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ModBase is a template project for split disassemblies of old videogame software. This particular template targets GBA software using the armips assembler.

Requirements

  • Latest version of armips
  • Python (any version)
  • make (probably GNU make)

Getting started

If you wish to disassemble a game with multiple versions, please take a look at the section entitled Multiple Base ROMs.

First, download this project's .zip file and create a new repository from it. Do not create a GitHub fork for your repository - new build code committed here is not guaranteed to be relevant or compatible with your project. Do, however, create a new Git repository to store your code.

Place your base ROM in base/ and alter lines 12-13 of the Makefile to point at the file. For example, if your file is base/Tumiki_Fighters.gba, then change

BASEROM := ${BASE_DIR}/baserom.gba
BUILDROM := ${BUILD_DIR}/baserom.gba

to

BASEROM := ${BASE_DIR}/Tumiki_Fighters.gba
BUILDROM := ${BUILD_DIR}/Tumiki_Fighters.gba

You may now add code to src/ representing your game. As you add code, it will automatically be included in the project, compiled, and verified against the base ROM. If you introduce a mistake into your project, you will recieve an error, such as:

cmp build/Frozen_Bubble.gba base/Frozen_Bubble.gba
build/Frozen_Bubble.gba base/Frozen_Bubble.gba differ: char 14, line 23

You can diagnose these errors using hexdump -C, cmp --verbose, or any graphical hex editor with a compare feature.

Convention is important!

By our default Makefile configuration, src/ holds all disassembled game code. Furthermore, standard convention is to store all code and assets within specific component folders. For example, if your game had a titlescreen with image assets and UI code, then you would have the following files:

src/titlescreen/state_machine.asm
src/titlescreen/resources.asm
src/titlescreen/background_gfx.png
src/titlescreen/sprite_gfx.png

The purpose of grouping code and coupled assets together in component directories is to clearly indicate the relationship between the two. For the same reason, any labels declared in *.asm files should be prefixed with the name of the component directory the file exists within. This is so that references from other *.asm files to this symbol will always indicate what directory a symbol's code is. Furthermore, exported labels should be CamelCased with a single _ separating the component from the rest of the label; while local labels should be always lowercase with an _ in lieu of spaces between words. Don't be afraid of long labels.

Avoid long files with large amounts of code. The longer the file, the harder for people to scan through it. At the same time, a single *.asm file must have a clearly indicated purpose. If your game has a state machine, then you create a state_machine.asm file and put the state table and code in there. If your game has a custom scripting language with 300 opcodes, you may want to split up their implementations according to purpose. Perhaps something like:

src/scriptvm/opcodes/arithmetic.asm
src/scriptvm/opcodes/resource_ldr.asm
src/scriptvm/opcodes/sprite_choreo.asm
src/scriptvm/opcodes/playfield.asm

This also demonstrates how you can group files within a subdirectory of a component. Just as long files are a detriment to readability, so are long directories. You do not have to prefix labels with both directories, however.

Code within an *.asm file must be formatted correctly. Labels should be always indicated; there should be no memory locations scattered throughout the code. All code should be indented with four spaces. An empty line should be added after every string of instructions that write memory or alter control flow. Conditional branches should include a label for the branch not taken. Don't be afraid to rename labels or do other sweeping refactors if the current set of labels don't accurately describe the function of the code. Use comments, but only to explain the purpose of an exported label, or where the behavior of the code isn't obvious from the labels and instructions in use.

What looks more readable to you?

.gba
.open "build/Gunroar.gba", 0x80000000
start:
bl 0x802B3710
bl statemachine
ldr r1, =0x0300C420
beq @@nothing_todo
bl linktx
@@nothing_todo:
bx R0
.pool
 .close

Or?

.gba
.open "build/Gunroar.gba", 0x80000000
GameLoop:
    bl LCDC_ExecuteDMA
    bl Game_StateMachine
    ldr r1, =I_SIO_Connected
    beq @@no_link_connection
    
@@link_connection:
    bl SIO_RunLinkTxDriver
    
@@no_link_connection:
    bx R0
.pool
 .close

Multiple Base ROMs

Some games may share code across multiple ROM images. For example, the game may have a 1.1 revision, or was released in multiple versions, or have multiple translations. In this case, you will need to modify the build system to be aware of multiple builds of the ROM image.

First, you will need to declare a BASEROM and BUILDROM for each version of the game you want to build. Because of some armips quirkiness which we will get to shortly, this particular iteration of ModBase treats BASEROM and BUILDROM as lists:

BASEROM := ${BASE_DIR}/Tumiki_Fighters_(U).gba ${BASE_DIR}/Tumiki_Fighters_(E).gba
BUILDROM := ${BUILD_DIR}/Tumiki_Fighters_(U).gba ${BUILD_DIR}/Tumiki_Fighters_(E).gba

No further modification to the Makefile is necessary, because armips is an overlay assembler. asm files directly reference whichever files they intend to modify, and there's no object format. Hence, we don't need to modify the Makefile to include or ignore specific asm files in different ROMs. The asm files will include or ignore themselves.

Adding image resources

To be determined.