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- The frame buffer is automatically erased before each frame is drawn. Erasing is performed by writing (filling) the code specified in the register to the frame buffer. This erase and write operation is called erase write .
- Access
- Reading and writing VRAM, system register, and frame buffer data from the CPU and VDP1 is called access . Arbitration is performed when accessing the VRAM, system register, and frame buffer. Arbitration means assigning a priority ( Priority ) when two or more devices are accessed simultaneously. Normally, writing has priority over reading for drawing.
DMA controller allows VRAM access to be performed continuously at high speed with burst transfer . However, do not burst access the registers.
- Data
- bit (bit) is the smallest data unit representing 0 or 1. 8 bits are collectively called byte (Byte), and 16 bits (or 2 bytes) is called word . When the upper and lower bytes are divided into 4 bits, each is called a nibble .
- Command
- command has a drawing command, clipping coordinate setting command, and relative coordinate setting command. Clipping cuts out the figure outside the set area. Relative coordinates changes the drawing position using the coordinates specified by the drawing command as relative coordinates.
The command is defined in VRAM as command table . In addition to the command table, there are a character pattern table, a Gouraud shading table, and a color lookup table.
* The table is defined and stored from the address of 8HByte boundary or 20HByte boundary on VRAM. Boundary means that when the address is divided by the values 8H and 20H, there is no remainder and it is divisible. The register is 2HByte (word) boundary.
VDP1 fetches the command table one after another and draws it according to the command. Fetch means reading from VRAM to process a command.
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