4.19 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:

    VMRS    r10,FPSCR              ; copy FPSCR into the general-purpose r10
    BIC     r10,r10,#0x00370000    ; clear STRIDE bits[21:20] and LEN bits[18:16]
    ORR     r10,r10,#0x00030000    ; set bits[17:16] (STRIDE =1 and LEN = 4)
    VMSR    FPSCR,r10              ; copy r10 back into FPSCR

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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