Excalibur electronic A-MNL-NIOSPROG-01.1 Exception Handling Overview, Exception Vector Table

Overview

Exceptions

The topics in this section include a description of the following:

■Exception vector table

■How external hardware interrupts, internal exceptions, register window underflow, register window overflow and TRAP instructions are handled

■Direct software exceptions (TRAP) and exception processing sequence

Exception Handling Overview

The Nios processor allows up to 64 vectored exceptions. Exceptions can be enabled or disabled globally by the IE control-bit in the STATUS register, or selectively enabled on a priority basis by the IPRI field in the STATUS register. Exceptions can be generated from any of three sources: external hardware interrupts, internal exceptions or explicit software TRAP instructions.

The Nios exception-processing model allows precise handling of all internally generated exceptions. That is, the exception-transfer mechanism leaves the exception-handling subroutine with enough information to restore the status of the interrupted program as if nothing had happened. Internal exceptions are generated if a SAVE or RESTORE instruction causes a register-window underflow or overflow, respectively.

Exception-handling subroutines always execute in a newly opened register window, allowing very low interrupt latency. The exception handler does not need to manually preserve the interruptee’s register contents.

Exception Vector Table

The exception vector table is a set of 64 exception-handler addresses. On a 32-bit Nios CPU each entry is 4 bytes and on a 16-bit Nios CPU each entry is 2 bytes. The base-address of the exception vector table is configurable. When the Nios CPU processes exception number n, it fetches the nth entry from the exception vector table, doubles the fetched value and then loads the results into the PC.

The exception vector table can physically reside in RAM or ROM, depending on the hardware memory map of the target system. A ROM exception vector table will not require run-time initialization.

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