Bus Copy

The System RAM serves as a bridge between the CPU's main busses and the STIC's private graphics bus. During the VBlank Interval, the System RAM enters Bus Copy mode to allow the CPU access to the STIC's registers, as well as the GRAM and GROM. The System RAM exits Bus Copy mode when it sees a BUSAK signal from the CP-1610 CPU. This usually occurs just before the STIC starts active display at the end of VBlank Period 2.