2.3.2. Accessing CP15 registers

The terms and abbreviations shown in Table 2.4 are used throughout this section.

Table 2.4. CP15 abbreviations

Term

Abbreviation

Description

Unpredictable

UNP

For reads, the data returned when reading from this location is unpredictable. It can have any value.

For writes, writing to this location causes unpredictable behavior, or an unpredictable change in device configuration.

Should be zero

SBZ

When writing to this location, all bits of this field should be 0.

In all cases, reading from, or writing any data values to any CP15 registers, including those fields specified as unpredictable or should be zero, does not cause any permanent damage.

All CP15 register bits that are defined and contain state, are set to zero by BnRES except the V bit in register 1, which takes the value of macrocell input VINITHI when BnRES is asserted.

You can only access CP15 registers with MRC and MCR instructions in a privileged mode. The instruction bit pattern of the MCR and MRC instructions is shown in Figure 2.1. The assembler for these instructions is:

MCR/MRC{cond} P15,opcode_1,Rd,CRn,CRm,opcode_2

Figure 2.1. CP15 MRC and MCR bit pattern

Instructions CDP, LDC, and STC, together with unprivileged MRC and MCR instructions to CP15, cause the undefined instruction trap to be taken. The CRn field of MRC and MCR instructions specifies the coprocessor register to access. The CRm field and opcode_2 fields specify a particular action when addressing registers. The L bit distinguishes between an MRC (L=1) and an MCR (L=0).

Note

Attempting to read from a nonreadable register, or to write to a nonwritable register causes unpredictable results.

The opcode_1, opcode_2, and CRm fields should be zero, except when the values specified are used to select the desired operations, in all instructions that access CP15. Using other values results in unpredictable behavior.

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