2.1.5 Events

The exception handler built into the N64 operating system decodes interrupts and other exceptions and maps them to system events. If the system event is one that may be handled by the game itself, a message is sent to an associated event mailbox and the game application is notified. In this way, the game designer can provide an interrupt handler to deal with the exception as required by the game requirements.

Once the cause of an interrupt (or other exception) has been determined, the N64 operating system maps it to one of the following 14 events defined for the N64 system:

SW1 - System software interrupt 1 asserted
SW2 - System software interrupt 2 asserted
CART - Peripherial has generated an interrupt
COUNTER - Internal counter reached terminal count
SP - RCP (reality co-processor) SP (signal processor) interrupt; Task Done/Task Yield
SI - RCP SI (serial interface) interrupt; controller input available
AI - RCP AI (audio interface) interrupt; audio buffer swap
VI - RCP VI (video interface) interrupt; vertical retrace
PI - RCP PI (peripheral interface) interrupt; ROM to RAM DMA done
DP - RCP DP (display processor) interrupt; RDP processing done
PRE-NMI - An NMI (nonmaskable interrupt) has been requested and will occur in 0.5 seconds
CPU_BREAK - R4300i CPU has hit a breakpoint
SP_BREAK - RCP SP (signal processor) interrupt; RCP has hit a breakpoint
FAULT - R4300i CPU has faulted
THREAD_STATUS - Thread was created or destroyed

Threads, messages, and events work together to form the core of the N64 operating system. N64 applications run under a small, multi-threaded operating system. Often a thread needs to wait for an exception such as an interrupt. Exceptions are trapped by the operating system and turned into events. Threads may register to receive notification of system events by requesting that the operating system send them a message whenever a system event occurs.