SEGA SATURN DEVELOPER'S MANUAL

VRAM

VRAM is 4Mbit DRAM.
VRAM defines a command table, sprite character pattern table, color lookup table, and Gouraud shading table. Each data can be anywhere in VRAM or nested. Specify each address for reference.
The command table is always fetched from the beginning of VRAM (000000H).
VRAM fetch is

Command table

Gouraud shading table (only when Gouraud shading is used)
Color lookup table (only when using lookup table method)
Character pattern table (only when drawing sprites)

It is repeated in the order.
If the command table fetch exceeds the VRAM end address (07FFFFH), it will be fetched back to the VRAM start address (000000H).
Both byte access and word access are possible from the CPU.
Priority is given to read / write access from the system controller (system controller IC) to VRAM and parameter read and pattern read access from VDP1.
VRAM access priority is always
Syscon (system controller IC)> Drawing
The order.
Because it is accessed with priority, depending on the timing, the CPU operating clock may wait about 10 or more clocks. CPU operating clock is 28MHz.
In order not to interrupt drawing, please access from CPU when not drawing. Whether or not drawing is in progress is determined by polling the system register or using an interrupt signal. Access to the VRAM and frame buffer should be done in a short time using burst transfer.