Video (TG-16)

From Data Crystal
Jump to: navigation, search

[code]

*****************************************************************
*      PC-Engine Video Display Controller Documentation         *
*      .                                              .         *
*   ---+----------------------------------------------+---      *
*      |   MOST COMPLETE HuC6270 INTERNAL WORKINGS    |         *
*      |    DOCUMENT. IF YOU HAPPEN TO FIND *ANY*     |         *
*      |   WRONG INFORMATION, PLEASE CONTACT ME VIA   |         *
*      |  EMAIL AS SOON AS POSSIBLE SO I CAN FIX IT.  |         *
*   ---+----------------------------------------------+---      *
*      :                                              :         *
*                                                               *
*      document revision 0.3 (3rd release)                      *
*                                                               *
*      written by Emanuel Schleussinger in Feb 1998             *
*                 ( eschleus@luva.lb.bawue.de )                 *
*      Thanks to:                                               *
*          DAVID MICHEL for LOTS of information!!!!!!!!!! ;)    *
*          JENS CHR. RESTEMEIER for his EXCELLENT PCE-docu      *
*          DAVE SHADOFF for his emails and his TGSim source     *
*          NIMAI MALLE for his VDC explanations                 *
*          VIDEOMAN for his excellent Hacking Web-Page          *
*                   and some documents in there                 *
*          PAUL CLIFFORD for an excellent HuC6270 register docu *
*****************************************************************
Revision reference:
-----+-----------------------------+---------------+----------
| rev 0.3:
|   - improved the VDC register table A LOT thanks to the
|     help of PAUL CLIFFORD. Thx for that cool doc, dude!
|       (all those nasty 'unknown's are now eliminated)
|   - more examples here and there.
|   - fixed some docu bugs with help of David Michel.
|   - Added Video Color Encoder reference.
|   - Sprite storage description was WRONG, corrected now.
|
| rev 0.2:
|   - Added Sprite information.
|   - Fixed some major bugs in the docu.
|   - Registers updated.
|
| rev 0.1 (initial release):
|   - Still missing sprite docu, lots of undocumented registers.
-----+-----------------------------+---------------+----------

[/code]

Preface

[code]

  This document has been created for both beginners and advanced
  programmers. There may be some information that you may well
  consider 'unnecessary' (such as the introduction to planar image
  storage), but please think of people who would really like to
  program the PC-Engine, but dont have a clue on how some basic
  techniques (like planar) work.
  This document is in very early state and may well contain
  a lot of information not being correct. For any wrong in-
  formation in this document you may discover, please write
  me a mail at eschleus@luva.lb.bawue.de so I can fix it and
  release a new version.
  The latest version of this document can always be obtained
  at my homepage located at:
         www.classicgaming.com/aec/
  or just write me an email and ask me to send you the latest
  revision.
  Any help on improving this document is highly appreciated!
  I think its the most complete one out there at this time.
  Yours,
  Manuel
  eschleus@luva.lb.bawue.de
  www.classicgaming.com/aec/

[/code]

Purpose and operation of the VDC

[code]

 The VDC (Video Display Processor), also known as the HuC6270,
 is the main graphics processing unit in the PC-Engine. Despite
 the CPU of the PC-Engine is only 8-bit, the VDC is a full 16-bit
 processor with very powerful capabilities. Its accessible from
 the main system via 3 special opcodes that write/read data from/
 into the Video Display. The VDC is connected to another chip known
 as the HuC6260 VCE (Video Color Encoder), which supplies the color
 palette data for the Video System.
 The VDC in the PC-Engine has two modes of operation:
   1. Background character processing
   2. Sprite processing
 The 64 kB VRAM that the VDC is connected to does NOT contain one big
 bitmap with all the display information stored pixel by pixel like
 on a Amiga or PC, the Graphics are stored tile-based. In case you do
 not know what tile-based graphics are, be sure to read section 2
 very carefully.

[/code]

The VRAM structure / encoding VRAM data

[code]

 The VRAM of the PC-Engine is 64 kBytes in size. No chip other
 than the VDC can access it. It contains all the important data
 needed for the display generation.
 The way graphical data is organized in the VRAM is called 'tile
 based'. This means there is NOT a huge bitmap containing a color
 index for every pixel, but only a list of pointers to small,
 rectangular areas in the VRAM that will, aligned to each other,
 make up the display. Explanation follows.
 Think of it like this:
 We have a 512*256 pixel 256 color screen. On a PC, for instance,
 we would have to have the following VRAM structure:
        +---------------------------------------+
        |       <--512 pixels across -->        |
        |                                       |
        |                                       | 256
        |                                       | pixels
        |                                       | down
        |                                       |
        |                                       |
        |                                       |
        |                                       |
        |                                       |
        +---------------------------------------+
 The color depth is 8 bit ^= 256 colors
 This would result in        512 * 256 * 8
                           = 1.048.576 bit
                           = 130 kbytes (roughly)
 So, if the PC-Engine would do it the same way, it would not be
 able to have such high resolutions due to the lack of VRAM.
 Thats why data is stored in the VRAM as follows:
 The screen background area is made up out of 8*8 pixels large
 blocks, called the 'tiles', each tile having a color palette of
 16 colors. There are 16 different palettes to choose from for
 each tile, resulting in 256 different colors for the background
 generation (the other 256 colors are reserved for sprite usage
 which will be described later).
 In the background, colour 0 of all palettes are equal. Colour 0 of
 palette 0 determines colour 0 of all the background palettes. Even
 though these colour CAN be set independently, the screen will not
 reflect these settings.

Now, how are those tiles aligned to each other?
 Starting at the very beginning of the VRAM ($0) there is the so-
 called BAT (Block Attribute Table), which is a list of pointers
 to tiles stored in the Video RAM. The amount of pointers varies
 depending on how big the actual screen is. (As I told you, you
 have 8*8 pixel tiles, so if the screen is larger, theres more
 tiles). For our test screen (512*256), we would need:
                            512 / 8     = 64 tiles per line
                            256 / 8     = 32 tiles vertical
                             64 * 32    = 2048 tiles
 That means, we would be in need of a BAT 2048 words in lenght.

WHY WORDS? How does a BAT pointer to the VRAM look like?
 A Pointer to a tile in the VRAM must contain palette information
 as well as the actual VRAM address where to find the tile. This
 ONE WORD LONG index pointer looks like this:
     PPPPAAAAAAAAAAAA
      |       |
      |       |
      |       +------- 12 lower bits:   Index of the tile
      |
      +--------------- 4  upper bits:   Palette number (0-15)
 If you multiply the tile index by 32 (LSL #5 ;-), you will get
 the actual VRAM pointer address.
 The pointers in the BAT are ordered from the left to the right
 and top to down.
 ----->Small example:<-----
 Here is the first few words of data in the VRAM of HATRIS, just having
 the intro screen up. If you look closely, note how VRAM was saved using
 the same tiles over and over again in the BAT:



HOW CAN I SET THE SIZE OF THE BAT?
 Easy, theres a VDC register dedicated to it, called the MWR register.
 (find more about the MWR in SECTION 4)
   MWR register mask:
 xxxxxxxxxxHWWxxx (16 bits)
                 | |
                 | +--- width in tiles/pixels
                 |      00 = 32/256
                 |      01 = 64/512
                 |      10 = 128/1024
                 |      11 = 128/1024
                 |
                 +----- height in tiles/pixels
                        0  = 32/256
                        1  = 64/512
 If you understood everything, you should now be asking:
  "No TV can display a resolution of 1024 pixels across, so whats
   this mode for?"
 Answer: Check out the BXR and BYR registers used for SCROLLING ;)
         (see SECTION 4)



HOW DOES THE TILE ITSELF LOOK LIKE IN THE VRAM?
 Well, the tile itself is a piece of memory sized like this:
      8 * 8 * 4 bits = 256 bits
      |   |   |
      |   |   +------- color index (4 bits per pixel)
      |   +----------- height in pixels
      +--------------- width in pixels
 On this issue, David Michel posted me a VERY good explanation on
 how the data of a single tile is organized in the VRAM:
   The PC-Engine use a planar mode rather than the well known chunky
   mode of PCs, if you already have some experience decoding Atari ST
   or Amiga gfx, you should easily understand the following.
   In planar mode the 4 bits that form the color index are stored
   in 4 separate bytes, let's say that we want to extract the color
   index for the third pixel from the left :
     color index
      3rd pixel
   +---+---+---+---+           +---+---+---+---+---+---+---+---+
   | 3 | 2 | 1 | 0 |   byte 1  | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
   +---+---+---+---+           +---+---+---+---+---+---+---+---+
     |   |   |   |                       |
     |   |   |   +-----------------------+
     |   |   |
     |   |   |                 +---+---+---+---+---+---+---+---+
     |   |   |         byte 2  | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
     |   |   |                 +---+---+---+---+---+---+---+---+
     |   |   |                           |
     |   |   +---------------------------+
     |   |
     |   |                     +---+---+---+---+---+---+---+---+
     |   |             byte 3  | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
     |   |                     +---+---+---+---+---+---+---+---+
     |   |                               |
     |   +-------------------------------+
     |
     |                         +---+---+---+---+---+---+---+---+
     |                 byte 4  | 7 | 6 | 5 | 4 | 3 | 2 | 1 | 0 |
     |                         +---+---+---+---+---+---+---+---+
     |                                   |
     +-----------------------------------+
   It's as simple as that :)
   The funny part is that those 4 bytes are not placed in order,
   they are interleaved. Byte 1 & 2 are stored first, and bytes
   2 & 3 are stored 16 bytes after, here is another nice drawing:
 VRAM OFFSET
       on pointer
       (in bytes)
                  +---------------------+
             0    | byte 1 & 2 of line 1|
                  +---------------------+
             2    | byte 1 & 2 of line 2|
                  +---------------------+
             4    | byte 1 & 2 of line 3|
                  +---------------------+
             6    | byte 1 & 2 of line 4|
                  +---------------------+
             8    | byte 1 & 2 of line 5|
                  +---------------------+
            10    | byte 1 & 2 of line 6|
                  +---------------------+
            12    | byte 1 & 2 of line 7|
                  +---------------------+
            14    | byte 1 & 2 of line 8|
                  +---------------------+
                  +---------------------+
            16    | byte 3 & 4 of line 1|
                  +---------------------+
            18    | byte 3 & 4 of line 2|
                  +---------------------+
            20    | byte 3 & 4 of line 3|
                  +---------------------+
            22    | byte 3 & 4 of line 4|
                  +---------------------+
            24    | byte 3 & 4 of line 5|
                  +---------------------+
            26    | byte 3 & 4 of line 6|
                  +---------------------+
            28    | byte 3 & 4 of line 7|
                  +---------------------+
            30    | byte 3 & 4 of line 8|
                  +---------------------+
  I think everyone should have got that right now. Thx David!
  If you ask yourself what this was about, consider reading part
  2 again. Part 3 won't be better ;-)

3. Accessing the VDC from the CPU

[code] ---HOW CAN I TRANSFER DATA INTO THE VRAM?

 Well, there are three memory locations involved that can be read/
 written by the CPU to supply the VDC with data / read data from
 the VDC (all in the I/O Memory Segment $FF):
 Full address    Access Purpose
 $1FE000          R/W    VDC Register select
 $1FE002    R/W    Low Data register
 $1FE003          R/W    High Data register
   The first of the three locations here is the so-called REGISTER
   SELECT. The VDC has 19 Registers (several of them being totally
   unknown, btw) to access. To tell the VDC to which register you
   want to write the value contained in $1FE002 (and $1FE003), simply
   write the number of the register to write to into the low 5 bits
   of $1FE000. As the VDC is a 16 bit processor (ALL VDC registers
   are one word wide) in most cases you will need to supply both
   of the data values.
   Detailed description of the VDC ports by Videoman (slightly changed):
   Address   | Access | Description
   (Mapped   |  mode  |
    to $FF)  |        |
     $0000   |   R    | 6270 Status register
             |        |
             |        |   Different bits flag different conditions.
             |        |   Not all are known.
             |        |   (Note: can use special ST0 opcode to store
             |        |   an immediate value.)
             |        |     b 7 = 0
             |        |     b 6 = 'BSY' flag
             |        |         I believe this is '1' when a DMA transfer
             |        |         is happening
             |        |     b 5 = 'VD' flag
             |        |         I believe this is a '1' when Vertical Sync
             |        |         happens, otherwise a '0' (uncertain)
             |        |     b 4 = 'DV' flag (unknown)
             |        |     b 3 = 'DS' flag (unknown)
             |        |     b 2 = 'RR' flag
             |        |         Set during a Scanline interrupt (see RCR
             |        |         register)otherwise '0'
             |        |     b 1 = 'OR' flag (unknown)
             |        |     b 0 = 'CR' flag (unknown)
             |        |
     $0000   |   W    | 6270 Address register
             |        |
             |        |     b 7-5 = ignored
             |        |     b 4-0 = 6270 register number to access using
             |        |             the 6270 data registers
             |        |             ($0002 and $0003). Please see 6270
             |        |             register list (SECTION 4) for details.
             |        |
     $0002   |  R/W   | 6270 data LSB
             |        |
             |        |   Note: can use special ST1 opcode to store
             |        |         an immediate value.)
             |        |
     $0003   |  R/W   | 6270 data MSB
             |        |
             |        |   Note: can use special ST2 opcode to store
             |        |         an immediate value.)


 ----->One short example on this one:<------
   To read the contents of Register 2 (VRAM-Read-Register) simply use the
   following line of code:
      ST0 #2
      ...and then the two data values will sort of 'mirror' the value in
      this VDC register.

[/code]

4. The VDC registers

[code]

This huge and very complete list has been taken from Videomans
hardware map document,  Jens' PCE documentation, and some information
to it was added by me.


REG ACCESS DESCRIPTION+ NO. MODE DETAILS


0 R?/W MAWR - 'Memory Address Write Register'

       b 15-0  this is the internal
               register used as an address-counter when writing to VRAM.
               All bits used (although no VRAM above $7FFF).

1 R?/W MARR - 'Memory Address Read Register'

       b 15-0  this is the internal
               register used as an address-counter when reading from VRAM.
               All bits used (although no VRAM above $7FFF).

2 R VRR - 'VRAM Read Register'

       b 15-0  this is the only valid read-access
               from the data port.  It reads the value from VRAM at the
               address specified by the MARR.  When the value is read
               from the second byte-port at $0003, the MARR register
               (ie. the 'address to read from') is auto-incremented
               (although this may be a configurable behaviour).
               All bits used.

2 W VWR - 'VRAM Write Register'

               (write-access version of the above)
       b 15-0  Write value to VRAM at the address specified by the MAWR.
               When the value is written to the second byte-port at $0003,
               the MAWR register (ie. the 'address to write to') is
               auto-incremented (although this may be a configurable behaviour).

3  ? (unused) ?

4  ? (unused) ?

5  ? CR - 'Control Register'

                    b 15-13 unused
                    b 12-11 'IW' Address register auto-Increment
                                 of the MAWR register
                                 00 - normal increment (+1)
                                 01 - +32
                                 10 - +64
                                 11 - +128
                    b 10    'DR' Dynamic RAM refresh (unknown by me though)
                    b 9-8   'TE' Selection of DISP terminal outputs
                                 00 - DISP output "H" during display
                                 01 - BURST colour burst inserting position is
                                      indicated by output "L"
                                 10 - INTHSYNC internal horizontal synchronous
                                      signal
                                 11 - not used
                    b 7     'BB' background (on/off)     --+
                                 1  - display background   |
                                 0  - no background         > gets effective in
                    b 6     'SB' sprites (on/off)          |  next horizontal
                                 1  - display sprites      |  display period.
                                 0  - no sprites         --+
                    b 5-4   'EX' (name unknown by me)
                                 00 - vsync and hsync inputs
                                 01 - vsync input, hsync output
                                 10 - not used
                                 11 - vsync and hsync outputs
                    b 3     irq  (on/off)
                                 0 = disabled
                                 1 = enabled
                    b 2     rcr  (on/off)
                                 0 = disabled
                                 1 = enabled
                    b 1     Enable interrupt for excess number detection of
                            sprites.
                                 0 = disabled
                                 1 = enabled
                    b 0     Enable interrupt for sprite collision detection.
                                 0 = disabled
                                 1 = enabled
                    Editor's note: bits 3-0 sound suspiciously like
                    interrupt-enable flags. Given what we know about
                    the interrupt vector table, is it logical to assume
                    that the remaining two IE bits stand for the
                    remaining two interrupt vectors? Then again, maybe not.
                     $FFFC-$FFFD     NMI Vector
                     $FFFA-$FFFB     TIMER Vector
                     $FFF8-$FFF9     IRQ1 Vector (for Video)
                     $FFF6-$FFF7     IRQ2 Vector (for BRK)


6 R RCR - 'Raster Counter Register'

                   b 15-10 ?
                   b 9-0   The rcr bit controls the generation of a raster
                           counter IRQ. The VDC generates an IRQ, when the
                           scanline specified in the RCR register is displayed.
                           You need to add 64 to the RCR register to get the
                           correct scanline.

7 R?/W BXR - 'Background X-Scroll Register'

                   b 15-10 (not used)
                   b 9-0   when the background map is a larger virtual
                           size than the viewing screen shows, this is
                           the viewing screen's x-offset (in pixels)
                           from the origin of the virtual background map.

8 R?/W BYR - 'Background Y-Scroll Register'

                   b 15-9  (not used)
                   b 8-0   when the background map is a larger virtual
                           size than the viewing screen shows, this is
                           the viewing screen's y-offset (in pixels)
                           from the origin of the virtual background map.

9 R?/W MWR - 'Memory-access Width Register'

                   Used to configure the size of the virtual background
                   map.
                   b 15-8  (not used)
                   b 7     'CM' (unknown - presumably 'Color Mode')
                   b 6-4   'SCREEN' These bits control virtual map size
                           as noted below.
                           b 6     virtual screen height
                                   0 = 256 pixels / 32 tiles
                                   1 = 512 pixels / 64 tiles
                           b 5-4   virtual screen width
                                   00 = 256 pixels / 32 tiles
                                   01 = 512 pixels / 64 tiles
                                   10 = 1024 pixels / 128 tiles
                                   11 = 1024 pixels / 128 tiles
                                      Complete lookup of
                                      available sizes in tiles:
                                      -------------------------
                                        000 -  32 x 32
                                        001 -  64 x 32
                                        010 - 128 x 32
                                        011 - 128 x 32
                                        100 -  32 x 64
                                        101 -  64 x 64
                                        111 - 128 x 64


                   b 3-2   Sprite pixel width
                   b 1-0   VRAM pixel width

10($A)  ? HSR - 'Horizontal Sync Register'

                   b 15    (not used)
                   b 14-8  'HDS' Horizontal display start position -1.
                   b 7-5   (not used)
                   b 4-0   'HSW' Horizontal synchronous pulse width.
                   Mask = $7F1F

11($B)  ? HDR - 'Horizontal Display Register'

                   b 15    (not used)
                   b 14-8  'HDE' Horizontal display ending period -1.
                   b 7     (not used)
                   b 6-0   'HDW' Horizontal display width in tiles -1.
                   Mask = $7F7F
                   added from Jens' PCE-documentation: Lower half of HDR:
                     It controls the horizontal width of display generation.
                     The value in this register is the number of horizontal
                     tiles minus one. Normal values are 31, for 32 tiles
                     and 256 pixel horizontally, 39, for 40 tiles or 320
                     pixel and 63, for 64 tiles or 512 pixel.

12($C)  ? VPR - 'Vertical synchronous register'

                   b 15-8  'VDS' Vertical display start position -2.
                   b 7-5   (not used)
                   b 4-0   'VSW' Vertical synchronous pulse width.
                   Mask = $FF1F


13($D)  ? VDW - 'Vertical display register'

                   b 15-9  (not used)
                   b 8-0   Vertical display width in pixels -1.
                   NOTE:
                       Unlike the HDR register, the information on the
                       vertical display width is split up in two registers,
                       this one storing the vertical width, and the next one
                       (VCR) containing the vertical display end position.


14($E)  ? VCR - 'Vertical display END position register'

                   b 15-8  (not used)
                   b 7-0   Vertical display end position.

15($F)  ? DCR - 'DMA Control Register'

   The DCR, SOUR, DESR and LENR registers control
   DMA operations.
   The DMA operation starts, as soon as the length
   is written into the LENR register
                                              b 15-5 (not used)
                                              b 4 -  DSR DMA (VRAM-SATB transfer repetition)
                   b 3 -  Increment (0)/decrement (1) of
                          destination address.
                   b 2 -  Increment (0)/decrement (1) of
                          source address.
                   b 1 -  Enable interrupt at completion of
                          VRAM-VRAM transfer.
                          Checked on completion of transfer.
                   b 0 -  Enable interrupt at completion of
                          VRAM-SATB transfer.
                          Checked on completion of transfer.

16($10) R?/W SOUR - '(DMA) Source Address Register'

                    b 15-0  This register sets the source address
                            for DMA transfers.
                            All bits used (address pointer).

17($11) R?/W DESR - '(DMA) Destination Address Register'

                    b 15-0  This register sets the destination
                            address for DMA transfers.
                            All bits used
                            (although no VRAM above $7FFF).

18($12) R?/W LENR - '(DMA) Block Length Register'

                    b 15-0  This register sets the length of
                            the DMA transfer.
                            All bits used
                            (although no VRAM above $7FFF).

19($13) R?/W SATB - 'Sprite Attribute Table'

                    b 15-0  This register points to the start address
                            of the sprite attribute table.
                            All bits used
                            (although no VRAM above $7FFF).


[/code]

5. The Sprites in the VRAM

[code]

 Well, I will not try to explain what Sprites are here. Basically,
 all of the PC-Engines' sprites are 16*16 to 32*64 pixels in size,
 and have a sprite palette of 16 colors.
 There are 16 separate sprite palettes available. (remember, there
 was 16*16 colors for the background processing, those colors are
 INDEPENDENT from the sprite palettes).
 In the Sprites colours, colour 0 is transparent in all palettes,
 although it does peek it's head in a peculiar place; beyond the display
 width of the BG.
 Explanation:
 The background display area (in it's most often used setting) is 256x216.
 The display width of a television may be adjusted to squash the screen
 vertically, or horizontally. Even normal TVs show a little more that 256
 TG-16 pixel wide, leaving a black border on the sides. This border colour is
 actually controlled by sprite colour 0. The programmer can actually set the
 screen width more narrow or vertically shorter, showing more of this area.
 It's only use that I've ever implemented was in measuring the CPU load of
 the TG-16 during development.

--HOW ARE SPRITES STORED IN THE VRAM?

 For the sprite characters the principe is the same as for the background
 tiles, but in place of using bytes (8 pixels) they use words (16 pixels).
 Note that the words still use the same encoding as all word data within the
 PC-Engine, this means that the first byte of the word is the lower byte.
 Sprite data is stored like in the following drawing:
           Byte           Data
          offset
                  +-------------------+
             0    | plane 1 of line 1 |
                  +-------------------+
             2    | plane 1 of line 2 |
                  +-------------------+
                  .                   .
                  :                   :
            30    | plane 1 of line 16|
                  +-------------------|
            32    | plane 2 of line 1 |
                  +-------------------+
            34    | plane 2 of line 2 |
                  +-------------------+
            36    | plane 2 of line 3 |
                  +-------------------+
                  .                   .
                  :                   :
            46    | plane 2 of line 16|
                  +-------------------+
            48    | plane 3 of line 1 |
                     ......and so on.


 Not only you can display sprites, you can do some sort of funny stuff
 with them, like mirroring, for instance. All this is controlled in the
 so-called SPRITE ATTRIBUTE TABLE.


[/code]

6. The Sprite attribute table (SATB)

[code]

 The sprites' positions and attributes are defined in the so-
 called SPRITE ATTRIBUTE TABLE (SATB). The SATB can be contained any-
 where in the VRAM ($0000-$7FFF).

--HOW DOES THE VDC KNOW WHERE THE SATB IS TO BE FOUND?

 The VDC has a special register containing nothing but the start
 address of the SATB in the VRAM. This is register 19 (SEE SECTION 4)
 The actual sprite attributes are stored at the address mentioned
 above. For aech sprite, there is a 4 word long attribute section,
 which looks as follows:
 Word   | Access | Description
 offset |  mode  |

   0     R/W     Y position
               b 15-10   (unused)
               b 9-0     y position (relative to
                              virtual-screen origin)
   1     R/W     X position
               b 15-19   (unused?)
               b 9-0     x position (relative to
                              virtual-screen origin)
   2     R/W     Pattern address
               b 15-11   (unused?)
               b 10-0      sprite data VRAM address shifted
                              right 5 bits(Shift left 6 bits to
                              get real VRAM address)
   3     R/W     Sprite attributes
               b 15     y-invert flag (upside-down)
               b 14     unused
               b 13-12  'CGY'
                     00 = sprite is 1 'cell' (16 pixels) high
                     01 = sprite is 2 cells high (32 pixels)
                     10 = invalid
                     11 = sprite is 4 cells high (64 pixels)
               b 11     x-invert flag (left-right invert)
               b 10-9     unused
               b 8   'CGX'
                    0 = sprite is 1 'cell' wide (16 pixels)
                    1 = sprite is 2 cells wide (32 pixels)
               b 7   'SPBG'; is sprite in foreground (in front
                                    of CG) or background (behind CG)
               b 6-4    unused
               b 3-0    sprite colour (i.e. which of 16 sprite
                                            palettes to use)

[/code]