SEGA SATURN DEVELOPER'S MANUAL


● ACH (W): This register stores the upper 16 bits of the 48-bit data representing the ALU operation result. It is also an input data storage register that stores the upper 16 bits of the input data A (48 bits) of the ALU calculator.
● ACL (W): This register stores the lower 32 bits of the 48-bit data representing the ALU operation result. It is also an input data storage register that stores the lower 32 bits of the input data A (48 bits) of the ALU calculator.
D0 bus: 32-bit data bus for external access. Operates at 28MHz. Used to access the main CPU.
X bus, Y bus: This is a 32-bit data bus used to obtain arithmetic unit input data. Operates at 14MHz.
RA0 (W): 32-bit external address register used for DMA transfer from external to DSP. Since the value is in 4 byte units, set the external address by shifting it to the right by 2 bits.
WA0 (W): DSP 32-bit external address register used for external DMA transfer. Since the value is in 4 byte units, set the external address by shifting it to the right by 2 bits.