Cortex™-A8 Technical Reference Manual 3.2.73. c13, Thread and Process ID Registers

3.2.73. c13, Thread and Process ID Registers

The purpose of the Thread and Process ID Registers is to provide locations to store the IDs of software threads and processes for OS management purposes.

The Thread and Process ID Registers are:

three read/write registers banked for Secure and Nonsecure states:
- user read/write Thread and Process ID Register
- user read-only Thread and Process ID Register
- privileged only Thread and Process ID Register.
accessible in different modes:
- the user read/write Thread and Process ID Register is read/write in User and privileged modes
- the user read-only Thread and Process ID Register is read-only in User mode, and read/write in privileged modes
- the privileged only Thread and Process ID Register is only accessible in privileged modes, and is read/write.

Table 3.146 shows the results of attempted access to each register for each mode.

Table 3.146. Results of access to the Thread and Process ID Registers^[62]

	Secure privileged		Nonsecure privileged		Secure User		Nonsecure User
Register^[1]	Read	Write	Read	Write	Read	Write	Read	Write
User read/write	Secure data	Secure data	Nonsecure data	Nonsecure data	Secure data	Secure data	Nonsecure data	Nonsecure data
User read-only	Secure data	Secure data	Nonsecure data	Nonsecure data	Secure data	Undefined	Nonsecure data	Undefined
Privileged only	Secure data	Secure data	Nonsecure data	Nonsecure data	Undefined	Undefined	Undefined	Undefined
^[1]Each row refers to a Thread and Process ID Register. For example, the User read/write row refers to the User read/write Thread and Process ID Register.

To access the Thread and Process ID Registers, read or write CP15 with:

                    MRC p15, 0, <Rd>, c13, c0, 2 ; Read User read/write Thread and Process ID Register

                    MCR p15, 0, <Rd>, c13, c0, 2 ; Write User read/write Thread and Process ID Register

                    MRC p15, 0, <Rd>, c13, c0, 3 ; Read User read-only Thread and Process ID Register

                    MCR p15, 0, <Rd>, c13, c0, 3 ; Write User read-only Thread and Process ID Register

                    MRC p15, 0, <Rd>, c13, c0, 4 ; Read Privileged only Thread and Process ID Register

                    MCR p15, 0, <Rd>, c13, c0, 4 ; Write Privileged only Thread and Process ID Register

Reading or writing the Thread and Process ID Registers has no effect on the processor state or operation. These registers provide OS support and must be managed by the OS.

You must clear the contents of all Thread and Process ID Registers on process switches to prevent data leaking from one process to another. This is important to ensure the security of secure data.

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