The microprocessor instruction code must have some bits to identify every possible source register and destination register . 微处理机的指令代码就必须要有若干位，以便能够识别出每个可能的源寄存器和每个可能的目的地寄存器。
2.
Destination register can mask which components are written to 我们可对目的寄存器进行掩码操作。
3.
Selects the destination register for the result of the current stage identified by 标识的当前贴图层的结果选择目标寄存器。
