Overview

This article provides an overview on the implementation of The Legend of Zelda: Breath of the Wild.

Framework and engine

Breath of the Wild is a modern Nintendo game which was released in March 2017 for the Wii U and the Switch. Thus, it shares a lot of code with contemporary first-party Nintendo games such as Splatoon 2 and Super Mario Odyssey, notably the sead (Nintendo's in-house framework and utilities for first-party titles) and NintendoWare (GFX, audio, effects, and UI) libraries.

BotW is the first major Nintendo game to use XLink (EffectLink and SoundLink) as a system to make briding code and graphics or sound effects easier, and also marks the first significant use of event flows and the EventFlow/evfl library to implement in-game events such as cutscenes and NPC interactions.

A completely custom engine – which Nintendo seemingly calls KingSystem – is used for Breath of the Wild. It appears to have been written from scratch as nothing significant is shared with previous Zelda games or even older Nintendo games. In particular, Breath of the Wild does not use LunchPack or SMO's engine, even though games using the former appear to use a BotW-inspired ROM structure.

The game relies on the Havok physics engine for ragdoll physics, collision (Physics2012), navigation (NavMesh), and cloth modeling. Breath of the Wild does not use Nintendo's KCL library.

Architecture

Just like any other Switch game, executable code is stored in the ExeFS – with the game code in the main executable – and content files or assets are stored in the RomFS. The latter is referred to as the game ROM.

Code

BotW is written in C++ with almost no hand-written assembly code and with no usage of the C++ standard library. Standard structures such as strings and associative maps are implemented in the sead library. Standard RTTI is not used; instead the game relies on Nintendo's own implementation of RTTI in sead, which does not leak symbols at all.

The codebase appears to be compiled with high levels of optimisation enabled for both Wii U and Switch releases, with code on the latter platform being better optimised. Since 1.6.0 (Switch only), the game is additionally built with LTO enabled, which results in massive amounts of code bloat. All known public releases are stripped, i.e. don't contain any debugging symbol.

The game is divided into several components and makes heavy use of managers, which are singletons that are usually responsible for one specific task (e.g. PlacementMgr for actor placement, EventMgr for in-game events, etc.). Unlike older Zelda games, most managers are dynamically allocated, not stored in BSS. In general, Breath of the Wild makes use of the heap and virtual functions a lot more.

In total, there are 130+ subsystems.

Concepts

While Breath of the Wild shares core concepts such as actors, scenes and flags with most (all?) Zelda games since Ocarina of Time, most of them are implemented in radically different ways. New concepts have also been introduced.

Actors

Actors are in-game objects. Link, enemies, scenery, weapons, NPCs, shrine interiors are all examples of actors.

In Breath of the Wild, actors are implemented through a combination of code and data such as parameter files (ActorParam), models, physics data, etc. Unlike actors in older Zelda games, BotW actors are composed of reusable components and large parts of functionality can be configured via parameter files.

File formats

AAMP and BYML are extremely common file formats, as they are most notably used for actor parameter (ActorParam) files, map units and more generally configuration files. They can be found in the ROM with various file extensions.

SARC (along with the Yaz0 compression format) is also omnipresent as it is used for all data archives which BotW heavily relies on. SARC archives have several file extensions.

Large numbers of BFEVFL (binary event flows) files are stored in Content/EventFlow.

Other commonly seen file formats include BFRES (models), BFSTM (sound streams), BFLYT (layouts) and various other standard NintendoWare formats.