4.3.29. c13, Thread and Process ID Registers

The Thread and Process ID Registers provide locations to store the IDs of software threads and processes for Operating System (OS) management purposes.

The Thread and Process ID Registers are:

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 Proc. ID Register
                    MCR p15, 0, <Rd>, c13, c0, 2 ; Write User read/write Thread and Proc. ID Register
                    MRC p15, 0, <Rd>, c13, c0, 3 ; Read User Read Only Thread and Proc. ID Register
                    MCR p15, 0, <Rd>, c13, c0, 3 ; Write User Read Only Thread and Proc. ID Register
                    MRC p15, 0, <Rd>, c13, c0, 4 ; Read Privileged Only Thread and Proc. ID Register
                    MCR p15, 0, <Rd>, c13, c0, 4 ; Write Privileged Only Thread and Proc. ID Register

Reading or writing the Thread and Process ID registers has no effect on processor state or operation. These registers provide OS support, and the OS must manage them.

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 data. The reset value of these registers is 0.

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