slightly strange order, but there it is. The order of the partial CUIs loaded in each case is determined by the order in which they appear in the parent CUI files, which is determined by the order in which you attached them. If you don’t like this order, you can attach and reattach them in the CUI interface, or you can do the same thing much quicker with a text editor if you feel confident enough. If there are LISP files associated with these CUI files, they are not loaded yet. You’ll need to wait a few paragraphs for that.

Next, if you have created a file called acad.rx in AutoCAD’s search path, any ARX files listed in that file will be loaded. There are other ways in which developers can load their ARX files at startup, but I won’t go into that here.

Following that, the acad*.lsp files are loaded. First, Autodesk’s acad2009.lsp file is loaded. Next, if you have created a file called acad.lsp, that is loaded. These two files are only loaded at first startup, unless the ACADLSPASDOC system variable is set to 1, in which case the acad.lsp file is reloaded with each new drawing. Next comes Autodesk’s acad2009doc.lsp and any acaddoc.lsp file you may have created, in that order. These two files are loaded at startup and with every new drawing session.

It’s worth pointing out here that the acad200x.lsp and acaddoc200x.lsp files are Autodesk’s and are not intended to be modified by users. You can modify them, and adding things in there works fine, but updates and hotfixes can overwrite these files, leaving you to patch things up again afterwards. The acad.lsp and acaddoc.lsp files are yours, and that is where you are best advised to put your additions.

I hesitate to mention VBA because I have long avoided that development environment and my knowledge in that area is very limited, but if you’re a VBA developer and have created an acad.dvb file in AutoCAD’s search path, it gets loaded at this point.

Once the acad*.* files are loaded, then come any LISP files associated with the CUI files that were loaded at the beginning. For each CUI file, if there is a *.mnl file of the same name, that will be loaded first (*.mnl files are just *.lsp files renamed). After that, any LISP files that are specified in the CUI file will be loaded, in the order in which they appear in the CUI file itself. This order can be modified in the same ways that the partial CUI loading sequence can be modified; “delete” and “load” (detach and attach, really) the files within the CUI interface, or hack the CUI file with a text editor.

The CUI-associated LISP files are loaded as described in the above paragraph for each CUI file in turn, in the same order as the CUI files themselves: Enterprise, then Main, then partials to Main, then partials to Enterprise.

The Appload command provides a Startup Suite facility, where you can specify any number of files to load (*.arx, *.lsp, *.dvb, *.dbx, *.vlx or *.fas). If you have done so, those files are loaded at this point, in the order in which they appear in the Startup Suite list.

That’s all the actual loading done, but we’re not finished yet. At this point AutoCAD’s environment should be all ready to do pretty much anything, including things that modify the drawing database, including invoking commands. This was not true earlier on, so if you want to do things like change the drawing or run commands, this should be done using a startup routine rather than called directly at load time from any of the files loaded above.

If you’ve defined a VBA sub called AcadStartup(), it will be called now. If starting a new drawing, any sub called AcadDocument_Activate() will be called instead. The caveat about my VBA ignorance still applies here.

If a LISP function called (S::STARTUP) has been defined, it will be called next. Where could that be defined? Anywhere in any of the LISP files mentioned above, or in any LISP or other files that are loaded by any of those files, or by any files that are loaded by any of those files, and so on ad infinitum. It could even be defined in one of the ARX files loaded at any point. This would be unusual, but is quite possible.

If there are multiple (S::STARTUP) functions defined in various places, which one wins? Whichever one loaded last. That’s why the load order can be important, but it’s also why you should never have an unconditional (defun S::STARTUP …) definition in your LISP code. Instead, you should append your startup code to any existing (S::STARTUP) function. That way, your startup can cooperate with any others in your environment rather than walking all over it. If there is some interest in that subject, I can cover it in more detail in a future post.

In summary, here is the AutoCAD startup sequence:

A. CUI files loaded:
1. Enterprise
2. Main
3. Partials to Main
4. Partials to Enterprise

B. acad*.* files loaded:
1. Files listed in acad.rx
2. acad2009.lsp
3. acad.lsp
4. acad2009doc.lsp
5. acaddoc.lsp
6. acad.dvb

C. CUI-associated MNL and LSP files loaded:
1. Enterprise named MNL
2. Enterprise loaded LSP and MNL
3. Main named MNL
4. Main loaded LSP and MNL
5. Partials to Main named MNLs
6. Partials to Main loaded LSPs and MNLs
7. Partials to Enterprise named MNLs
8. Partials to Enterprise loaded LSPs and MNLs

D. Startup suite files loaded

E. Startup routines run:
1. AcadStartup() called (AutoCAD startup)
2. AcadDocument_Activate() called (Drawing startup)
3. (S::STARTUP) called

ARXs and DLLs are loaded from HKEY_CURRENT_USER\Software\Autodesk\AutoCAD\R17.2\ACAD-7001:409\Applications and HKEY_LOCAL_MACHINE\SOFTWARE\Autodesk\AutoCAD\R17.2\ACAD-7001:409\Applications.

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