Cortex-A8 Technical Reference Manual - ARM Information Center

System Control Coprocessor
Table 3-144 shows how the bit values correspond with the Context ID Register functions.
Bits Field Function
3.2.73 c13, Thread and Process ID Registers
Table 3-145 shows the results of attempted access for each mode.
The current ASID value in the Context ID Register is exported to the MMU.
To access the Context ID Register, read or write CP15 with:
MRC p15, 0, , c13, c0, 1 ; Read Context ID Register
MCR p15, 0, , c13, c0, 1 ; Write Context ID Register
Table 3-144 Context ID Register bit functions
[31:8] PROCID Extends the ASID to form the process ID and identifies the current process. The reset value is 0.
[7:0] ASID Holds the ASID of the current process to identify the current ASID. The reset value is 0.
Table 3-145 Results of access to the Context ID Register a
Secure privileged Nonsecure privileged Secure User Nonsecure User
Read Write Read Write Read Write Read Write
Secure
data
Secure
data
Nonsecure
data
Nonsecure
data
Undefined Undefined Undefined Undefined
a. An entry of Undefined in the table means that the access gives an Undefined Instruction exception when the
coprocessor instruction is executed.
You must ensure that software executes a Data Synchronization Barrier operation before
changes to this register. This ensures that all accesses are related to the correct context ID.
You must execute an IMB instruction immediately after changes to the Context ID Register. You
must not attempt to execute any instructions that are from an ASID-dependent memory region
between the change to the register and the IMB instruction. Code that updates the ASID must
execute from a global memory region.
You must program each process with a unique number to ensure that the ETM and debug logic
can correctly distinguish between processes.
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
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