Excalibur electronic A-MNL-NIOSPROG-01.1 manual Register Window Overflow

Overview

When a SAVE instruction causes a register window underflow exception, CWP is decremented only once before control is passed to the exception- handling subroutine. The underflow exception handler will see CWP = LO_LIMIT – 1. The register window underflow exception is exception number 1. The CPU will not process a register window underflow exception if interrupts are disabled (IE=0) or the current value in IPRI is less than or equal to 1.

The action taken by the underflow exception-handler subroutine depends upon the requirements of the system. For systems running larger or more complex code, the underflow (and overflow) handlers can implement a virtual register file that extends beyond the limits of the physical register file. When an underflow occurs, the underflow handler might (for example) save the current contents of the entire register file to memory and re-start CWP back at HI_LIMIT, allowing room for code to continue opening register windows. Many embedded systems, on the other hand, might wish to tightly control stack usage and subroutine call-depth. Such systems might implement an underflow handler that prints an error message and exits the program.

The programmer determines the nature of and actions taken by the register window underflow exception handler. The Nios software development kit (SDK) includes, and automatically installs by default, a register window underflow handler that virtualizes the register file using the stack as temporary storage.

A register window underflow exception can only be generated by a SAVE instruction. Directly writing CWP (via a WRCTL instruction) to a value less than LO_LIMIT will not cause a register window underflow exception. Executing a SAVE instruction when CWP is already below LO_LIMIT will not generate a register window underflow exception.

Register Window Overflow

A register window overflow exception occurs whenever the highest valid register window is in use (CWP = HI_LIMIT) and a RESTORE instruction is issued. Control is transferred to an exception-handling subroutine before the instruction following RESTORE is executed.

When a register window overflow exception is taken, the exception handler will see CWP at HI_LIMIT. You can think of CWP being incremented by the RESTORE instruction, but then immediately decremented as a consequence of normal exception processing. The register window overflow exception is exception number 2.

18 Altera Corporation