ARM 学习笔记之7： The Read-Modify-Write operation

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本文详细介绍了如何通过读改写操作安全地修改系统寄存器中的特定位，以避免因错误修改而导致程序行为异常。文章提供了具体的指令示例，包括复制系统寄存器到通用寄存器、清除指定位、设置指定位以及将通用寄存器的值写回系统寄存器的过程。

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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.
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.