Is there a way to make this work through 4swordshax or sudokuhax? Or a flashcart updater homebrew? (Similar to the NTRBoothax installer?)
TWL_FIRM MUST be sig patched before this will boot! The new patches to TWL were recently added to rxTools and should make it's way into a nightly build soon. The sig patches added also patch out DS Cart White List and save chip detection. So if your DS Flashcart already works thanks to that patch, then you don't need this. rxTools isn't specifically needed. If you happen to have a prepatched TWL_FIRM installed, that will work too. Note that rxTools does not yet support patching TWL_FIRM on n3DS. The prepatched TWL_FIRM cia is your only option (for now) on the n3DS.
Many older DS flash carts get blocked by CTR mode (most likely by Process9 of NATIVE_FIRM). You can tell if you see it error almost instantly when trying to boot your flashcart. This app gets around that! Any flashcart that hasn't worked since 1.0 launch firmware can be used with this launcher. Note that some flashcarts may work without this launcher with just the TWL_FIRM patches alone. Test that first before using this launcher. If your card works with just the patches, do not report that as that no longer pertains to my launcher. This launcher is intended for DS flashcarts that get blocked by CTR mode.
You can tell if yours is blocked by CTR mode by attempting to boot it from home menu. If you see the "Error has occured" screen pop up almost immediately without the screens flickering on and off, then it is a card blocked by CTR mode. That's what this launcher is for!
This should work on n3DS too once sig patched TWL_FIRM is available for it.
To use this, simply insert your previously blocked DS flashcart into the game card slot, then boot this app.
That is all you have to do!
Older flashcarts that don't have valid icon/banner data still will not work. Notable examples are the original R4 and older hardware revisions of the original Action Replay for DS.
Anything that behaves like a normal cartridge should be bootable with this method.
How this DSi app works is simple. It's a stripped down DSi Extended header with dummy data for the arm7/arm9 sections (they aren't needed as it launches slot-1 before ever getting to load them). Inserted a custom banner and set the file type at offset 0x234 to 00. This tells TWL_FIRM that it's a cartridge. (even though it's not. ).
This tricks it into booting older flashcarts because the 3DS boots into TWL_FIRM expecting to load this this DSiWare SRL. But because the file type in the TID field is set to cartridge, it immediately boots what ever is in slot-1. Neat little oversight thanks to Nintendo.
Download link: TWLSlot1Launcher_v3.rar
Update: Download link updated. Minor changes to header and brought SRL closer to proper SRL spec.
Confirmed list of carts working with this:
R4i SDHC Dual Core
Supercard DS One v3 SDHC
If you get an error on boot, double check that you have TWL_FIRM patched either via preinstalled CIA or via rxTools (or a future CFW that add the new patches) before reporting that your flashcard doesn't work with this. Thanks!
Credit goes to TuxSH and Steveice10 for finding the sig patches for TWL_FIRM! Also credit to TheStoneBanana for accidently discovering this odd little quirk of TWL FIRM.