The Read-Modify-Write operation
The read-modify-write operation ensures that you modify only the specific bits in a system register that you want to change.
Individual bits in a system register control different system functionality. Modifying the wrong bits in a system register might cause your program to behave incorrectly.
The following example shows how to use the read-modify-write procedure to change some bits in the NEON and VFP system register FPSCR, without affecting the other bits:
; copy FPSCR into the general-purpose r10 VMRS r10,FPSCR ; clear STRIDE bits[21:20] and LEN bits[18:16] BIC r10,r10,#0x00370000 ; set bits[17:16] (STRIDE =1 and LEN = 4) ORR r10,r10,#0x00030000 ; copy r10 back into FPSCR VMSR FPSCR,r10
To read-modify-write a system register, the instruction sequence is:
The first instruction copies the value from the target system register to a temporary general-purpose register.
The next one or more instructions modify the required bits in the general-purpose register. This can be one or both of:
BIC to clear to 0 only the bits that must be cleared.
ORR to set to 1 only the bits that must be set.
The final instruction writes the value from the general-purpose register to the target system register.