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Slide 12.4: Controlling ALU instructions Slide 12.6: Controlling the execution of load Home         Print version

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Adding Data Memory to Datapath

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The figure shows the datapath including data memory for load and store (I-type) instructions where

• The ALU is also used to calculate data memory address.
• BusB is connected to Data_in of data memory for store instructions.

It also includes three multiplexors:

• The left one selects RW as either rt (I-type) or rd (R-type).
• The middle one selects the second ALU input as either source register rt (R-type) data on BusB or the extended immediate (I-type).
• The right one selects data on BusW as either ALU result (R- or I- type) or memory Data_out (I-type).

• ALUCtrl is derived from either the op (I-type) or funct (R-type) field.
• RegWrite enables the writing of the ALU result or Data_out.
• ExtOp controls the extension of the 16-bit immediate.
• RegDst selects the destination as either rt (I-type) or rd (R-type).
• ALUSrc selects the second ALU source as BusB (R-type) or extended immediate (I-type).
• MemRead is for load instructions.
• MemWrite is for store instructions.
• MemtoReg selects data on BusW as ALU result (R- or I- type) or memory Data_out (I-type).

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Q: What room does a ghost not need?           A: A living room!