2.3.1. Accessing CP15 registers

Table 2.2 shows the terms and abbreviations used in this section.

Table 2.2. CP15 abbreviations






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.



An instruction that accesses CP15 in the manner indicated takes the undefined instruction trap.

Should be zero


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

Should be one


When writing to this location, all bits of this field should be 1.

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 HRESETn except V-Bit in register 1, that takes the value of macrocell input VINITHI when HRESETn is asserted.

I-SRAM and D-SRAM sizes in register 9 reflect the physical I-SRAM and D-SRAM sizes.

CP15 registers can only be accessed 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.

Figure 2.1. CP15 MRC and MCR bit pattern

The assembler for these instructions is:

MCR/MRC{cond} p15,opcode_1,Rd,CRn,CRm,opcode_2

Instructions CDP, LDC, and STC, along 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 field specify a particular action when addressing registers.

Attempting to read from a nonreadable register, or writing 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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