The SH7305 used in the Prizm contains one Interrupt Controller (INTC). This component is tasked with prioritizing interrupt sources and controlling interrupt requests to the CPU. The priority of each interrupt can be set using the INTC registers. This interrupt controller is compatible with the one found in the SH7724 and shares the same base address, 0xA4080000.
The OS uses bit 1 of the Interrupt Mask Register 9 (IMR9), at address 0xA40800A4, to mask the USB interrupt.