Chapter 1 Overview

1.1 SCU overview

SCU (System Control Unit) is equipped with CPU I / F, A-Bus I / F, and B-Bus I / F, and interfaces of multiple processors connected to each I / F via a bus Is something that runs smoothly. Furthermore, a DMA controller, interrupt controller, and DSP are installed inside.
∙ The DMA controller controls the DMA transfer of a total of 4 channels of internal level 2-0 and DSP, and can freely transfer data between CPU, A-Bus, and B-Bus. In addition, the CPU can access the work area using the CPU-Bus during A-Bus and B-Bus DMA execution. However, the DSP area must be used for data transfer requests from the DSP. For example, a DMA transfer between A-Bus and B-Bus that does not use the DSP area cannot request a data transfer from the DSP. .
∙ The interrupt controller controls all interrupts in the SCU, including interrupts from A-Bus, B-Bus, and SMPC. In addition, it supports timer interrupts and can generate interrupts in sync with the screen.
The DSP plays a role in realizing processing that is difficult to realize because the main CPU is overloaded. The DSP operates at half the operating frequency of the main CPU. Therefore, 1 step is executed in about 70nsec.

System configuration diagram

Figure 1.1 shows the system configuration. WORK RAM-H, WORK RAM-L, BACKUP-RAM, IPL ROM, and SMPC are connected to CPU-Bus, and system reset signals and control pad control are executed.
∙ A-Bus is connected to a medium that supplies software such as CD and cartridge as an external system. VDP1, VDP2, and SCSP are connected to B-Bus to control video and audio.

Figure 1.1 System configuration diagram