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22 Altera Corporation
Overview
Complex Exception Handlers
An exception handler is considered complex if it violates any of the
requirements of a simple exception handler, listed above. Complex
exception handlers allow nested exception handling and the execution of
more complex code (e.g. subroutines that SAVE and RESTORE). A
complex exception handler has the following additional responsibilities:
■ It must preserve the contents of ISTATUS before re-enabling
interrupts. For example, ISTATUS could be saved on the stack.
■ It must check CWP before re-enabling interrupts to be sure CWP is at
or above LO_LIMIT. If CWP is below LO_LIMIT, it must take an
action to open up more available register windows (e.g., save the
register file contents to RAM), or it must signal an error.
■ It must re-enable interrupts subject to the above two conditions
before executing any SAVE or RESTORE instructions or calling any
subroutines that execute any SAVE or RESTORE instructions.
■ Prior to returning control to the interruptee, it must restore the
contents of the ISTATUS register, including any adjustments to CWP
if the register-window has been deliberately shifted.
■ Prior to returning control to the interruptee, it must restore the
contents of the interruptee’s register window.
Pipeline
Implementation
This topics in this section include a description of the following:
■ Nios CPU pipeline
■ Exposed pipeline branch delay and direct CWP manipulation
Figure 4. Nios CPU Block Diagram