OSDev.org

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void scroll(size_t lines) {
    if(!lines) return; /* nothing to scroll */
 
    /* copy memory from "X position = top+lines" to top */
    memcpy(video_mem,
           video_mem+((SCREENCOLS*lines)<<1),
           ((SCREENROWS-lines)*SCREENCOLS)<<1);
 
    /* Then zero the other lines below the copied lines, which
       is the same amount as the number lines that we've scrolled. */
    memsetw(video_mem+(((SCREENROWS-lines)
                        *SCREENCOLS)<<1), /* multiply by 2 */
            ((k_text_color_get() & 0x70)<<8) /* fill with bgcolor */,
            ((SCREENCOLS*lines)) /* amount of lines to copy */);
 
    /* If we're not yet at the top of the screen,
     * move the "Y position" up by X lines. */
    if(screen_ypos >= lines) screen_ypos -= lines;
}

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k_text_color_get()

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((k_text_color_get() & 0x70)<<8)