||reg 0: request pointer
||To make an IOS request, the physical address of an IOS command struct is written here by the Broadway. Then, Broadway sets bit 0 of IPC reg 1 to indicate a request is ready.
||reg 1: semaphore flags
||Broadway sets bits here as "doorbells" to indicate status; Starlet responds by setting flags here.
||reg 2: Reply pointer
||When an IOS request has completed, IOS will modify the original command struct passed in IPC reg 0, copy that pointer to reg 2, then set reg 1 to 0x14 to indicate a reply is ready.