ARM Cortex-M4 - Mrs

The Cortex-M4 Instruction Set
ARM DUI 0553A Copyright © 2010 ARM. All rights reserved. 3-163
ID121610 Non-Confidential
3.12.6 MRS
Move the contents of a special register to a general-purpose register.
Syntax
MRS{cond} Rd, spec_reg
where:
cond Is an optional condition code, see Conditional execution on page 3-18.
Rd Specifies the destination register.
spec_reg Can be any of: APSR , IPSR , EPSR , IEPSR , IAPSR , EAPSR , PSR , MSP , PSP , PRIMASK , BASEPRI ,
BASEPRI_MAX , FAULTMASK , or CONTROL .
Note
All the EPSR and IPSR fields are zero when read by the MRS instruction.
Operation
Use
MRS in combination with MSR as part of a read-modify-write sequence for updating a PSR,
for example to clear the Q flag.
In process swap code, the programmers model state of the process being swapped out must be
saved, including relevant PSR contents. Similarly, the state of the process being swapped in
must also be restored. These operations use
MRS in the state-saving instruction sequence and MSR
in the state-restoring instruction sequence.
Note
BASEPRI_MAX is an alias of BASEPRI when used with the MRS instruction.
See MSR on page 3-164.
Restrictions
Rd must not be SP and must not be PC.
Condition flags
This instruction does not change the flags.
Examples
MRS R0, PRIMASK ; Read PRIMASK value and write it to R0