ML Fatal Error A1000
cannot open file: filename

The assembler was unable to open a source, include, or output file.

One of the following may be a cause: 

The file does not exist. 
The file is in use by another process. 
The filename is not valid. 
A read-only file with the output filename already exists. 
The current drive is full. 
The current directory is the root and is full. 
The device cannot be written to. 
The drive is not ready.