Read-Modify-Write procedure

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:

  1. The first instruction copies the value from the target system register to a temporary general-purpose register.

  2. 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 just the bits that need to be cleared to 0

    • ORR to set just the bits that need to be set to 1.

  3. The final instruction writes the value from the general-purpose register to the target system register.

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This 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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Reference

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