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Computer Architecture
Single-Cycle Implementation
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Single-Cycle Datapath
PCSrc 0MUX1 Add ALU 4 Add result RegWrite Shift left 2 Instruction [25-21] Read register 1 Read data 1 MemWrite Read address PC Instruction [20-16] Read register 2 ALUSrc MemtoReg Zero Instruction 0MUX1 Read data 2 ALU ALU [31-0] Write register 0MUX1 result Address Read data 1MUX0 Instruction Instruction [15-11] Write data memory Registers Data memory Write data RegDst 16 32 Instruction [15-0] Sign extend ALU control MemRead Instruction [5-0] ALUOp This datapath supports the following instructions: add, sub, and, or, slt, lw, sw, beq
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Single-Cycle Control RegDst Select destination register
RegWrite Specify if the destination register is written ALUSrc Select whether source is register or immediate ALUOp Specify operation for ALU MemWrite Specify whether memory is to be written MemRead Specify whether memory is to be read MemtoReg Select whether memory or ALU output is used PCSrc Select whether next PC or computed address is used
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R-format Instruction Dataflow
For add, sub, and, or, slt instructions PCSrc 0MUX1 Add ALU 4 Add result RegWrite Shift left 2 Instruction [25-21] Read register 1 Read data 1 MemWrite Read address PC Instruction [20-16] Read register 2 ALUSrc MemtoReg Zero Instruction 0MUX1 Read data 2 ALU ALU [31-0] Write register 0MUX1 result Address Read data 1MUX0 Instruction Instruction [15-11] Write data memory Registers Data memory Write data RegDst 16 32 Instruction [15-0] Sign extend ALU control MemRead Instruction [5-0] ALUOp
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R-format Instruction Control
Control signal summary RegDst 1 to select Rd RegWrite 1 to enable writing Rd ALUSrc 0 to select Rt value from register file ALUOp Dependent on operation (see below) MemWrite 0 to disable writing memory MemRead 0 to disable reading memory MemtoReg 0 to select ALU output to register PCSrc 0 to select next PC ALUOp add OP (add) and OP (and) sub OP (sub) or OP (or) slt OP (slt)
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I-format Load Instruction Dataflow
For lw instruction PCSrc 0MUX1 Add ALU 4 Add result RegWrite Shift left 2 Instruction [25-21] Read register 1 Read data 1 MemWrite Read address PC Instruction [20-16] Read register 2 ALUSrc MemtoReg Zero Instruction 0MUX1 Read data 2 ALU ALU [31-0] Write register 0MUX1 result Address Read data 1MUX0 Instruction Instruction [15-11] Write data memory Registers Data memory Write data RegDst 16 32 Instruction [15-0] Sign extend ALU control MemRead Instruction [5-0] ALUOp
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I-format Load Instruction Control
Control signal summary RegDst 0 to select Rt RegWrite 1 to enable writing Rt ALUSrc 1 to select immediate field value from instruction ALUOp add MemWrite 0 to disable writing memory MemRead 1 to enable reading memory MemtoReg 1 to select memory output to register PCSrc 0 to select next PC
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I-format Store Instruction Dataflow
For sw instruction PCSrc 0MUX1 Add ALU 4 Add result RegWrite Shift left 2 Instruction [25-21] Read register 1 Read data 1 MemWrite Read address PC Instruction [20-16] Read register 2 ALUSrc MemtoReg Zero Instruction 0MUX1 Read data 2 ALU ALU [31-0] Write register 0MUX1 result Address Read data 1MUX0 Instruction Instruction [15-11] Write data memory Registers Data memory Write data RegDst 16 32 Instruction [15-0] Sign extend ALU control MemRead Instruction [5-0] ALUOp
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I-format Store Instruction Control
Control signal summary RegDst x (don’t care) RegWrite 0 to disable writing a register ALUSrc 1 to select Rt value from register file ALUOp add MemWrite 1 to enable writing memory MemRead 0 to disable reading memory MemtoReg x (don’t care) PCSrc 0 to select next PC
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I-format Branch Instruction Dataflow
For beq instruction 0MUX1 Add ALU 4 Add result RegWrite Shift left 2 PCSrc Instruction [25-21] Read register 1 Read data 1 MemWrite Read address PC Instruction [20-16] Read register 2 ALUSrc MemtoReg Zero Instruction 0MUX1 Read data 2 ALU ALU [31-0] Write register 0MUX1 result Address Read data 1MUX0 Instruction Instruction [15-11] Write data memory Registers Data memory Write data RegDst 16 32 Instruction [15-0] Sign extend ALU control MemRead Instruction [5-0] ALUOp
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I-format Branch Instruction Control
Control signal summary RegDst x (don’t care) RegWrite 0 to disable writing a register ALUSrc 0 to select Rt value from register file ALUOp sub MemWrite 0 to disable writing memory MemRead 0 to disable reading memory MemtoReg x (don’t care) PCSrc zero
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Single-Cycle Control Signals Summary
Signal R-fmt I-fmt (lw) I-fmt (sw) I-fmt (beq) RegDst x x RegWrite ALUSrc ALUOp OP add add sub MemWrite MemRead MemtoReg x x PCSrc zero
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More Details on Control Signal Generation
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Review and Projection Single-Cycle implementation is easy
Control is based solely on the operation (and results!) dictates ALU operation controls multiplexor selection enables/disables storage elements Processor signals (control and data) stabilize and then any state (register and/or memory) change takes place when the clock cycle ends Multi-Cycle implementation explained in the next class Instruction processing takes multiple steps, one step per cycle Within a given clock cycle, signals stabilize and their local state change takes place when the clock cycle ends
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