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CS61C L27 Single-Cycle CPU Control (1) Garcia, Spring 2010 © UCB
inst.eecs.berkeley.edu/~cs61c UC Berkeley CS61C : Machine Structures
Lecture 27 Single-cycle CPU Control
2010-04-02
Success! The large Hadron
collider @ CERN is finally able to get subatomic particles colliding.
No Higgs boson particle yet, but stay tuned. 16 years and 10 G$!
Lecturer SOE Dan Garcia
www.cs.berkeley.edu/~ddgarcia
www.nytimes.com/2010/03/31/science/31collider.html
CS61C L27 Single-Cycle CPU Control (2) Garcia, Spring 2010 © UCB
Review: A Single Cycle Datapath
32
ALUctr
clk
busW
RegWr
32
32busA
32
busB
5 5
Rw Ra Rb
RegFile
Rs
Rt
Rt
RdRegDst
Exten
der
3216imm16
ALUSrcExtOp
MemtoReg
clk
Data In32
MemWrzero
01
0
1
=
AL
U 0
1
WrEn Adr
DataMemory
5
Instruction<31:0><21:25>
<16:20>
<11:15>
<0:15>
Imm16RdRtRs
nPC_sel instrfetchunitclk
• We have everything except control signals
CS61C L27 Single-Cycle CPU Control (3) Garcia, Spring 2010 © UCB
Recap: Meaning of the Control Signals• nPC_sel: “+4” 0 PC <– PC + 4
“br” 1 PC <– PC + 4 + {SignExt(Im16) , 00 }
• Later in lecture: higher-level connection between mux and branch condition
“n”=next
imm16
clk
PC
00
4nPC_sel
PC
Ext
Ad
derA
dder
Mu
x
Inst Address
0
1
CS61C L27 Single-Cycle CPU Control (4) Garcia, Spring 2010 © UCB
Recap: Meaning of the Control Signals• ExtOp: “zero”, “sign”
• ALUsrc: 0 regB; 1 immed
• ALUctr: “ADD”, “SUB”, “OR”
° MemWr: 1 write memory° MemtoReg: 0 ALU; 1 Mem° RegDst: 0 “rt”; 1 “rd”° RegWr: 1 write register
32
ALUctr
clk
busW
RegWr
32
32busA
32
busB
5 5
Rw Ra Rb
RegFile
Rs
Rt
Rt
RdRegDst
Exten
der 3216imm16
ALUSrcExtOp
MemtoReg
clk
Data In
32
MemWr01
0
1
AL
U 0
1
WrEn Adr
DataMemory
5
CS61C L27 Single-Cycle CPU Control (5) Garcia, Spring 2010 © UCB
RTL: The Add Instruction
add rd, rs, rt•MEM[PC] Fetch the instruction
from memory•R[rd] = R[rs] + R[rt] The actual operation
•PC = PC + 4 Calculate the next instruction’s address
op rs rt rd shamt funct
061116212631
6 bits 6 bits5 bits5 bits5 bits5 bits
CS61C L27 Single-Cycle CPU Control (6) Garcia, Spring 2010 © UCB
Instruction Fetch Unit at the Beginning of Add• Fetch the instruction from Instruction memory: Instruction = MEM[PC]
• same for all instructions
imm16
clk
PC
00
4nPC_sel
PC
Ext
Ad
derA
dder
Mu
x
Inst Address
InstMemory Instruction<31:0>
CS61C L27 Single-Cycle CPU Control (7) Garcia, Spring 2010 © UCB
The Single Cycle Datapath during Add
R[rd] = R[rs] + R[rt]op rs rt rd shamt funct
061116212631
32
ALUctr=ADD
clk
busW
RegWr=1
32
32busA
32
busB
5 5
Rw Ra Rb
RegFile
Rs
Rt
Rt
RdRegDst=1
Exten
der
3216imm16
ALUSrc=0ExtOp=x
MemtoReg=0
clk
Data In32
MemWr=0
zero01
0
1
=
AL
U 0
1
WrEn Adr
DataMemory
5
Instruction<31:0><21:25>
<16:20>
<11:15>
<0:15>
Imm16RdRtRs
nPC_sel=+4 instrfetchunitclk
CS61C L27 Single-Cycle CPU Control (8) Garcia, Spring 2010 © UCB
Instruction Fetch Unit at the End of Add• PC = PC + 4
• This is the same for all instructions except: Branch and Jump
imm16
clk
PC
00
4nPC_sel=+4
PC
Ext
Ad
derA
dder
Mu
x
Inst Address
InstMemory
CS61C L27 Single-Cycle CPU Control (10) Garcia, Spring 2010 © UCB
• R[rt] = R[rs] OR ZeroExt[Imm16]op rs rt immediate
016212631
Single Cycle Datapath during Or Immediate?
32
ALUctr=OR
clk
busW
RegWr=1
32
32busA
32
busB
5 5
Rw Ra Rb
RegFile
Rs
Rt
Rt
RdRegDst=0
Exten
der3216
imm16
ALUSrc=1ExtOp=zero
MemtoReg=0
clk
Data In32
MemWr=0
zero01
0
1
=
AL
U 0
1
WrEn Adr
DataMemory
5
Instruction<31:0><21:25>
<16:20>
<11:15>
<0:15>
Imm16RdRtRs
nPC_sel=+4 instrfetchunitclk
CS61C L27 Single-Cycle CPU Control (12) Garcia, Spring 2010 © UCB
The Single Cycle Datapath during Load
• R[rt] = Data Memory {R[rs] + SignExt[imm16]}op rs rt immediate
016212631
32
ALUctr=ADD
clk
busW
RegWr=1
32
32busA
32
busB
5 5
Rw Ra Rb
RegFile
Rs
Rt
Rt
RdRegDst=0
Exten
der3216
imm16
ALUSrc=1ExtOp=sign
MemtoReg=1
clk
Data In32
MemWr=0
zero01
0
1
=
AL
U 0
1
WrEn Adr
DataMemory
5
Instruction<31:0><21:25>
<16:20>
<11:15>
<0:15>
Imm16RdRtRs
nPC_sel=+4 instrfetchunitclk
CS61C L27 Single-Cycle CPU Control (14) Garcia, Spring 2010 © UCB
The Single Cycle Datapath during Store
• Data Memory {R[rs] + SignExt[imm16]} = R[rt]op rs rt immediate
016212631
32
ALUctr=ADD
clk
busW
RegWr=0
32
32busA
32
busB
5 5
Rw Ra Rb
RegFile
Rs
Rt
Rt
RdRegDst=x
Exten
der3216
imm16
ALUSrc=1ExtOp=sign
MemtoReg=x
clk
Data In32
MemWr=1
zero01
0
1
=
AL
U 0
1
WrEn Adr
DataMemory
5
Instruction<31:0><21:25>
<16:20>
<11:15>
<0:15>
Imm16RdRtRs
nPC_sel=+4 instrfetchunitclk
CS61C L27 Single-Cycle CPU Control (16) Garcia, Spring 2010 © UCB
The Single Cycle Datapath during Branch
• if (R[rs] - R[rt] == 0) then Zero = 1 ; else Zero = 0op rs rt immediate
016212631
32
ALUctr=SUB
clk
busW
RegWr=0
32
32busA
32
busB
5 5
Rw Ra Rb
RegFile
Rs
Rt
Rt
RdRegDst=x
Exten
der3216
imm16
ALUSrc=0ExtOp=x
MemtoReg=x
clk
Data In32
MemWr=0
zero01
0
1
=
AL
U 0
1
WrEn Adr
DataMemory
5
Instruction<31:0><21:25>
<16:20>
<11:15>
<0:15>
Imm16RdRtRs
nPC_sel=br instrfetchunitclk
CS61C L27 Single-Cycle CPU Control (17) Garcia, Spring 2010 © UCB
Instruction Fetch Unit at the End of Branch• if (Zero == 1) then PC = PC + 4 + SignExt[imm16]*4 ;
else PC = PC + 4
op rs rt immediate
016212631
• What is encoding of nPC_sel?
• Direct MUX select?• Branch inst. / not branch
• Let’s pick 2nd option
nPC_sel zero? MUX0 x 01 0 01 1 1
Adr
InstMemory
nPC_selInstruction<31:0>
Zero
nPC_sel
Q: What logic gate?
imm16 clk
PC
00
4
PC
Ext
Ad
derA
dder
Mu
x
0
1
MUX ctrl
CS61C L27 Single-Cycle CPU Control (19) Garcia, Spring 2010 © UCB
Step 4: Given Datapath: RTL Control
ALUctrRegDst ALUSrcExtOp MemtoRegMemWr
Instruction<31:0>
<21:25>
<16:20>
<11:15>
<0:15>
Imm16RdRsRt
nPC_sel
Adr
InstMemory
DATA PATH
Control
Op
<0:5>
Fun
RegWr
<26:31>
CS61C L27 Single-Cycle CPU Control (20) Garcia, Spring 2010 © UCB
A Summary of the Control Signals (1/2)inst Register Transfer
add R[rd] R[rs] + R[rt]; PC PC + 4
ALUsrc = RegB, ALUctr = “ADD”, RegDst = rd, RegWr, nPC_sel = “+4”
sub R[rd] R[rs] – R[rt]; PC PC + 4
ALUsrc = RegB, ALUctr = “SUB”, RegDst = rd, RegWr, nPC_sel = “+4”
ori R[rt] R[rs] + zero_ext(Imm16); PC PC + 4
ALUsrc = Im, Extop = “Z”,ALUctr = “OR”, RegDst = rt,RegWr, nPC_sel =“+4”
lw R[rt] MEM[ R[rs] + sign_ext(Imm16)]; PC PC + 4
ALUsrc = Im, Extop = “sn”, ALUctr = “ADD”, MemtoReg, RegDst = rt, RegWr, nPC_sel = “+4”
sw MEM[ R[rs] + sign_ext(Imm16)] R[rs]; PC PC + 4
ALUsrc = Im, Extop = “sn”, ALUctr = “ADD”, MemWr, nPC_sel = “+4”
beq if ( R[rs] == R[rt] ) then PC PC + sign_ext(Imm16)] || 00 else PC PC + 4
nPC_sel = “br”, ALUctr = “SUB”
CS61C L27 Single-Cycle CPU Control (21) Garcia, Spring 2010 © UCB
A Summary of the Control Signals (2/2)
add sub ori lw sw beq jump
RegDst
ALUSrc
MemtoReg
RegWrite
MemWrite
nPCsel
Jump
ExtOp
ALUctr<2:0>
1
0
0
1
0
0
0
x
Add
1
0
0
1
0
0
0
x
Subtract
0
1
0
1
0
0
0
0
Or
0
1
1
1
0
0
0
1
Add
x
1
x
0
1
0
0
1
Add
x
0
x
0
0
1
0
x
Subtract
x
x
x
0
0
?
1
x
x
op target address
op rs rt rd shamt funct
061116212631
op rs rt immediate
R-type
I-type
J-type
add, sub
ori, lw, sw, beq
jump
func
op 00 0000 00 0000 00 1101 10 0011 10 1011 00 0100 00 0010Appendix A10 0000See 10 0010 We Don’t Care :-)
CS61C L27 Single-Cycle CPU Control (22) Garcia, Spring 2010 © UCB
Boolean Expressions for Controller
RegDst = add + subALUSrc = ori + lw + swMemtoReg = lwRegWrite = add + sub + ori + lw MemWrite = swnPCsel = beqJump = jump ExtOp = lw + swALUctr[0] = sub + beq (assume ALUctr is 00 ADD, 01: SUB, 10: OR)ALUctr[1] = or
where,
rtype = ~op5 ~op4 ~op3 ~op2 ~op1 ~op0, ori = ~op5 ~op4 op3 op2 ~op1 op0 lw = op5 ~op4 ~op3 ~op2 op1 op0 sw = op5 ~op4 op3 ~op2 op1 op0
beq = ~op5 ~op4 ~op3 op2 ~op1 ~op0 jump = ~op5 ~op4 ~op3 ~op2 op1 ~op0
add = rtype func5 ~func4 ~func3 ~func2 ~func1 ~func0
sub = rtype func5 ~func4 ~func3 ~func2 func1 ~func0
How do we implement this in
gates?
CS61C L27 Single-Cycle CPU Control (23) Garcia, Spring 2010 © UCB
Controller Implementation
add
sub
ori
lw
sw
beq
jump
RegDstALUSrcMemtoRegRegWriteMemWritenPCselJumpExtOpALUctr[0]ALUctr[1]
“AND” logic “OR” logic
opcode func
CS61C L27 Single-Cycle CPU Control (24) Garcia, Spring 2010 © UCB
Peer Instruction
1) MemToReg=‘x’ & ALUctr=‘sub’. SUB or BEQ?
2) ALUctr=‘add’. Which 1 signal is different for all 3 of: ADD, LW, & SW? RegDst or ExtOp?
32
ALUctr
Clk
busW
RegWr
3232
busA
32busB
55 5
Rw Ra Rb32 32-bitRegisters
Rs
Rt
Rt
Rd
RegDst
Exten
der
Mu
x
Mux
3216imm16
ALUSrc
ExtOp
Mu
x
MemtoReg
Clk
Data InWrEn
32Adr
DataMemory
32
MemWr
AL
U
InstructionFetch Unit
Clk
Zero
Instruction<31:0>
0
1
0
1
01
<21:25>
<16:20>
<11:15>
<0:15>
Imm16RdRsRt
nPC_sel
12a) SRb) SEc) BRd) BE
CS61C L27 Single-Cycle CPU Control (25) Garcia, Spring 2010 © UCB
°5 steps to design a processor• 1. Analyze instruction set datapath requirements• 2. Select set of datapath components & establish clock
methodology• 3. Assemble datapath meeting the requirements• 4. Analyze implementation of each instruction to
determine setting of control points that effects the register transfer.
• 5. Assemble the control logic• Formulate Logic Equations• Design Circuits
Summary: Single-cycle Processor
Control
Datapath
Memory
ProcessorInput
Output
CS61C L27 Single-Cycle CPU Control (27) Garcia, Spring 2010 © UCB
32
ALUctr =
Clk
busW
RegWr =
3232
busA
32busB
55 5
Rw Ra Rb32 32-bitRegisters
Rs
Rt
Rt
RdRegDst =
Exten
der
Mu
x
Mux
3216imm16
ALUSrc =
ExtOp =
Mu
x
MemtoReg =
Clk
Data InWrEn
32Adr
DataMemory
32
MemWr =A
LU
InstructionFetch Unit
Clk
Zero
Instruction<31:0>
0
1
0
1
01<
21:25>
<16:20>
<11:15>
<0:15>
Imm16RdRsRt
• New PC = { PC[31..28], target address, 00 }
nPC_sel=
The Single Cycle Datapath during Jump
op target address02631
J-type jump25
Jump=
<0:25>
TA26
CS61C L27 Single-Cycle CPU Control (28) Garcia, Spring 2010 © UCB
The Single Cycle Datapath during Jump
32
ALUctr =x
Clk
busW
RegWr = 0
3232
busA
32busB
55 5
Rw Ra Rb32 32-bitRegisters
Rs
Rt
Rt
RdRegDst = x
Exten
der
Mu
x
Mux
3216imm16
ALUSrc = x
ExtOp = x
Mu
x
MemtoReg = x
Clk
Data InWrEn
32Adr
DataMemory
32
MemWr = 0A
LU
InstructionFetch Unit
Clk
Zero
Instruction<31:0>
0
1
0
1
01<
21:25>
<16:20>
<11:15>
<0:15>
RdRsRt
• New PC = { PC[31..28], target address, 00 }
nPC_sel=?
Jump=1
Imm16
<0:25>
TA26
op target address02631
J-type jump25
CS61C L27 Single-Cycle CPU Control (29) Garcia, Spring 2010 © UCB
Instruction Fetch Unit at the End of Jump
Adr
InstMemory
Ad
derA
dder
PC
Clk
00Mu
x
4
nPC_sel
imm
16
Instruction<31:0>
0
1
Zero
nPC_MUX_sel
• New PC = { PC[31..28], target address, 00 }op target address
02631J-type jump
25
How do we modify this
to account for jumps?
Jump
CS61C L27 Single-Cycle CPU Control (30) Garcia, Spring 2010 © UCB
Instruction Fetch Unit at the End of Jump
Adr
InstMemory
Ad
derA
dder
PC
Clk00
Mu
x
4
nPC_sel
imm
16
Instruction<31:0>
0
1
Zero
nPC_MUX_sel
• New PC = { PC[31..28], target address, 00 }op target address
02631J-type jump
25
Mu
x1
0
Jump
TA
4 (MSBs)
00
Query• Can Zero still
get asserted?
• Does nPC_sel need to be 0? • If not, what?
26