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Multicart engine for NES
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Action 53

The Action 53 ROM builder chews up a bunch of ROMs and spits out a multicart.


The menu has up to five pages, each represented by a tab at the top of the screen, and each page may have up to 20 activities. From this list, the player can view a more detailed description including basic play instructions.

For each ROM added to the collection, the ROM builder patches each bank with a small piece of code that allows the activity to reset back to the menu. NROM games use one of two patches: a 10-byte one that fits at $BFF0-$BFF9 or $FFF0-$FFF9, or a 20-byte one that fits anywhere in the PRG bank. (Larger games use more involved patches that change more of the mapper's state.) Then it stores the ROM's PRG data, its CHR data compressed with PB53, and information used by the extractor to reverse the reset patch.

The config file may specify ranges of addresses within $8000-$FFFF that are not used by the ROM. The reset patch is placed here, and other programs' CHR ROM and screenshot tile data may be placed here. For an NROM-128 program, this unused area automatically includes the half of $8000-$BFFF where the reset vector does not point.

For each activity, the builder stores information needed to display it in the menu: title, author, year of publication, number of players, a description with up to 16 lines of play instructions, and a screenshot as defined below. It also stores information needed to start the activity after the user has chosen it: which PRG and CHR banks it uses and where in the program to start running.

A ROM may have multiple entry points within one PRG bank, one for each activity. This allows a ROM with multiple separate activities to become several items in the menu. Examples of these from the commercial NES library include Duck Hunt ("Duck Hunt" and "Clay Shooting") and Donkey Kong Classics ("Donkey Kong" and "Donkey Kong Jr."). In fact, packing activities into "submultis", or ROMs with multiple activities sharing a PRG bank, is the most efficient way to fit a large number of activities into a collection. If no entry point is specified, the builder uses the reset vector from the ROM.

A ROM may have multiple CHR banks. If you specify one, the menu will load it into CHR RAM before launching your activity. Some developers have used defective startup code that tries to clear the nametables ($2000-$27FF), but it loops too much and ends up clearing the pattern tables too, so don't do that. The builder provides a patching mechanism that allows the config file to fix minor bugs, such as scribbling over the pattern tables, switching CHR banks, or a wrong version number.

ROM requirements

The builder produces a ROM, usually 128 to 2048 KiB. It can be configured to use an extended-BNROM mapper (iNES #34) with vertical mirroring or a custom Action 53 mapper (iNES #28). Each ROM inserted into a BNROM collection:

  • MUST work with vertical mirroring.
  • MUST initialize all hardware and memory needed to run from each activity's declared entry point.
  • MUST NOT write to $8000-$FFFF after the declared entry point.
  • MUST NOT blindly overwrite CHR data if using CHR ROM.
  • MUST declare an unused range of the ROM to hold the reset patch if the PRG ROM is at least 32768 bytes.
  • MUST NOT play samples if the PRG ROM is 16384 bytes or smaller and has a reset vector in $8000-$BFFF.

Most of these restrictions are lifted for collections using the Action 53 mapper, which may use common discrete mappers: UNROM (iNES #2 or #180), CNROM (iNES #3), AOROM (iNES #7), or BNROM. But if a game actually uses CNROM, as opposed to being a submulti with a separate CHR ROM bank for each activity, the cartridge will need to have 32 KiB of CHR RAM instead of 8 KiB.

Preparing graphics

A collection needs a title screen and a screenshot for each activity.

The title screen is 30 rows of 32 tiles, each 8 by 8 pixels. A picture can use up to 256 different tiles. Each 2 by 2 tile (16 by 16 pixel) area can use one of four color sets in the palette. Each color set has three colors plus a shared backdrop color.

To save ROM space, text may be overlaid on top of the title screen. Each 8x8-pixel area covered by text reduces available tiles by 1. For example, three 128-pixel lines of text take 16 tiles each, leaving 208 for graphics.

The title screen's palette is specified with a 32-nibble string representing a hex dump of NES palette memory ($3F00-$3F0F). The following example specifies a black ($0F) backdrop, color set 0 as grays ($00, $10, $20), color set 1 as reds ($06, $16, $26), color set 2 as greens ($0A, $1A, $2A), and color set 3 as blues ($02, $12, $22). By convention, the ignored bytes match the backdrop color.

||||||||||||||||||||||||||++++++- Color set 3
||||||||||||||||||||||||++------- Ignored
||||||||||||||||||++++++--------- Color set 2
||||||||||||||||++--------------- Ignored
||||||||||++++++----------------- Color set 1
||||||||++----------------------- Ignored
||++++++------------------------- Color set 0
++------------------------------- Backdrop color

An activity's screenshot is 7 rows of 8 tiles, each 8 by 8 pixels. Each tile can use one of two color sets in the palette. Each color set has three colors plus four grays ($0F, $00, $10, and $20). If a screenshot fails to meet this restriction, the builder will fail and print a diagnostic. If no screenshot is assigned to an activity, the builder assigns a fallback image: ../tilesets/screenshots/default.png


To build a collection, you'll need Python 3 and Pillow (Python Imaging Library). To rebuild the menu itself, you'll also need GNU Make, GNU Coreutils, cc65, and optionally the SoX audio converter. Instructions to install most of this are at


After you've installed Python and Pillow, make a config file and put it in a folder with your ROMs. Then run this command, specifying the filename of the config file and ROM file:

tools/ example.cfg example.nes

If no fatal errors occurred, a53games.nes should appear in the top level folder.

Warning: The following mechanism is broken.
The package also includes a tool to extract ROMs from the collection. If you have an a53games.nes file, you can extract ROMs, screenshots, and the skeleton of a.cfg file by running Thus, a collection is an "aggregate" under the GNU General Public License in the same way that a bootable disc image is. One caveat is that the only prgunused entries in the resulting configuration file correspond to the reset patches.

The configuration file

The configuration file uses a name-value pair syntax similar to that of .ini files but allowing multiple-line values. See the docstring at the top of tools/ for detailed syntax information.

A sample config file with one activity follows:

# BEGIN example.cfg


text=Hi Mom!

title=Wrecking Ball Boy
author=Justin Bailey
Use your grappling hook
to swing over gaps and
through obstacles.

+ Move
A: Fire or retract
  grappling hook
players=1-2 alt
screenshot=screenshots/Wrecking Ball Boy.png
rom=roms/Wrecking Ball Boy.nes

# END example.cfg

Let's take that apart, with comments this time:


Commands related to the title screen must appear in this section.


The title screen is a 256x240 pixel image as described above. This and other paths are relative to the directory containing the config file.


Sets the palette for converting the title screen to an NES background image.

text=Hi Mom!

Adds a line of text. Multiple-line values are allowed, but behavior is undefined if any line is longer than 128 pixels, which is about 28 characters.


Sets the top left X,Y coordinate for the latest line of text. X is pixel precise, but Y is rounded down to a multiple of 8.


Sets the foreground color for the latest line of text to color 2 and the background color to 0. One of the colors must be 0 or 3, and the other must be 1 or 2. They appear with whatever attribute was already at that position in the PNG.


Commands related to activities must appear in this section.


Begins a new page called "Platform". The page need only be specified once, before the first activity on the page. If you try to put more than 20 activities on a page, the builder will print a diagnostic.

title=Wrecking Ball Boy

Begins a new activity with this name.

author=Justin Bailey

The author or copyright owner of the activity.


The year in which the activity was first published.

Use your grappling hook
to swing over gaps and
through obstacles.

+ Move
A: Fire or retract
  grappling hook

Up to 16 lines, each about 25 to 28 characters, describing the basics of how to play a game. A colon instead of an equal sign starts a multiple-line value, and a dot on its own line ends it. The first dot on any line is stripped off. (This escaping method is the same used in SMTP.)

players=1-2 alt

How many players can play at once. Acceptable values are 1, 2, 1-2, 1-2 alt, 1-3, 1-4, 2-4 alt, 2-6 alt, and 2-4. 2 means the game is for two players only, like "Fire Breathers" from Action 52. If omitted, uses 1.

screenshot=screenshots/Wrecking Ball Boy.png

A 64x56 pixel screenshot of the activity as described above.

rom=roms/Wrecking Ball Boy.nes

The path to the ROM containing the activity.


Ranges of bytes in the PRG bank that are unused. The builder will usually place an exit patch here and may fill these with CHR data, screenshots, etc. This MUST be specified for any 32 KiB or bigger PRG ROM, but it need only be specified in one activity within each bank of the PRG ROM.

The following are useful primarily for submultis, or ROMs containing more than one activity:


Advanced: Overrides the detected mapper with a mapper configuration code. Bit 7 is 0 for CNROM or 1 for other, bits 5-4 control the game's PRG ROM size (32, 64, 128, or 256 KiB), and bits 3-0 have nearly the same meaning as on MMC1's control register. Useful primarily for submultis.


Which 32 KiB PRG bank within the PRG ROM is associated with an activity. If omitted, uses the first bank for fixed-$8000 (such as UNROM configured as mapper 180) or the last bank for other mappers.


Which 8 KiB bank is associated with an activity. If omitted, uses the first bank (0).


Where the activity's code starts if the PRG bank's reset vector doesn't point there.


One-line fixes to make an activity boot correctly. These apply to all activities in one PRG ROM.


These control how the game is patched to jump to the menu when Reset is pressed. If your game has a menu option to exit, use none; otherwise, the valid options are nrom, unrom, cnrom, aorom, unrom, and unrom180.


Sets the reset vector to $FF00 instead of automatically patching the ROM to exit to the menu. Use this if an activity has an entrypoint and is already patched to exit to the main menu on Reset.


Q. I've added a header to a53menu.prg. Why does it just hang?

This file contains the code for the menu. It contains no activities and thus will not work unless activities are added using

Q. How do I use a collection with more than a dozen or so ROMs on a PowerPak?

The PowerPak has only enough RAM to simulate 512 KiB of PRG ROM. You'll need to rebuild some of your activities into submultis and split your collection into smaller collections. Action 53 volume 3 (2017), for example, had to be played with two.

Q. How do I play games that use two Zappers?

The menu supports a Zapper in port 2 or a standard controller or Super NES Mouse in port 1. PowerPak users can use controller 1 to navigate to Action 53, swap to the desired controller at the title screen, press Reset to get the NES to recognize the newly connected controller, and then use the menu.

Q. Why can't I use [x] controller for the menu?

The vast majority of the menu program is the work of one person, and he has an NES, not a Famicom. To get a new NES controller supported in the menu, make a homebrew game supporting both that controller and the standard controller, and notify that you want that game included.


This manual, as well as the menu and builder, are distributed under the following terms:

Copyright 2012-2018 Damian Yerrick

Copying and distribution of this file, with or without modification, are permitted in any medium without royalty provided the copyright notice and this notice are preserved in all source code copies. This file is offered as-is, without any warranty.

(End of terms)

This product is not endorsed by Nintendo or Active Enterprises.

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