Specifications

• Application context—The status of the Nios II processor and the HAL during normal program
execution, outside of exception funnels and handlers.
• Context switch—The process of saving the Nios II processor’s registers on a software exception or
hardware interrupt, and restoring them on return from the exception handling routine or ISR.
• Exception—A transfer of control away from a program’s normal flow of execution, caused by an event,
either internal or external to the processor, which requires immediate attention. Exceptions include
software exceptions and hardware interrupts.
• Exception context—The status of the Nios II processor and the HAL after a software exception or
hardware interrupt, when funnel code, a software exception handler, or an ISR is executing.
• Exception handling system—The complete system of software routines that service all exceptions,
including hardware interrupts, and pass control to software exception handlers and ISRs as necessary.
• Exception (or interrupt) latency—The time elapsed between the event that causes the exception (such
as an unimplemented instruction or interrupt request) and the execution of the first instruction at the
exception (or interrupt vector) address.
• Exception (or interrupt) response time—The time elapsed between the event that causes the exception
and the execution of the handler.
• Exception overhead—Additional processing required to service a software exception or hardware
interrupt, including HAL-specific processing and RTOS-specific processing if applicable.
• Funnel code—HAL-provided code that sets up the correct processor environment for an exception-
specific handler, such as an ISR.
• Handler—Code specific to the exception type. The handler code is distinct from the funnel code,
which takes care of general exception overhead tasks.
• Hardware interrupt—An exception caused by an explicit hardware request signal from an external
device. A hardware interrupt diverts the processor’s execution flow to a ISR, to ensure that a hardware
condition is handled in a timely manner.
• Implementation-dependent instruction—A Nios II processor instruction that is not supported on all
implementations of the Nios II core. For example, the mul and div instructions are implementation-
dependent, because they are not supported on the Nios II/e core.
• Interrupt—Hardware interrupt.
• Interrupt controller—Hardware enabling the Nios II processor to respond to an interrupt by transfer‐
ring control to an ISR.
• Interrupt request (IRQ)—Hardware interrupt.
• Interrupt service routine (ISR)—A software routine that handles an individual hardware interrupt.
• Invalid instruction—An instruction that is not defined for any implementation of the Nios II
processor.
• Maskable exceptions—Exceptions that can be disabled with the status.PIE flag, including internal
hardware interrupts, maskable external hardware interrupts, and software exceptions, but not
including nonmaskable external interrupts.
• Maximum disabled time—The maximum amount of continuous time that the system spends with
maskable exceptions disabled.
• Maximum masked time—The maximum amount of continuous time that the system spends with a
single interrupt masked.
• Miscellaneous exception—A software exception which is neither an unimplemented instruction nor a
trap instruction.
For more information, refer to the “Miscellaneous Exceptions” chapter.
• Nested interrupts—See pre-emption.
• Pre-emption—The process of a high-priority interrupt taking control when a lower-priority ISR is
already running. Also: nested interrupts.
8-2
Exception Handling Terminology
NII5V2
2015.05.14
Altera Corporation
Exception Handling
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