Altera Nios II User Manual page 66

3–26
1
When shadow register sets and the EIC interface are implemented on the Nios II core,
you must ensure that your software is built with the Nios II EDS version 9.0 or later.
Earlier versions have an implementation of the eret instruction that is incompatible
with shadow register sets.
Shadow register sets are typically used in conjunction with the EIC interface. This
combination can substantially reduce interrupt latency.
f
For details of EIC interface usage, refer to the Exception Processing section.
System software can read from and write to any shadow register set by setting
status.PRS and using the rdprs and wrprs instructions.
f
For details of the rdprs and wrprs instructions, refer to the
chapter of the Nios II Processor Reference Handbook.
The sstatus Register
The value in the sstatus register preserves the state of the Nios II processor during
external interrupt handling. The value of sstatus is undefined at processor reset.
Some bits are exclusively used by and available only to certain features of the
processor.
The sstatus register is physically stored in general-purpose register r30 in each
shadow register set. The normal register set does not have an sstatus register, but
each shadow register set has a separate sstatus register.
Table 3–33. sstatus Control Register Fields
31 30 29 28 27 26 25
Reserved
Table 3–34. sstatus Control Register Field Descriptions (Part 1 of 2)
Bit
SRS is the switched register set bit. The processor sets SRS to 1 when
(2) an external interrupt occurs, if the interrupt required the processor to
SRS
switch to a different register set.
RSIE
NMI
PRS
CRS
IL
IH
EH
U
Nios II Processor Reference Handbook
24 23 22 21 20 19 18 17
PRS
Description
(1)
(1)
(1)
(1)
(1)
(1)
(1)
(1)
16 15 14 13 12 11 10 9
CRS
Access
Read/Write
Read/Write
Read/Write
Read/Write
Read/Write
Read/Write
Read/Write
Read/Write
Read/Write
Chapter 3: Programming Model
Registers
Instruction Set Reference
8 7 6 5 4 3 2 1
IL
Reset Available
EIC interface
and shadow
Undefined
register sets
only
Undefined (1)
Undefined (1)
Undefined (1)
Undefined (1)
Undefined (1)
Undefined (1)
Undefined (1)
Undefined (1)
February 2014 Altera Corporation
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