When ALP is invoked, if the full path name of the ALP executable was provided by the operating system, then the "path" portion is isolated and used to construct the value of the BasePATH internal variable. For instance, if the user invoked ALP and the name of the executable was made available as "C:\TOOLS\ALP.EXE", then the BasePATH internal variable would contain the value "C:\TOOLS\". The value of BasePATH can be used to locate ALP-specific components in the environment (such as the ALP messages file) without the need to store this information in an alternate environment variable such as DPATH. Check your operating system documentation to see if it feasible to use the BasePATH method of locating ALP components.
If the file name of the ALP executable is not available at run time, then the value of BasePATH is NULL.