Excalibur electronic A-MNL-NIOSPROG-01.1 manual Pipeline Implementation

Overview

Pipeline Implementation

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.

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

22 Altera Corporation