Altera Nios II User Manual page 59

Chapter 3: Programming Model
Registers
The DBL Flag
During a general exception, the processor sets DBL to one when a double TLB miss
condition exists. Otherwise, the processor clears DBL to zero.
The DBL flag indicates whether the most recent exception is a double TLB miss
condition. When a general exception occurs, the MMU sets DBL to one if a double TLB
miss is detected, and clears DBL to zero otherwise.
The BAD Flag
During a general exception, the processor sets BAD to one when a bad virtual address
condition exists, and clears BAD to zero otherwise. The following exceptions set the BAD
flag to one:
■ Supervisor-only instruction address
■ Supervisor-only data address
■ Misaligned data address
Misaligned destination address
■
Refer to Nios II Exceptions (In Decreasing Priority Order) table for more information on these
exceptions.
The PERM Flag
During a general exception, the processor sets PERM to one for a TLB permission
violation exception, and clears PERM to zero otherwise.
The D Flag
The D flag indicates whether the exception is an instruction access exception or a data
access exception. During a general exception, the processor sets D to one when the
exception is related to a data access, and clears D to zero for all other nonbreak
exceptions.
The following exceptions set the D flag to one:
■ Fast TLB miss (data)
■ Double TLB miss (data)
■ TLB permission violation (read or write)
Misaligned data address
■
Supervisor-only data address
■
The badaddr Register
When the extra exception information option is enabled, the Nios II processor
provides information useful to system software for exception processing in the
exception and badaddr registers when an exception occurs. When your system
contains an MMU or MPU, the extra exception information is always enabled. When
no MMU or MPU is present, the Nios II Processor parameter editor gives you the
option to have the processor provide the extra exception information.
For information about controlling the extra exception information option, refer to the
Instantiating the Nios II Processor
February 2014 Altera Corporation
chapter of the Nios II Processor Reference Handbook.
3–19
Nios II Processor Reference Handbook