This is a demo for the Nintendo DS framebuffer which uses the libnds library.

framebuffer_demo.c

//---------------------------------------------------------------------------------
// Framebuffer demo for Nintendo DS, using the libnds library
//---------------------------------------------------------------------------------

#include <nds.h>

void framebufferDemoDrawPattern() {
    int color = 0;
    for (int pixelPosition = 0; pixelPosition < SCREEN_HEIGHT*SCREEN_WIDTH; pixelPosition++) {
        VRAM_A[pixelPosition] = RGB15(color, color, color);
        
        if (color < 32)
            color++;
        else
            color = 0;
    }
}

void framebufferDemoDrawRectangle(int color, int x, int y, int size_x, int size_y) {
    for (int pos_y = y; pos_y < SCREEN_HEIGHT && pos_y < y+size_y; pos_y++) {
        for (int pos_x = x; pos_x < SCREEN_WIDTH && pos_x < x+size_x; pos_x++) {
            VRAM_A[SCREEN_WIDTH*pos_y+pos_x] = color;
        }
    }
}

void framebufferDemoDrawRectangleHollow(int color, int x, int y, int size_x, int size_y) {
    for (int offset = 0; offset+y < SCREEN_HEIGHT && offset <= size_y; offset++) {
        VRAM_A[(y+offset)*SCREEN_WIDTH+x] = color;
        VRAM_A[(y+offset)*SCREEN_WIDTH+x+size_x] = color;
    }
    
    for (int offset = 0; offset+x < SCREEN_WIDTH && offset <= size_x; offset++) {
        VRAM_A[y*SCREEN_WIDTH+x+offset] = color;
        VRAM_A[(y+size_y)*SCREEN_WIDTH+x+offset] = color;
    }
}

u32 color, x, y, rect_size;
u32 old_x, old_y, old_size;

void framebufferDemoVblankHandler() {
    framebufferDemoDrawRectangleHollow(RGB15(0, 0, 0), old_x, old_y, old_size, old_size);
    framebufferDemoDrawRectangle(RGB15(0, 0, 0), old_x+1, old_y+1, old_size-1, old_size-1);
    framebufferDemoDrawRectangleHollow(color, x, y, rect_size, rect_size);
    framebufferDemoDrawRectangle(RGB15(31,31,31), x+1, y+1, rect_size-1, rect_size-1);
}

void framebufferDemo() {
    powerOn(POWER_ALL);
    vramSetBankA(VRAM_A_LCD);
    videoSetMode(MODE_FB0);
    irqSet(IRQ_VBLANK, framebufferDemoVblankHandler);
    lcdSwap();
    
    touchPosition touch_pos;
    rect_size = 60;
    
    while(1) {
        old_x = x;
        old_y = y;
        old_size = rect_size;
        
        color = RGB15(0, 0, 31);
        
        scanKeys();
        
        if (keysCurrent() & KEY_RIGHT)
            x++;
        if (keysCurrent() & KEY_DOWN)
            y++;
        if (keysCurrent() & KEY_LEFT)
            x--;
        if (keysCurrent() & KEY_UP)
            y--;
        
        if (keysCurrent() & KEY_L) {
            rect_size -= 2;
            x++;
            y++;
        }
        
        if (keysCurrent() & KEY_R) {
            rect_size += 2;
            x--;
            y--;
        }
        
        if (keysCurrent() & KEY_TOUCH) {
            touchRead(&touch_pos);
            x = touch_pos.px - rect_size/2;
            y = touch_pos.py - rect_size/2;
        }
        
        swiWaitForVBlank();
    }
}

int main(void) {
    framebufferDemo();
    return 0;
}