SEGA SATURN DEVELOPER'S MANUAL

Details of the SH-2 interface register are shown below.
COMREG (W): COMmand REGister: 8-bit register for receiving various commands from SH-2. As soon as the command is written, SMPC analyzes and executes the command. SMPC provides reset system management commands, non-reset system management commands, and RTC commands. Byte access from SH-2.
SR (R): Status Register: 8-bit register for displaying status after command execution by SMPC. Regardless of command issuance, it is always possible to read from SH-2. During peripheral control, various statuses of peripheral control are displayed. Byte access from SH-2.
SF (R / W): Status Flag: Flag for managing command issuance. The flag is set by the SH-2 before issuing the command, and the SMPC is reset when the command ends. Only set is possible from SH-2. When setting, write 01H. When reading, bits other than bit0 are undefined. This flag can be used to manage the double issue of commands. Byte access from SH-2.
IREG0 ~ IREG6 (W): Input REGister 0 ~ 6: 8-bit register for receiving command parameters from SH-2. SMPC has 7 IREGs. Byte access from SH-2.
OREG0 to OREG31 (R): Output REGister 0 to 31: 8-bit register to output result parameters and peripheral data to SH-2. SMPC has 32 OREGs, which are used to obtain cartridge code, area code, peripheral data, current time, etc. Byte access from SH-2.