some problem about the DLX instruction set and the corresponding architecture

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zhengyudennis

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Hello,
when I learning the DLX instruction set and its corresponding architecture, there are a mux in ID stage to select the one from three candidate numbers to calculate the new address. Two of them are immediate numbers, to which I find the corresponding instruction. However, the last candidate is from a register. I have searched through the whole DLX instructions, but failed to find a instruction that uses the number from the register to calculate the address.
what is the use of third candidate. Can anyone give me some explanation? Thank very much!
 

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