Graphics updates



19 : VRAM Buffer - Write Flag (if != 0, then write VRAM Buffer to VRAM)



1A : VRAM Buffer - Write Flag (Vertical NameTable Updates) (if != 0, then write VRAM Buffer to VRAM, NameTable writes are done in Vertical direction. )



58-59 : Graphics being loaded - Dest. VRAM Address Pointer



9A : VRAM update suspend flag. Used to synchronize sprite DMA updates and VRAM updates.



780-7FE : VRAM Buffer.

  (Used to write to Name Table VRAM, either when

   screen is off or in NMI when screen is on.)

  Format:

    The buffer is divided in chunks of strings.

    If the first byte in the chunk is $80 or more (usually $FF),

    it means to stop reading chunks from the buffer.

    Format of string chunk:

      byte 0: PPU VRAM Address High (if >= $80, stop reading buffer)

      byte 1: PPU VRAM Address Low

      byte 2: Number of Letters/Bytes to write (minus 1; if

        the value is 0, it means to write 1 tile, if value is 1,

        it means to write 2 tiles, and so on..)

      From byte 3 follows the variable-length string of bytes to write

      in sequence, (length is the value of [Byte 2] + 1)

      This is the end of the string chunk.

  Format, explained by interpretion in C code style: {

    unsigned char a, x, y;

    unsigned char vram_hi, vram_lo;

    x = 0;

   loop:

    a = *(0x780 + x);

    if ((a & 0x80) == 0x80) { return (0); }

    vram_hi = a;

    a = *(0x781 + x);

    vram_lo = a;

    y = *(0x782 + x);

    for ( ; y >= 0; y--) {

      a = *(0x783 + x);

      NES_PPU_VRAM[(vram_hi << 8) | vram_lo] = a;

      x++;

    }

    x += 3;

    goto loop;

  }