Integration with CC65/ASM6

When building homebrew software in assembly or C, it is possible to export the labels used in your code and import them into Mesen to simplify the debugging process. This allows the debugger to know which portions of the ROM correspond to which functions in your code, as well as display your code’s comments inside the debugger itself.

CC65 / CA65

CC65/CA65 are able to produce .DBG files which can be imported into Mesen’s debugger.
To make CC65/CA65 create a .DBG file during the compilation, use the --dbgfile command line option.

To import the .DBG file, use the File→Workspace→Import Labels command in the debugger window.

You can also enable the Auto-load DBG/MLB files to make Mesen load any .DBG file it finds next to the ROM whenever the debugger is opened or the power cycle button is used.
Note: For this option to work, the ROM file must have the same name as the DBG file (e.g MyRom.nes and MyRom.dbg) and be inside the same folder.

Source View

When a .DBG file is loaded, 2 additional options appear in the code window’s right-click menu:

  • Switch to Source View: This turns on Source View mode, which allows you to debug the game using the original code files, rather than the disassembly. This can be used for both assembly and C projects.
  • Show source code as comments: When enabled, the debugger will show the original source code to the right of the disassembly, as comments.


Integration with ASM6 is possible by using freem’s branch of ASM6 named ASM6f.
This fork contains 2 additional command line options that are useful when using Mesen: -m and -c

-m produces a .mlb (Mesen labels) file that can be imported manually using the File→Workspace→Import Labels command.
-c produces a .cdl (Code Data Logger) file which can be imported manually using the Tools→Code/Data Logger→Load CDL file command.

Additionally, you can use the Auto-load DBG/MLB files and Auto-load CDL files options in the File→Workspace menu to automatically load MLB and CDL files present in the same folder as the current ROM, with the same filename (e.g MyRom.nes,, MyRom.cdl).


Mesen can also import the .fns symbol files that NESASM produces. However, due to limitations in the .fns format, these labels can only be reliably imported for games containing exactly 32kB of PRG ROM. If you are creating a larger game, CC65/CA65 offers the best integration features at the moment.