Migrating from devkitARM

In most cases, porting devkitARM projects to BlocksDS should be relatively easy. BlocksDS includes most of the NDS functionality provided by devkitARM. For simple projects, only minor changes should be required.

This is the biggest difference between devkitARM and BlocksDS. Makefiles provided by devkitARM are more complicated: they call themselves recursively from the build directory. They also hide a lot of compilation rules from the user, as they are provided by sub-makefiles in the devkitARM system directory.

The Makefiles of BlocksDS include all available rules so that its behaviour is easier to understand and customize. This allows for example creating a new build system based on them, tailored to your project (for example, with CMake or Meson). They also do not rely on self-recursion.

As a BlocksDS user, you need to edit a few paths and variables, same as with devkitARM. Open the Makefile of your devkitARM project and check this part (some variables may be missing if you’re not using them):

TARGET      := $(notdir $(CURDIR))   # Name of the resulting NDS file
SOURCES     := source source/common  # Directories with files to compile
INCLUDES    := include               # Directories with files to #include
GRAPHICS    := graphics              # Folder with images and .grit files
MUSIC       := audio                 # Folder with audio files for maxmod
DATA        := data                  # Folder with .bin files
NITRODATA   := nitrofs               # Root of your NitroFS filesystem

Copy the Makefiles from the rom_arm9_only or rom_combined to your project, and open it. You have to copy the values to the following part, and leave them empty if you aren’t using them:

SOURCEDIRS  := source
INCLUDEDIRS := include
GFXDIRS     := graphics
BINDIRS     := data
AUDIODIRS   := audio
NITROFSDIR  := nitrofs

Important notes:

• SOURCEDIRS searches all directories recursively. If you don’t like this behaviour, go to the SOURCES_S, SOURCES_C and SOURCES_CPP lines and add -maxdepth 1 to the find command.
• Paths to directories that are outside of the root of the project is not supported.

Note that TARGET is not part of this group. The top of the Makefile has this other group of variables that you can also set to your own values:

NAME          := template_arm9     # Name of the resulting NDS file

# Banner and icon information
GAME_TITLE    := Combined ARM7+ARM9 template
GAME_SUBTITLE := Built with BlocksDS
GAME_AUTHOR   := github.com/blocksds/sdk
GAME_ICON     := icon.bmp

Once this has been adapted to your desired values, you will need to link with the libraries used by your program.

This is how it looks like in a devkitARM project:

LIBS := -ldswifi9 -lmm9 -lnds9 -lfat -lfilesystem

LIBDIRS := $(LIBNDS)

This would be the equivalent in a BlocksDS project:

LIBS    := -ldswifi9 -lmm9 -lnds9
LIBDIRS := $(BLOCKSDS)/libs/dswifi \
           $(BLOCKSDS)/libs/maxmod \
           $(BLOCKSDS)/libs/libnds

You can remove the DSWiFi or Maxmod libraries if you aren’t using them.

The reason for this additional complexity with LIBS and LIBDIRS is to allow the user as much flexibility as possible when mixing and matching libraries.

devkitARM and BlocksDS also heavily differ with regards to the structure of their filesystem access libraries. For most users, this should not lead to major code chagnes.

devkitARM uses libfat and libfilesystem, connected to a modified version of its C library newlib through the devoptab interface. These are devkitPro’s additions, and are not replicated in BlocksDS.

Instead, BlocksDS uses the picolibc C library’s “tiny” stdio implementation. It also uses a modified version of Elm’s FatFS library in place of libfat, as well as a custom implementation of NitroFS.

To adapt, you have to remove -lfat and -lfilesystem from LIBS in your Makefile. As all filesystem support is included in libnds, no additional libraries are required.

From the point of view of the source code, you can use the same includes as when using libfat and libfilesystem:

#include <fat.h>
#include <filesystem.h>

Beyond the limitations listed below, filesystem support should work identically. Please report any behaviour that isn’t the same. If any other functionality your homebrew program requires is missing, please report that as well.

Some minor implementation differences exist between libfilesystem and BlocksDS’s implementation:

• In BlocksDS, NitroFS files can only be opened for reading. This means that, for example, fopen("nitro:/file.dat", "rb+"); will always fail, as the rb+ mode allows writing.
• While the behaviour of nitroFSInit(NULL); is identical, for non-NULL arguments, the provided value is now treated as an input path to the .nds file, as opposed to an output base path. To retrieve the base path, it is required to use the fatGetDefaultCwd() function instead.

readdir() returns a struct dirent pointer with the field d_type. This field can be used to determine if an entry is a directory or a file. I’ve seen that some programs use it like this:

struct dirent *cur = readdir(dirp);
if (cur->d_type & DT_DIR)
    printf("This is a directory\n");
else if (cur->d_type & DT_REG)
    printf("This is a file\n");

However, this is incorrect. The right way to compare it is by checking if the value is equal:

struct dirent *cur = readdir(dirp);
if (cur->d_type == DT_DIR)
    printf("This is a directory\n");
else if (cur->d_type == DT_REG)
    printf("This is a file\n");

• To cut down on RAM, code size and complexity, BlocksDS omits the devoptab interface. As homebrew does not generally need to modify the device list, this should not affect them.

The newlib C library provides faster and smaller integer versions of stdio.h functions, such as iprintf() or siscanf(). These are not provided by picolibc. Replace any calls to them by the standard names of the functions: printf(), sscanf(), etc.

In newlib, including both iprintf() and printf() in the same codebase led to including both versions of the relevant stdio.h functions, unnecessarily increasing code size. picolibc opts for a different approach - it allows you to decide, project-wide, if you want to use integer-only or float-compatible versions of the printf() and scanf() functions. Note that by default, for compatibility reasons, the float-compatible versions are provided.

picolibc’s printf/scanf replacement functionality is documented further in the optimization guide.

scanKeys() updates the internal state of the key handling code. This is then used by keysHeld() and keysDown().

However, BlocksDS also requires the user to call scanKeys() before any of the following functions: touchRead(), keyboardGetChar(), keyboardUpdate() and the deprecated touchReadXY().

This is unlikely to be a problem in most projects, as the normal thing to do is to both scan the keys and read the touchscreen status, not just read the touchscreen.

The reason is that the functions scanKeys() and touchRead() aren’t synchronized in any way, which creates a race condition. Consider this code:

while (1)
{
    scanKeys();
    if (keysHeld() & KEY_TOUCH)
    {
        touchPosition touchPos;
        touchRead(&touchPos);
        printf("%d, %d\n", touchPos.px, touchPos.py);
    }
    swiWaitForVBlank();
}

The state of the X and Y buttons, as well as the state of the touch screen, is passed from the ARM7 with a FIFO message. This message can technically happen in between scanKeys() and touchRead(), which means that it’s possible that keysHeld() & KEY_TOUCH is true, but the coordinates read by touchRead() are (0, 0) because the user has stopped pressing the screen right at that point, and scanKeys() read the outdated values while touchRead() read the updated values.

In BlocksDS, scanKeys() is used to latch the current state of the keys and the touch screen. This forces the developer to call scanKeys(), but it also ensures that there are no race conditions, as scanKeys() will read all the state atomically.

Occasionally, various components of the devkitARM toolchain introduced breaking changes of their own, which lead to the unfortunate situation of homebrew being stuck on legacy, buggier and less reliable versions of toolchains. These issues also need to be resolved before updating to BlocksDS. Some known issues and their remedies are documented here; note that this list is not exhaustive.

In libnds, while the ARM9 initializes console hardware in a separate function called before main(), the ARM7 does so as part of its code. Over time, these initializations have been modified. Not adhering to them can cause issues - for example, a missing touchInit(); can lead to the touch screen not functioning as expected.

When updating old homebrew with custom ARM7 binaries, it is recommended to study the latest version of the default ARM7 binary to apply any necessary changes.

Many old libnds-based homebrew extend the TransferRegion memory area to facilitate communication between the ARM9 and ARM7 CPUs. With the introducion of the FIFO message queue system, this approach is discouraged for message/command passing. For sharing buffers, one can use the FIFO system to send pointers to main RAM.

Old versions of libnds used alternate name defines for the DS console’s memory mapped registers. They are primarily distinguished by not having the REG_ prefix; for example, SUB_BLEND_CR has been replaced by REG_BLDCNT_SUB in newer versions.

A translation table is provided in #include <nds/registers_alt.h>; this can be used as a stopgap to compile the project and update all uses of legacy register names.

For ARM/Thumb interwork (calling ARM functions from Thumb and vice versa) to work correctly in more recent versions of the GNU toolchain, functions must be appropriately marked in the generated object file. While they are annotated automatically for code generated by GCC, it has to be done manually for assembly code. For example, a snippet of assembly as follows:

   .global myFunction
myFunction:
   // code

should be updated to read as follows:

   .global myFunction
   .type myFunction, %function
myFunction:
   // code

The important part is the addition of the .type annotation. Alternatively, the BEGIN_ASM_FUNC macro may be used:

#include <nds/asminc.h>

BEGIN_ASM_FUNC myFunction
   // code

Hardcoding areas at the end of RAM at 0x27FFFFF is not compatible with the DSi, as it has more than 8 MB of RAM. Using 0x2FFFFFF works on both DS and DSi consoles. Likewise, operations like using | 0x400000 or | 0x800000 to mark uncached variants of cached addresses should be replaced with the memUncached() helper or by using cached pointers alongside cache flushing/invalidation functions to ensure the ARM7 can see the changes; these transformations are different in DS and DSi modes.

Note that, as all accesses omit the ARM9 cache on the ARM7 CPU, it is recommended to send pointers to it which have not been processed by memUncached().

• irqInit() is now called on the ARM9 CPU before main(). As the FIFO system registers its own IRQ handlers, and irqInit() clears them, extraneous calls to it should be removed.
• DIR_ITER*, diropen, dirnext and dirclose are non-standard libfat extensions; they should be replaced by DIR*, opendir, readdir and closedir, respectively.
• GCC 10 and above default to -fno-common, which can cause issues if multiple global variables with the same name are defined in the same project without using extern. More information is available here.
• GCC 14 and above consider as errors some situations which were previously treated as warnings, such as implicit function definitions. More information is available here.