Integration with compilers

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.

Integration with compilers

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

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.

Importing and exporting labels

Workspace Menu

Mesen can also import and export labels in .mlb format. This is the same format as the label files produced by ASM6f. The ability to import labels can be used to integrate the debugger with your own workflow (e.g by creating your own scripts that produce .mlb files)

Mesen Label Files (.mlb)

The .mlb files used by Mesen to import/export labels is a simple text format. For example, this defines a label and comment on byte $100 of PRG ROM:

P:100:MyLabel:This is a comment

The format also supports multi-byte labels, defined by giving specifying an address range:


The first letter on each row is used to specify the label’s type:

P: PRG ROM labels
R: RAM labels (for the NES' internal 2kb RAM)
S: Save RAM labels
W: Work RAM labels
G: Register labels (these are used to define PPU/APU/Mapper registers, etc.)

Import Settings

Import Settings

For fine-grain control over the DBG/MLB file imports, the Import Settings window can be used.
This allows you to configure which types of labels/comments should be imported, as well as choosing whether or not Mesen should delete all existing labels before importing DBG/MLB files.