Think of this as a specialized version of the C library function select, but for DMA transfers.
可以把它看作C库函数select的特殊版本，但是针对的是DMA传输。
The function select keys are positioned for easy use, allowing quick access to the desired function.
功能选择键的位置，便于使用，使快速访问所需的功能。
The function select keys are positioned for easy use, allowing quick access to the desired function.
功能选择键的位置，便于使用，使快速访问所需的功能。
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