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 drawing much faster than conventional systems, greatly increases RAM capacity, and can display more sprites (characters).

● System configuration

VRAM (4Mbit DRAM) and two-sided frame buffer (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.
The drawing data is sent from the CPU to VDP1 via the syscon (system control IC) and written to VRAM. Parts written to VRAM are drawn in the framebuffer in 16 or 8bit / pixel format. The drawn frame buffer data is displayed on the display device via the priority circuit in VDP2.
The priority circuit prioritizes the scroll plane and the priority plane. There are two frame buffers, and drawing and display are switched for each frame.
The information that controls drawing is set in the system register of VDP1 from the CPU via the syscon. System registers control drawing.

Figure 1.1 System configuration

● Functions of VDP1

VDP1 features include drawing parts (characters and lines), specifying colors, Gouraud shading color operations, specifying clipping and relative coordinates, and controlling the display of the framebuffer. The parts, colors, and coordinates are controlled by the command table in VRAM, and the frame buffer display is controlled by the system registers.

◆ Parts

• Fixed sprite
• Rectangular sprite (with zoom)
• Deformed sprites (including rotation)
• Polygon (quadrangle)
• Polyline (4 lines in a quadrangle)
• line

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

◆ Color

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

◆ Special functions

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

◆ Coordinate control

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

◆ Frame buffer display control

• Framebuffer Enlargement / reduction and rotation of the entire screen
• Specifying the TV display mode
• Specifying framebuffer switching mode, switching trigger and drawing trigger
• Designation of double-dense interlace
• Specifying the fill area and fill data during erase light
• End of transfer to frame buffer as help information during program development