The AOYUNCP: Video System

Overview of CASSY

The Cool Ass Sprite SYstem is the video subsystem of the As Of Yet Unnamed Console Project. The current design of the video system for the console provides a wide variety of simple acceleration features to programmers. While at the heart of the system is a normal linear framebuffer, we have added a sprite engine to reduce the overhead of bitmap oriented games and simple animation. In addition to the sprite manipulation features, CASSY incorporates four (4) layers to allow true parallax scrolling, tile based playfields, and programmer control of transparency and intensity. All access to CASSY is done via normal memory reads and writes to the addresses mapped into the processor address space (i.e. a memory mapped device).

Definitions of Terms

In the discussion of CASSY, we will use the following terms to describe technical aspects of the design and programmer's view of the system.

Tile: A tile is a 16x16 graphic element consisting of a 768 bytes of color information (16*16*3RGB) and the TID of the next Tile in sequence (this is used by the sprite animator built into CASSY). The format of a Tile is as follows:
```
    T[0] = R0,     T[1] = G0,     T[2] = B0
    T[3] = R1,     T[4] = G1,     T[5] = B1
         .              .              .
         .              .              .
         .              .              .
    T[765] = R255, T[766] = G255, T[767] = B255
    T[768 && 769] = NTILE
```
CASSY has 1Mbyte of memory reserved for Tiles that the software defines and uses. This equates to about 1360 Tiles registered at one time. Tiles are overwritten by simply specifying a new TID and Tile data. Most graphic operations in CASSY manipulate these registered Tiles. Registration of is the process of simply moving the pixel data and NTILE value to the appropriate memory locations. (Note: This might change.)
Playfield: A Playfield is an array in video memory that stores information regarding what tile is being displayed at a given screen location. There are a fixed number (currently four) of playfields in CASSY. Each Playfield has associated control information on the transparency and visibility of the Playfield. If a Playfield is transparent, the any pixel data that is (R,G,B)=(0,0,0) will not be moved to the framebuffer. If a Playfield is not enabled, all tile information is ignored and the given Playfield does not mutate the framebuffer.
Sprite: A Sprite is position, velocity, orientation and tile information for placing a Tile at a pixel precise position in the framebuffer. If CASSY's sprite animator is enabled, the hardware will automatically update X,Y and Tile information without software intervention. This allows the programmer to do initial setup for simple animation sequences and then (through small updates) force the sprite through the animations. There are also associated attributes that determine the horizontal and vertical orientation of the image.
Sprite Bank: There are four (4) banks of Sprites in CASSY that allow the programmer to specify four levels of Sprite priority. When initializing a Sprite, the progammer specifies which Sprite Bank to place the data into. Using this method, the system simulates priorities found on other systems. Each sprite bank is blit after its associated Playfield (i.e. PL0, SB0, PL1, SB1, ...). Sprite Banks are basically pixel precise versions of Playfields. Usage of the Sprite Banks allows the programmer to do image/movement setup and then turn actual animation of the images over to the hardware.

Some Block Diagrams

Below are the available diagrams explaining CASSY. Look for more in the future.

CASSY Datapath (PostScript ~20K)

Last Updated: pbleisch@uiuc.edu 9/19/96