IDT MIPS32 4Kc Processor Core
CP0 Registers
79RC32438 User Reference Manual
2 - 56
November 4, 2002
Notes
CP0 Registers
The CP0 registers provide the interface between the ISA and the architecture. Each register is
discussed below, with the registers presented in numerical order, first by register number, then by select
field number. For each register described below, field descriptions include the read/write properties of the
field and the reset state of the field. Table 2.26 summarizes the read/write properties of the field.
Index Register (CP0 Register 0, Select 0)
The Index register is a 32-bit read/write register that contains the index used to access the TLB for
TLBP, TLBR, and TLBWI instructions. The width of the index field is implementation-dependent as a func-
tion of the number of TLB entries that are implemented. The minimum value for TLB-based MMUs is
Ceiling(Log2(TLBEntries)). The operation of the processor is UNDEFINED if a value greater than or equal
to the number of TLB entries is written to the Index register. This register is only valid with the TLB.
IIndex Register Format
31 30
Read/Write
Notation
Hardware Interpretation
Software Interpretation
R/W
A field in which all bits are readable and writable by software and, potentially, by hard-
ware.
Hardware updates of this field are visible by software read. Software updates of this
field are visible by hardware read.
If the reset state of this field is “Undefined,” either software or hardware must initialize
the value before the first read will return a predictable value. This should not be con-
fused with the formal definition of UNDEFINED behavior.
R
A field that is either static or is updated
only by hardware.
If the Reset State of this field is either “0”
or “Preset”, hardware initializes this field
to zero or to the appropriate state,
respectively, on powerup.
If the Reset State of this field is “Unde-
fined”, hardware updates this field only
under those conditions specified in the
description of the field.
A field to which the value written by soft-
ware is ignored by hardware. Software
may write any value to this field without
affecting hardware behavior. Software
reads of this field return the last value
updated by hardware.
If the Reset State of this field is “Unde-
fined,” software reads of this field result in
an UNPREDICTABLE value except after
a hardware update done under the condi-
tions specified in the description of the
field.
0
A field that hardware does not update,
and for which hardware can assume a
zero value.
A field to which the value written by soft-
ware must be zero. Software writes of
non-zero values to this field may result in
UNDEFINED behavior of the hardware.
Software reads of this field return zero as
long as all previous software writes are
zero.
If the Reset State of this field is “Unde-
fined,” software must write this field with
zero before it is guaranteed to read as
zero.
Table 2.26 CP0 Register Field Types
4 3
0
P
0
Index