Resource system

In Breath of the Wild, the resource system is a subsystem that is responsible for managing resources, i.e. content files. It is composed of ResourceMgrTask, ResourceSystem and a few ancillary classes (such as "resource binders" and "handles").

BotW's resource system is also used in Animal Crossing: New Horizons and called "ares" or "ARes".

Concepts

Resource

Resources (in the context of the codebase) are C++ classes that are responsible for parsing raw data from content files and storing them in a more convenient data structure.

In general, there is one resource class per file type, not format. As an example, bgparamlist and bxml do not share the same class even though both are AAMP files.

In some cases, the resource is nothing more than a thin wrapper over the underlying file bytes. This is the case for BYMLs and other simple resource types that don't require any additional memory allocation. Such a wrapper is (unofficially) called ResourceBase.

Resource factory

Factories are C++ classes that create resource objects.

During application init, factory instances are created and registered with the resource system. When a resource is loaded, the system will look up the corresponding factory based on the file extension, load the entire file into memory, and pass the data to the factory, which then returns a Resource object on which parse() must be called.

List of resource factories

A complete list of resource factories can be found in the rstb project.

These values were extracted from the Switch and Wii U 1.5.0 executables.

Note that:

For ResourceLoadArg3 (used in model/bfres related code), the factory is hardcoded to be the ResourceBase factory.
For ResourceLoadArg2 (used for actor resources and physics stuff) and ResourceLoadArg (everything else), the factory is determined from the file extension.
Any file for which there isn't any specific factory will use the ResourceBase factory.

Special cases

Compressed files

Compressed files can be automatically decompressed and loaded if their extension starts with an 's'. The 's' prefix indicates that a file is Yaz0 compressed.

When a resource load is requested, the resource system will in most cases automatically prepend 's' to the extension and use sead::ResourceMgr::tryLoadWithDecomp to try loading a compressed version first. This means that any code that interacts with the resource loading functions must not include the prefix in resource paths.

Exceptions: bfevfl, bcamanim and barslist are always loaded uncompressed^[1].

Warning: This behaviour can be overridden by passing some flags to the resource load functions.

Add-on content

In order to load add-on content, the file device must be set to the AoC file device when calling a resource loading function.

This can be done by calling various aocManager functions that return the AoC file device if the resource path matches specific AoC path patterns and assigning the result to field 0x48 in the ResourceLoadArg class.

If no file device is explicitly set, the resource system will use the default file device (romfs on Switch or content directory on Wii U).

If the file device is set to the AoC file device, Aoc/0010/ will be prepended to the canonical resource path. This is done to avoid conflicts between the base and AoC versions of a resource. Warning: the AoC file device is not always set when loading resources from AoC archives. This is the case for the model and ActorParam code. Conversely, the AoC device is sometimes set even when it is unnecessary.

Cases where the AoC file device is used

If the AoC version is >= 0x200 (in most cases) or >= 0x300 (for DLC Pack 2 content), and if the canonical resource name (without the Aoc/0010/ prefix) matches any of the below patterns:

Terrain/A/AocField*
Any dungeon pack (Pack/%s.pack): shrines, divine beasts, Final Trial (for shrines, the dungeon number must be > 119)
UI/StaffRollDLC/*
Map/MainField/* (if Pack/AocMainField.pack was loaded successfully)
Map/MainFieldDungeon/*
Map/AocField/*
Map/CDungeon/* (for DungeonNNN with NNN > 119)
Physics/StaticCompound/AocField/*
Physics/StaticCompound/MainFieldDungeon/*
Physics/StaticCompound/CDungeon/* (for DungeonNNN with NNN > 119)
Movie/Demo6*
Game/AocField/*
NavMesh/AocField/*
NavMesh/MainFieldDungeon/*
NavMesh/CDungeon/* (for DungeonNNN with NNN > 119)
Physics/TeraMeshRigidBody/AocField/*
Voice/*/Stream_Demo6*/*.bfstm
System/AocVersion.txt

There are two other situations where the Aoc/0010/ prefix is supposed to be prepended:

If the load file device is set to the Pack/AocMainField.pack archive file device explicitly
If the global resource pack is an AoC dungeon pack

However, the first case is irrelevant because that archive only contains Map/MainField/ (a case that is already handled by the path checks) and the second check appears to never pass.

Failed resource load

If a resource loading failure is detected by the resource system, it will try again up to 30 times.^[3]

Memory allocation

Heap size

The size of the resource loading heap the system allocates every time a resource is loaded depends on the value that is listed in the #Resource size table (RSTB). If lookup fails, the game will fall back to the following formula (Switch on 1.5.0):

alignedFileSize = (actualFileSize + 31) & -32;
factory->getResourceSize()
+ factory->constant
+ factory->getLoadDataAlignment()
+ (signed int)(float)(factory->sizeMultiplier * alignedFileSize)
+ (factory->sizeMultiplier * alignedFileSize >= 0.0 &&
   (float)(signed int)(float)(factory->sizeMultiplier * alignedFileSize) != (float)(sizeMultiplier * alignedFileSize))
+ 0x750

Warning: Failure to add entries to the RSTB for a large number of resources may result in system instability. The purpose of this fallback appears to be to allow loading resources during development even without an entry in the resource size table. As such, the system will often allocate way more memory than needed to account for any dynamic allocation that the resource class may do.

Parent heap

If the heap size is smaller than 0x80000 bytes, the heap is created with the "arena for resource S" (small) heap as parent.

Otherwise, the parent heap is the "arena for resource L" (large).

Resource size table

The Resource Size Table (RSTB) contains a list of canonical resource names and their sizes. It is loaded from ResourceSizeTable.product.rsizetable and used to determine how much memory should be allocated when loading a resource.

Resource size calculation

According to an internal source, Nintendo automatically generates the Resource Size Table using development builds of the game that are believed to contain instrumentation code (perhaps in the Patrol component) This is consistent with the fact that the RSTB asset has the "product" suffix in its name, which is used for machine-generated files, and also with the fact that a similar solution is used for actor instance heap size measurement.

The resource loading heap is, for most factories, only used to allocate the file loading buffer (which is as large as the file to load), the C++ resource class, extra allocations depending on the resource class and some extra bytes to ensure data is aligned correctly in memory.

Therefore, the following formula should give a correct size value for all resource types:

(size rounded up to multiple of 32) + CONSTANT + sizeof(ResourceClass) + PARSE_SIZE

CONSTANT is 0x168 in the Switch version and 0xe4 on Wii U.

PARSE_SIZE is the amount of memory allocated from the resource heap in the Resource::parse function. Determining the exact value of PARSE_SIZE requires reversing that function and tracking calls to operator new() because the amount of dynamically allocated memory depends on the resource.

↑ See 0x710120A9F4 in Switch 1.5.0
↑ 0x71012087EC
↑ 0x710121087C in Switch 1.5.0 (ResourceBinder::retryLoading -- member function name is unofficial)

[1] See 0x710120A9F4 in Switch 1.5.0

[2] 0x71012087EC

[3] 0x710121087C in Switch 1.5.0 (ResourceBinder::retryLoading -- member function name is unofficial)

[1]

[2]

[3]