On supported platforms (currently Cocoa and Island), you can opt to see a Disassembly view of the underlying assembly code for a given Stack Frame. This option is available whether are present for the frame in question, or not.
When no Debug Symbols (or no Source) for a frame is available, you can invoke Disassembly directly from the Stack Frame view, by clicking the "Show Disassembly" button:
The view will switch from the generic view to a read-only text view that contains the assembly code for the stack frame. Execution Points will be highlighted, just as they would be for regular source code view – in this case indicating the exact machine instruction that the application is paused on:
As you can see, comments provide more detailed information about known symbols, where available.
Even if source is available for a method, sometimes it can be helpful to see the underlying assembly, to get a clearer understanding of what exactly is going on, under the hood. For this reason, Disassembly view is also available by right-clicking a stack frame in the Threads and Callstacks pane and choosing "Show Disassembly" from the menu:
To revert to viewing source code (or the generic Stack Frame view as shown in the first screenshot), you can right-click the frame again, and choose "Hide Disassembly".