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

Overview

3.IE is set to 0, disabling interrupts.

4.IPRI is set with the 6-bit number of the exception.

5.The address of the next non-executed instruction in the interrupted program is transferred into %o7.

6.The start-address of the exception handler is fetched from the exception vector table and written into the PC.

7.After the exception handler finishes a TRET instruction is issued to return control to the interrupted program.

Register Window Usage

All exception processing starts in a newly opened register window. This process decreases the complexity and latency of exception handlers because they are not responsible for maintaining the interruptee’s register contents. An exception handler can freely use registers %o0..%L7 in the newly opened window. An exception handler should not execute a SAVE instruction upon entry. The use of SAVE and RESTORE from within exception handlers is discussed later.

Because the transfer to exception handling always opens a new register window, programs must always leave one register window available for exceptions. Setting LO-LIMIT to 1 guarantees that one window is available for exceptions (The reset value of LO_LIMIT is 1). Whenever a program executes a SAVE instruction that would then use up the last register window (CWP = 0), a register-underflow trap is generated. The register-underflow handler itself will execute in the final window (with CWP = 0).

Correctly written software will never process an exception when CWP is 0. CWP will only be 0 when an exception is being processed, and exception handlers must take certain well-defined precautions before re-enabling interrupts. See “Simple and Complex Exception Handlers” on page 21 for more information.

Status Preservation: ISTATUS Register

When an exception occurs, the interruptee’s STATUS register is copied into the ISTATUS register. The STATUS register is then modified (IE set to 0, IPRI set, CWP decremented). The original contents of the STATUS register are preserved in the ISTATUS register. When exception processing returns control to the interruptee, the original program’s STATUS register contents are restored from ISTATUS by the TRET instruction.

20 Altera Corporation