DLDI in ARM7
DLDI (Dynamically Linked Disc Interface) is an interface designed for homebrew projects to be able to access the memory of flashcarts (usually micro SD cards) in a standardized way. https://www.chishm.com/DLDI/
In short, when you build an application, the DLDI stub is added to your application. Then you and your users have to patch your application with the DLDI driver your flashcarts. Most flashcarts patch homebrew NDS ROMs, so normally you don’t need to worry about it, it just works. Some old flashcarts (specially slot-2 flashcarts) don’t support this, so you have to patch ROMs yourself. DeSmuME doesn’t autopatch ROMS, for example, while melonDS does.
This is an archive of DLDI drivers: https://github.com/DS-Homebrew/DLDI
Reading files with DLDI is very slow because reading from SD cards is very slow. It’s not limited to DLDI. Regular cartridge read commands (like the ones used by offical games) are also slow. Traditionally this process happens in the ARM9.
This means that, if you have a game that has a 3D open world, and you want to move around the world while loading and unloading models and textures from the filesystem you can’t do it in the same CPU. The ARM9 is too busy with the game logic, it can’t just stop until it has finished reading a file.
The fix is to use the ARM7 to do the reads while the ARM9 is doing other things. Note that, at the time of writing this document, while DLDI in ARM7 is supported, there is no way to do asynchronous reads yet. For this, a cooperative multithreading scheduler is needed. This has a high priority in the to-do list of this SDK.
The main drawback of this approach is that not all flashcarts support this. Some DLDI drivers have been built for the ARM9 and will crash in the ARM7, for example. This is why DLDI in ARM7 isn’t enforced. The current code supports using DLDI from both ARM9 and ARM7 without rebuilding the application.
All you need to do is to make sure that your code is built for the ARM7 instead
of the ARM9. Use -mcpu=arm7tdmi instead of -mcpu=arm946e-s+nofp.
Then add the following flag to the features entry of your DLDI header:
#define FEATURE_ARM7_CAPABLE 0x00000100
BlocksDS’s fork of libnds will detect this flag and switch to using DLDI from the ARM7.
If your application doesn’t care about which CPU uses DLDI, you don’t have to do
anything. Any driver marked with FEATURE_ARM7_CAPABLE will run from the
ARM7, and any driver without the mark will run from the ARM9. If you want to
force a certain CPU to do it, you can do it this way:
#include <nds/arm9/dldi.h>
// Set the DLDI mode
dldiSetMode(DLDI_MODE_ARM9); // or DLDI_MODE_AUTODETECT or DLDI_MODE_ARM7
// Get the current mode. This is set to DLDI_MODE_ARM9 or DLDI_MODE_ARM7
// after initializing the FAT system.
DLDI_MODE dldi_mode = dldiGetMode(void);
This is a list of flashcarts that have been reported to work. Please, test your flashcart using this test and report your findings.
| flashcart | ARM9 | ARM7 | Notes |
|---|---|---|---|
| Acekard 2i | Yes | Yes | |
| SuperCard DSTWO | Yes | Yes | |
| Original R4 | Yes | Yes |