You can create an acad.lsp file if you regularly use specific AutoLISP routines. When you start AutoCAD, it searches the support file search path for an acad.lsp file. If an acad.lsp file is found, it is loaded into memory.
The acad.lsp file is loaded at each drawing session startup when AutoCAD is launched. Because the acad.lsp file is intended to be used for application-specific startup routines, all functions and variables defined in an acad.lsp file are only available in the first drawing. You will probably want to move routines that should be available in all documents from your acad.lsp file into the acaddoc.lsp file.
The recommended functionality of acad.lsp and acaddoc.lsp can be overridden with the system variable. If the ACADLSPASDOC system variable is set to 0 (the default setting), the acad.lsp file is loaded just once: upon application startup. If ACADLSPASDOC is set to 1, the acad.lsp file is reloaded with each new drawing.
The acad.lsp file can contain AutoLISP code for one or more routines, or just a series of load function calls. The latter method is preferable, because modification is easier. If you save the following code as an acad.lsp file, the files mysessionapp1.lsp, databasesynch.lsp, and drawingmanager.lsp are loaded every time you start AutoCAD.