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3.2.48. c9, Cycle Count Register

The purpose of the Cycle CouNT (CCNT) Register is to count the number of clock cycles since the register was reset. See bit [3] of the c9, Performance Monitor Control Register.

The CCNT Register is:

Table 3.94 shows the results of attempted access for each mode.

Table 3.94. Results of access to the Cycle Count Register[39]
EN[a]

Secure privileged

Nonsecure privileged

Secure User

Nonsecure User

ReadWriteReadWriteReadWriteReadWrite
0DataDataDataData

Undefined

Undefined

Undefined

Undefined

1DataDataDataDataDataDataDataData

[39] An entry of Undefined in the table means that the access gives an Undefined Instruction exception when the coprocessor instruction is executed.

[a] The EN bit in c9, User Enable Register enables User mode access of the Performance Monitor Registers.


To access the CCNT Register, read or write CP15 with:

MRC p15, 0, <Rd>, c9, c13, 0 ; Read CCNT Register
MCR p15, 0, <Rd>, c9, c13, 0 ; Write CCNT Register

The CCNT Register must be disabled before software can write to it. Any attempt by software to write to this register when enabled is Unpredictable.

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