Excalibur electronic A-MNL-NIOSPROG-01.1 Return-Address, Simple and Complex Exception Handlers

Overview

Interrupts are automatically disabled upon entry to an exception handler, so there is no danger of ISTATUS being overwritten by a subsequent interrupt or exception. The case of nested exception handlers (exception handlers that use or re-enable exceptions) is discussed in detail below. Nested exception handlers must explicitly preserve, maintain, and restore the contents of the ISTATUS register before and after enabling subsequent interrupts.

Return-Address

When an exception occurs, execution of the interrupted program is temporarily suspended. The instruction in the interrupted program that was preempted (i.e., the instruction that would have executed, but did not yet execute) is taken as the return-location for exception processing.

The return-location is saved in %o7 (in the exception handler’s newly opened register window) before control is transferred to the exception handler. The value stored in %o7 is the byte-address of the return- instruction right-shifted by one place. This value is suitable directly for use as the target of a TRET instruction without modification. Exception handlers will usually execute a TRET %o7 instruction to return control to the interrupted program.

Simple and Complex Exception Handlers

The Nios processor architecture permits efficient, simple exception handlers. The hardware itself accomplishes much of the status- and register-preservation overhead required by an exception handler. Simple exception handlers can substantially ignore all automatic aspects of exception handling. Complex exception handlers (for example, nested exception handlers) must follow additional precautions.

Simple Exception Handlers

An exception handler is considered simple if it obeys the following rules:

■It does not re-enable interrupts.

■It does not use SAVE or RESTORE (either directly or by calling subroutines that use SAVE or RESTORE).

■It does not use any TRAP instructions (or call any subroutines that use TRAP instructions).

■It does not alter the contents of registers %g0..%g7, or %i0..%i7.

Any exception handler that obeys these rules need not take special precautions with ISTATUS or the return address in %o7. A simple exception handler need not be concerned with CWP or register-window management.

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Overview

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