When you want to modify specific bits in a system register, you must ensure that you do not modify the other bits in the same register. This is because individual bits in a system register control different system functionality, and modifying them might cause your program to behave incorrectly. You must use a read-modify-write procedure to ensure that you modify only the bits you want to change.
To read-modify-write a system register, the instruction sequence must be:
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:
BICto clear just the bits that need to be cleared to 0ORRto set just the bits that need to be set to 1.
The final instruction writes the value from the general-purpose register to the target system register.
ExampleThis example shows the read-modify-write procedure to change some bits of a NEON and VFP system register FPSCR without affecting other bits.
VMRS r10,FPSCR ; copy FPSCR into the general-purpose r10
BIC r10,r10,#0x00370000 ; clears STRIDE bits[21:20] and LEN bits[18:16]
ORR r10,r10,#0x00030000 ; sets bits[17:16] (STRIDE =1 and LEN = 4)
VMSR FPSCR,r10 ; copy r10 back into FPSCR
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