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IDT MIPS32 4Kc Processor Core
CP0 Registers
79RC32438 User Reference Manual
2 - 63
November 4, 2002
Notes
Status Register (CP0 Register 12, Select 0)
The Status register (SR) is a read/write register that contains the operating mode, interrupt enabling,
and the diagnostic states of the processor. Fields of this register combine to create operating modes for the
processor, as follows:
Interrupt Enable: Interrupts are enabled when all of the following conditions are true:
IE = 1
EXL = 0
ERL = 0
DM = 0
If these conditions are met, the settings of the IM and IE bits enable the interrupt.
Operating Modes: If the DM bit in the Debug register is 1, the processor is in debug mode. Otherwise the
processor is in either kernel or user mode. The following CPU Status register bit settings determine user or
kernel mode.
User mode: UM = 1, EXL = 0, and ERL = 0
Kernel mode: UM = 0, or EXL = 1, or ERL = 1
Coprocessor Accessibility: The Status register CU bits control coprocessor accessibility. If any copro-
cessor is unusable, an instruction that accesses it generates an exception.
Coprocessor 0 is always enabled in kernel mode, regardless of the setting of the CU0 bit.
Status Register Format
31
28 27 26 25 24 23 22 21 20 19 18 17 16 15
8 7
5 4 3 2 1 0
CU3-CU0
RP R RE
0
BE
V
TS SR N
MI
0
0
IM7-IM0
R
U
M
R ER
L
EX
L
IE
Fields
Description
Read/
Write
Reset
State
Name
Bit(s)
CU3-CU0
31:28
Controls access to coprocessors 3, 2, 1, and 0,
respectively:
0: access not allowed
1: access allowed
Coprocessor 0 is always usable when the processor
is running in kernel mode, independent of the state of
the CU0 bit.
The core does not support coprocessors 1-3, but
CU3:1 can still be set. However, processor behavior
is unpredictable if a coprocessor instruction to copro-
cessors 1-3 is attempted with the corresponding
CU3:1 bit set.
R/W
Undefined
RP
27
Enables reduced power mode. The state of the RP
bit is available on the bus interface as the SI_RP sig-
nal.
R/W
0 for Cold
Reset
only.
R
26
This bit must be ignored on writes and read as zero.
R
0
Table 2.39 Status Register Field Description (Part 1 of 3)