**************
* Memory Map *
**************

$0000 - Cartridge ROM (32 KB)

$8000 - Character Data (4 KB)
$9000 - BG0 Data (2 KB)
$9800 - BG1 Data (2 KB)

$A000 - Working RAM (16 KB)

$E000 - Persistent RAM (256 B)

$FE00 - Sprite Registers (256 B)
$FF00 - Color Registers (32 B)
$FF20 - Video Registers
$FF40 - Audio Registers
$FF60 - I/O Registers


******************
* Character Data *
******************

A character is an 8x8-pixel image with 2 bits per pixel, with a
resulting size of 16 bytes. The video RAM has space for 256
characters.
The first 8 bytes of a character contain the low bits of all its
pixels, followed by 8 more bytes containing the high bits of all
pixels.


***********
* BG Data *
***********

A background is a map of 32x32 character cells. Each cell occupies two
bytes:
- Character number
- Attributes:
    Bit     Purpose
    0-2     Palette number
    3       Flip X
    4       Flip Y
    5       Priority
    6-7     Unused


********************
* Sprite Registers *
********************

There are 64 sprites available, each occupies 4 bytes:
- X position
- Y position
- Character number
- Attributes:
    Bit     Purpose
    0-2     Palette number
    3       Flip X
    4       Flip Y
    5       Priority
    6-7     Size:
            0: 1 character (8x8 pixels)
            1: 2x2 characters (16x16 pixels)
            2: 3x3 characters (24x24 pixels)
            3: 4x4 characters (32x32 pixels)


*******************
* Color Registers *
*******************

There are 8 palettes of each 4 colors. One color is one byte:
Bits    Component
0-1     Blue
2-3     Green
4-5     Red


*******************
* Video Registers *
*******************

$FF20 - Attributes:
    Bit     Purpose
    0       Sprites enabled
    1       BG0 enabled
    2       BG1 enabled

$FF21 - BG0 scroll offset X
$FF22 - BG0 scroll offset Y
$FF23 - BG1 scroll offset X
$FF24 - BG1 scroll offset Y
$FF25 - Raster line


*******************
* Audio Registers *
*******************

- Not yet defined -


*****************
* I/O Registers *
*****************

$FF60 - Gamepad 0 status
$FF61 - Gamepad 1 status

Gamepad status:
    Bit     Purpose
    0       Up
    1       Down
    2       Left
    3       Right
    4       Button A
    5       Button B

$FF62 - Last touch X position
$FF63 - Last touch Y position
$FF64 - Last pressed key (ASCII code)
$FF65 - Status:
    Bit     Purpose
    0       Pause button
    1       Touch