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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:

  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 to 0 only the bits that must be cleared.

    • ORR to set to 1 only the bits that must be set.

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

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