Chapter 1 Functions of VDP1

■1.1 What is VDP1?

VDP1 is a sprite drawing IC for Sega Saturn. VDP1 uses a frame buffer, which makes it much faster to draw than previous systems, and the RAM capacity is also significantly increased, allowing more sprites (characters) to be displayed.

●System configuration

VRAM (4Mbit DRAM) and two frame buffers (2Mbit DRAM per side) are connected to VDP1. Image data is defined from the CPU to VRAM and output to the display device via the frame buffer.
Drawing data is sent from the CPU to VDP1 via the system controller (system control IC) and written to VRAM. Parts written to VRAM are drawn to the frame buffer in 16 or 8 bit/pixel format. The drawn frame buffer data is displayed on the display device via the priority circuit in VDP2.
The priority circuit determines the priority between the scroll surface and the priority surface. There are two frame buffers, and drawing and display are switched for each frame.
Information that controls drawing is set in the VDP1 system register from the CPU via the system controller. System registers control drawing.

Figure 1.1 System configuration

●Functions of VDP1

VDP1 functions include drawing parts (characters and lines), specifying colors, color calculations for Gouraud shading, specifying clipping coordinates and relative coordinates, and controlling frame buffer display. Parts, colors, and coordinates are controlled by command tables in VRAM, and frame buffer display is controlled by system registers.

◆Parts

• shaped sprite
• Rectangular sprite (with zoom)
• Transform sprites (including rotation)
• polygon (rectangle)
• Polyline (a rectangle made up of 4 lines)
• line

Sprites are collectively called texture parts, and other polygons, polylines, and lines are called non-texture parts.

◆Color

• The number of colors per texture part can be 16, 64, 128, 256, 32768 colors.

◆Special features

• When using RGB code, half-brightness, semi-transparent, Gouraud shading, and shadow color calculations are possible.
• Mesh (tiling)

◆Coordinate control

• Two clipping settings available: system and user
• Relative coordinates can be specified when drawing

◆Frame buffer display control

• Scaling and rotating the entire frame buffer screen
• Specifying the TV display mode
• Specifying framebuffer switching mode, switching trigger and drawing trigger
• Specifying double-dense interlacing
• Specifying fill area and fill data during erase write
• End status of transfer to frame buffer as help information during program development