The use of Video and Screens in combination with memory pointers like BG_BMP_RAM(0) , BG_MAP_BASE(0) and vramSetBankA is often unclear when starting with the nds.
I made an example which illustrates the use of the memorybanks and the vramSetBank setting, touching with the stylus on the lower screen.
You can use the HelloWorld example Makefile.
You can experiment commenting out vramSetBankC(VRAM_C_MAIN_BG_0x06040000);
and see what happens.
The prog writes the memory for a screen in one color and then a screen in the next color and a third part of memory in a third color. We steer the “viewing” screen using the touch.px, we “see” the colored memory pass the real screen.
the code in a textfile for a single screen steered by the stylus
links for vram video and memory: