Question

Question 4: Single Cycle Datapath Control (15 points) We wish to add the hardware support for a special R-type instruction jlr Jump and Link Register) to the single-cycle datapath below. Though this is an R-type instruction, but it is a special one that has the opcode being 000001 (instead of 000000), so the control unit will be able to differentiate this jlr instruction from the other R-type instructions and generate a special set of controls for this instruction. Opcode rs rt rd Shamt funct (bits 31 - (bits 25 - (bits 20 - (bits 15 - (bits 10 - (bits 5- 26) 21) 16) 11) 6) 0) 000001 SSSSS ttttt 00000 00000 000000 w When we execute jlr rs, rt two things happen: (1) PC+4 will be put into register rt (Reg[rt]=PC+4), (2) PC will be overwritten with the target address in S (PC=Reg[rs]) a) Draw directly to the datapath above so that it can support jlr correctly. You can add wire(s) to the circuit but you are not allowed to add any new component(s) or extend any existing multiplexers. No mark will be given if you add any new component, extend any multiplexers or if your connections are not clearly shown. (8 points)

Answer 1

Here is final answer for question (a)

ALU Add result Add Shift left2 PCSO Instruction (31 26] RegDst Jump Branch MemRead MemtoReg Control ALUOP MemWrite ALUSrc RegWrite Instruction (2521) Read register 1 Read address PC Read data 1 2 Instruction (20-16) Read register 2 Zero Instruction (31-01 Registers Read Write data 2 ALU ALUI result Address Read data Instruction memory register Instruction (15-11) Write data Data memory NO Write data 16 32 Instruction (150) Sign lextend ALU control Instruction (5-0)

Explanation: Notice that the new connections made do not extend any multiplexer nor add a new component. The jump control signal signifies this instruction.

(b) For all other intructions we do not want jump to happen hence the value of jump for all other instructions is 0.

For jlr instruction here the values for all control signals in the order of the table.

jlr

0

x

10

1

0

0

0

x

x

1

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ALU Add result Add Shift left2 PCSO Instruction (31 26] RegDst Jump Branch MemRead MemtoReg Control ALUOP MemWrite ALUSrc RegWrite Instruction (2521) Read register 1 Read address PC Read data 1 2 Instruction (20-16) Read register 2 Zero Instruction (31-01 Registers Read Write data 2 ALU ALUI result Address Read data Instruction memory register Instruction (15-11) Write data Data memory NO Write data 16 32 Instruction (150) Sign lextend ALU control Instruction (5-0)