The source editor is used to open and modify MMIX code. When launching Visual MMIX a default mms document is open named untitled.mms which does not exist until save (or assemble which automatically calls save) is executed from the File drop down. At this time, a user is prompted to supply a name and location for the file.
A user may also open an existing file by selecting File->Open and browsing to its location. Only a file ending in .mms may be opened at this time. Note that if the opened file had previously be assembled (a .mmo file exists with the same name in the same location as the currently open .mms file) the run button will be enabled and you can execute the file without having to first assemble. Note that any text changes since the file was last assembled will not be taken into account.
Copy, cut, and paste can all be executed by either selecting their buttons or selecting them from the Edit drop down menu. The cut,copy, and paste buttons will only be enabled if text is currently selected (in the case of cut & copy) or has previously been cut or copied (in the case of paste). Text outside of Visual MMIX can also be pasted into the editor window and the standard window hot buttons can be used (Ctrl-C for copy, etc).
Finally, if a file has successfully be assembled (as indicated in the Main Console), the tab will automatically switch from the editor tab to the simulator tab. The assembled lines will then appear in the simulator tab. The text in the editor panel can be modified at this time however the changes will not be reflected in the simulator tab until the next successful assembly.
As noted above, this tab displays the assembly instructions corresponding to the MMIX source code entered in the text editor tab (corresponding to the last successful assembly).