Slide 12.3: Combining R- and I- type datapaths

Slide 12.2: Datapath for I-type ALU instructions
Slide 12.4: Controlling ALU instructions
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Combining R- and I- Type Datapaths

This slide shows the datapath by combining R-type datapath and I-type ALU datapath in the previous slides. The R- and I- type instruction formats and examples are listed again as follows:

   add  $t1, $t2, $t3
   # $t1 = $t2 + $t3
   #  rd =  rs +  rt

   addi  $10, $8, 9
   # $10 = $8 + 9
   #  rt = rs + imm16

The figure shows the datapath by combining R-type datapath and I-type ALU datapath. It includes the following two multiplexors:

The left mux selects RW as either rt (I-type) or rd (R-type).

The right mux selects the second ALU input as either source register rt (R-type) data on BusB or the extended immediate (I-type).

It also has the following control signals:

ALUCtrl is derived from either the op (I-type) or funct (R-type) field.

RegWrite enables the writing of the ALU result.

ExtOp controls the extension of the 16-bit immediate.

RegDst selects the register destination as either rt (I-type) or rd (R-type).

ALUSrc selects the second ALU source as BusB or extended immediate.

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Slide 12.2: Datapath for I-type ALU instructions
Slide 12.4: Controlling ALU instructions
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