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ECE 550D Fundamentals of Computer Systems and Engineering Fall 2016 Datapaths Tyler Bletsch Duke University Slides are derived from work by Andrew Hilton (Duke) and Amir Roth (Penn)
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ECE 550D Fundamentals of Computer Systems and Engineering

Fall 2016

Datapaths

Tyler Bletsch

Duke University

Slides are derived from work by Andrew Hilton (Duke) and Amir Roth (Penn)

2

Last time

• What did we do last time?

• MIPS Assembly • Practice translating C to assembly together

• Using functions

• Calling conventions

• jal (call)

• jr (return)

3

Now confluence of MIPS + digital logic

• Start of semester: Digital Logic • Building blocks of digital design

• Most recently: MIPS assembly, ISA • Lowest level software

• Now: where they meet… • Datapaths: hardware implementation of processors

• By the way: homework 4 = build a datapath

• With some components from the TAs…

4

Necessary ingredient: the ALU

• ALU: Arithmetic/Logic Unit

• Performs any supported math or logic operation on two inputs

• Which operation is chosen by a third input

ALU

A

B

op

out

5

S0 S1 Sn- 2 Sn- 1

Overflow

a0 a1 b0 b1 an- 2 bn- 2 an- 1 bn- 1

6

C in

C ou t

2

0

1

2

3

a

b

Q

A F Q

0 0 a + b

1 0 a - b

- 1 NOT b

- 2 a OR b

- 3 a AND b

ALU Slice

7

The ALU

ALU Slice ALU Slice ALU Slice ALU Slice

ALU control

a 0 b 0 a 1 b 1 a n-2 b n-2 a n-1 b n-1

Q 0 Q 1 Q n-2 Q n-1

Overflow Is non-zero?

ALU

A

B

op

out

8

Datapath for MIPS ISA

• Consider only the following instructions add \$1,\$2,\$3

lw \$1,4(\$3)

sw \$1,4(\$3)

beq \$1,\$2,PC_relative_target

j absolute_target

• Why only these? • Most other instructions are the same from datapath viewpoint

• The one’s that aren’t are left for you to figure out

9

Instruction

Fetch

Instruction

Decode

Operand

Fetch

Execute

Result

Store

Next

Instruction

Remember The von Neumann Model?

• Instruction Fetch: Read instruction bits from memory

• Decode: Figure out what those bits mean

• Operand Fetch: Read registers (+ mem to get sources)

• Execute: Do the actual operation (e.g., add the #s)

• Result Store: Write result to register or memory

• Next Instruction: Figure out mem addr of next insn, repeat

10

• Same for all instructions (don’t know insn yet)

• PC and instruction memory

• A +4 incrementer computes default next instruction PC

• Details of Insn Mem: later…

• For now: just assume a bunch of DFFs

P

C

Insn

Mem

+

4

11

• Add register file and ALU

P

C

Insn

Mem

Register

File

R-type

s1 s2 d

+

4

Decoding: Very easy in MIPS

Op(6) Rs(5) Rt(5) Rd(5) Sh(5) Func(6)

12

• Destination register can now be either Rd or Rt

• Add sign extension unit and mux into second ALU input

P

C

Insn

Mem

Register

File

S

X

Op(6) Rs(5) Rt(5) I-type Immed(16)

s1 s2 d

+

4

13

Third Instruction: lw

• Add register write data mux to select memory output or ALU output

P

C

Insn

Mem

Register

File

S

X

Op(6) Rs(5) Rt(5) I-type Immed(16)

s1 s2 d

Data

Mem d

+

4

a

14

Fourth Instruction: sw

• Add path from second input register to data memory data input

P

C

Insn

Mem

Register

File

S

X

Op(6) Rs(5) Rt(5) I-type Immed(16)

s1 s2 d

Data

Mem

a

d

+

4

15

Fifth Instruction: beq

• Add left shift unit and adder to compute PC-relative branch target

• Add PC input mux to select PC+4 or branch target

• Note: shift by fixed amount very simple

P

C

Insn

Mem

Register

File

S

X

Op(6) Rs(5) Rt(5) I-type Immed(16)

s1 s2 d

Data

Mem

a

d

+

4

<<

2

z

16

Sixth Instruction: j

• Add shifter to compute left shift of 26-bit immediate

P

C

Insn

Mem

Register

File

S

X

Op(6) J-type Immed(26)

s1 s2 d

Data

Mem

a

d

+

4

<<

2 <<

2

17

More Instructions…

• Figure out datapath modifications for

• jal (J-type)

• jr (R-type)

P

C

Insn

Mem

Register

File

S

X

s1 s2 d

Data

Mem

a

d

+

4

<<

2 <<

2

18

Jal

• For jal, need to get PC+4 to RF write mux

P

C

Insn

Mem

Register

File

S

X

s1 s2 d

Data

Mem

a

d

+

4

<<

2 <<

2

19

JR

• For JR need to get RF read value to next PC mux

P

C

Insn

Mem

Register

File

S

X

s1 s2 d

Data

Mem

a

d

+

4

<<

2 <<

2

20

Good practice: Try other insns

• Pick other MIPS instructions, contemplate how to add them

P

C

Insn

Mem

Register

File

S

X

s1 s2 d

Data

Mem

a

d

+

4

<<

2 <<

2

21

d

• Works because writes (PC, RegFile, DMem) are independent

• And because no read logically follows any write

P

C

Insn

Mem

Register

File

S

X

s1 s2 d

Data

Mem

a

+

4

Write PC

22

d

What Is Control?

• 8 signals control flow of data through this datapath • MUX selectors, or register/memory write enable signals

• A real datapath has 300-500 control signals

P

C

Insn

Mem

Register

File

S

X

s1 s2 d

Data

Mem

a

+

4

<<

2 <<

2

Rwe

ALUinB

DMwe

JP

ALUop

BR

Rwd

Rdst

23

• Control for an instruction: • Values of all control signals to correctly execute it

P

C

Insn

Mem

Register

File

S

X

s1 s2 d

Data

Mem

a

d

+

4

<<

2 <<

2

BR=0

JP=0

Rwd=0

DMwe=0 ALUop=0

ALUinB=0 Rdst=1

Rwe=1

24

Example: Control for sw

• Difference between sw and add is 5 signals • 3 if you don’t count the X (don’t care) signals

P

C

Insn

Mem

Register

File

S

X

s1 s2 d

Data

Mem

a

d

+

4

<<

2 <<

2

Rwe=0

ALUinB=1

DMwe=1

JP=0

ALUop=0

BR=0

Rwd=X

Rdst=X

25

d

Example: Control for beq

• Difference between sw and beq is only 4 signals

P

C

Insn

Mem

Register

File

S

X

s1 s2 d

Data

Mem

a

+

4

<<

2 <<

2

Rwe=0

ALUinB=0

DMwe=0

JP=0

ALUop=1

BR=1

Rwd=X

Rdst=X

26

You all figure LW

• How would these control signals be set for LW?

P

C

Insn

Mem

Register

File

S

X

s1 s2 d

Data

Mem

a

d

+

4

<<

2 <<

2

Rwe

ALUinB

DMwe

JP

ALUop

BR

Rwd

Rdst

27

Example: Control for LW

P

C

Insn

Mem

Register

File

S

X

s1 s2 d

Data

Mem

a

d

+

4

<<

2 <<

2

BR=0

JP=0

Rwd=1

DMwe=0 ALUop=0

ALUinB=1 Rdst=0

Rwe=1

28

d

How Is Control Implemented?

P

C

Insn

Mem

Register

File

S

X

s1 s2 d

Data

Mem

a

+

4

<<

2 <<

2

Rwe

ALUinB

DMwe

JP

ALUop

BR

Rwd

Rdst

Control?

29

Implementing Control

• Each insn has a unique set of control signals • Most are function of opcode

• Some may be encoded in the instruction itself

• E.g., the ALUop signal is some portion of the MIPS Func field

+ Simplifies controller implementation

• Requires careful ISA design

30

Control Implementation: ROM

• ROM (read only memory): think rows of bits • Bits in data words are control signals

• Lines indexed by opcode

• Example: ROM control for 6-insn MIPS datapath

• X is “don’t care”

BR JP ALUinB ALUop DMwe Rwe Rdst Rwd

add 0 0 0 0 0 1 0 0

addi 0 0 1 0 0 1 1 0

lw 0 0 1 0 0 1 1 1

sw 0 0 1 0 1 0 X X

beq 1 0 0 1 0 0 X X

j 0 1 0 0 0 0 X X

opcode

31

Control Implementation: Random Logic

• Real machines have 100+ insns 300+ control signals

• 30,000+ control bits (~4KB)

• Not huge, but hard to make faster than datapath (important!)

• Alternative: random logic (random = ‘non-repeating’)

• Exploits the observation: many signals have few 1s or few 0s

• Example: random logic control for 6-insn MIPS datapath

ALUinB

op

co

de

lw

sw

beq

j

BR JP DMwe Rwd Rdst ALUop Rwe

Yes, “random logic” is a very dumb and

misleading name for this concept. Sorry.

32

Datapath and Control Timing

P

C

Insn

Mem

Register

File

S

X

s1 s2 d

Data

Mem

a

d

+

4

Control ROM/random logic

Read DMEM Write DMEM Write Registers

Write PC

33

Single-Cycle Datapath Performance

• Goes against make common case fast (MCCF) principle + Low Cycles Per Instruction (CPI): 1

– Long clock period: to accommodate slowest insn

P

C

Insn

Mem

Register

File

S

X

s1 s2 d

Data

Mem

a

d

+

4

Control ROM/random logic

34

Interlude: Performance

• Previous slide alludes to something new: Performance • Don’t just want it to work…

• But want it to go fast!

• Three components to performance: Number of instructions

x Cycles per instruction (CPI)

x Clock Period (1 / Clock frequency)

Instructions Cycles Seconds Seconds

—————— x ————— x ————— = ——————

Program Instruction Cycle Program

35

Interlude: Performance

• Three components to performance: Number of instructions <- Compiler’s Job

x Cycles per instruction (CPI)

x Clock Period (1 / Clock frequency)

Instructions Cycles Seconds Seconds

—————— x ————— x ————— = ——————

Program Instruction Cycle Program

• Insns/Program: determined by compiler + ISA • Generally assume fixed program when doing micro-architecture

36

Micro-architectural factors

• Micro-architecture: • The details of how the ISA is implemented

• Affects CPI and Clock frequency

• Often will look at fixed program, and consider MIPS • Million Instructions Per Second

• MIPS = IPC * Frequency (in MHz)

• IPC = Instruction Per Cycle (1 / CPI)

• Gives “Bigger is better” number

Instructions Cycles Instructions

————— x ————— = ——————

Cycle Second Second

(IPC) (Frequency) (Throughput)

The use of “MIPS” to mean “Millions of

Instructions Per Second” has nothing to do with the CPU architecture also called “MIPS”,

which actually stands for “Microprocessor without

Interlocked Pipeline Stages”.

This fact that a major CPU

architecture shares a

name with an important

metric for performance is

incredibly confusing and

dumb, and I apologize.

I blame the cocaine-fueled

CPU architects of the

1980s.

37

“Best” IPC

• For now, best we can do: IPC = 1 (CPI = 1) • Do 1 instruction every cycle

• Later: • Real processors can do multiple instructions at once!

• Potentially: IPC > 1! (CPI < 1!)

• Best possible IPC depends on design

38

Performance vs ….

• 1990s: Performance at all cost • Actually more “clock frequency” at all cost…

• Now: Care about other things • Energy (electric bill, battery life)

• Power (cooling, also affects energy)

• Area (chip cost)

• Reliability (tolerance of transient faults: e.g., charge particle strikes)

• …

• Important metric these days “Performance / Watt” • Throughput divided by power consumption

• Why?

39

Performance Modeling and Analysis

• Speaking of performance • Making a processor takes time (years) and money (millions)

• Want to know it will perform well before you finish

• If its wrong, doing it all over is painful…

• Performance can be simulated in software

• Estimate what IPC will be

• Guide design

40

Single-Cycle Datapath Performance

• Goes against make common case fast (MCCF) principle + Low Cycles Per Instruction (CPI): 1

– Long clock period: to accommodate slowest insn

P

C

Insn

Mem

Register

File

S

X

s1 s2 d

Data

Mem

a

d

+

4

Control ROM/random logic

41

Alternative: Multi-Cycle Datapath

• Multi-cycle datapath: attacks high clock period • Cut datapath into multiple stages (5 here), isolate using FFs

• FSM control “walks” insns thru stages (by staging control signals)

+ Insns can bypass stages and exit early

P

C

Insn

Mem

Register

File

S

X

s1 s2 d Data

Mem

a

d

+

4

<<

2

I

R D O

B

A

s3

s3

s3 s4

s5

s5 s5

42

Multi-cycle Datapath FSM

• First state: Get a New Instruction • Output signals to fetch (e.g., read enable IMEM)

• Next State: Always Decode

Next

Insn

Decode

Insn

43

Multi-cycle Datapath FSM

• Second State: Decode • Output signals to decode instruction (RdEn RegFile)

• Go to Next Insn if NOP

• Otherwise Execute

Next

Insn

Decode

Insn Execute

Insn

NOP

44

Multi-cycle Datapath FSM

• Execute State • Execute Insn (varies by insn type)

• Next State: Also depends on insn type

• Branches: Next Insn

Next

Insn

Decode

Insn Execute

Insn Branch

NOP

45

Multi-cycle Datapath FSM

• Execute State • Execute Insn (varies by insn type)

• Next State: Also depends on insn type

• ALU op: write register

Next

Insn

Decode

Insn Execute

Insn Branch

Writeback

ALU

NOP

46

Multi-cycle Datapath FSM

• Execute State • Execute Insn (varies by insn type)

• Next State: Also depends on insn type

Next

Insn

Decode

Insn Execute

Insn Branch

Writeback

ALU

DMEM

NOP

47

Multi-cycle Datapath FSM

• Execute State • Execute Insn (varies by insn type)

• Next State: Also depends on insn type

• Store: Write Memory

Next

Insn

Decode

Insn Execute

Insn Branch

Writeback

ALU

DMEM

DMEM

Store

NOP

48

Multi-cycle Datapath FSM

• Next state is writeback

Next

Insn

Decode

Insn Execute

Insn Branch

Writeback

ALU

DMEM

DMEM

Store

NOP

49

Multi-cycle Datapath FSM

• Writeback state • Control signals enable regfile write

• Next state: Next Insn

Next

Insn

Decode

Insn Execute

Insn Branch

Writeback

ALU

DMEM

DMEM

Store

NOP

50

Multi-cycle Datapath FSM

• Write DMEM state • Control signals enable memory write

• Next state: Next Insn

Next

Insn

Decode

Insn Execute

Insn Branch

Writeback

ALU

DMEM

DMEM

Store

NOP

51

P

C

Insn

Mem

Register

File

S

X

s1 s2 d Data

Mem

a

d

+

4

<<

2

I

R D O

B

A

52

P

C

Insn

Mem

Register

File

S

X

s1 s2 d Data

Mem

a

d

+

4

<<

2

I

R D O

B

A

• Cycle 2: Decode + Read RF

53

P

C

Insn

Mem

Register

File

S

X

s1 s2 d Data

Mem

a

d

+

4

<<

2

I

R D O

B

A

• Cycle 2: Decode + Read RF

• Cycle 3: ALU

54

• Cycle 2: Decode + Read RF

• Cycle 3: ALU

• Cycle 4: Writeback + Increment PC

P

C

Insn

Mem

Register

File

S

X

s1 s2 d Data

Mem

a

d

+

4

<<

2

I

R D O

B

A

55

Multi-Cycle Datapath Performance

• Opposite performance split of single-cycle datapath + Short clock period

– High CPI

P

C

Insn

Mem

Register

File

S

X

s1 s2 d Data

Mem

a

d

+

4

<<

2

I

R D O

B

A

56

Multi-cycle Data-path CPI

• CPI depends on instructions • Branches / Jumps: 3 cycles

• ALU: 4 cycles

• Stores: 4 cycles

• Overall CPI is weighted average

• Example: • 20% loads, 15% stores, 20% branches, 45% ALU

57

Multi-cycle Data-path CPI

• CPI depends on instructions • Branches / Jumps: 3 cycles

• ALU: 4 cycles

• Stores: 4 cycles

• Overall CPI is weighted average

• Example: • 20% loads, 15% stores, 20% branches, 45% ALU

CPI= 0.20 * 5 +

58

Multi-cycle Data-path CPI

• CPI depends on instructions • Branches / Jumps: 3 cycles

• ALU: 4 cycles

• Stores: 4 cycles

• Overall CPI is weighted average

• Example: • 20% loads, 15% stores, 20% branches, 45% ALU

CPI= 0.20 * 5 + 0.15 * 4 +

59

Multi-cycle Data-path CPI

• CPI depends on instructions • Branches / Jumps: 3 cycles

• ALU: 4 cycles

• Stores: 4 cycles

• Overall CPI is weighted average

• Example: • 20% loads, 15% stores, 20% branches, 45% ALU

CPI= 0.20 * 5 + 0.15 * 4 + 0.20 * 3 + 0.45 * 4 = 4.0

60

Multi-cycle Datapath Performance

• Single-cycle • Clock period = 50ns, CPI = 1

• Performace = 50 ns/insn

• Multi-cycle • Clock period = 10ns

• CPI = (0.2*3+0.2*5+0.6*4) = 4

• Performance = 40 ns/insn

• But wait…

61

Multi-Cycle Datapath Performance

• Did not just cut up existing logic into 5 pieces

• Also added logic (flip flops) • So clock period not 1/5 of single cycle, but slightly longer

P

C

Insn

Mem

Register

File

S

X

s1 s2 d Data

Mem

a

d

+

4

<<

2

I

R D O

B

A

62

Multi-cycle Datapath Performance

• Single-cycle • Clock period = 50ns, CPI = 1

• Performace = 50 ns/insn

• Multi-cycle • Clock period = 12ns

• CPI = (0.2*3+0.2*5+0.6*4) = 4

• Performance = 48 ns/insn

• Better, but not as exciting… • Can we do better still?

• Have our cake (low CPI) and eat it too (high clock frequency)?

63

Clock Period and CPI

• Single-cycle datapath + Low CPI: 1

– Long clock period: to accommodate slowest insn

• Multi-cycle datapath + Short clock period

– High CPI

• Can we have both low CPI and short clock period? – No good way to make a single insn go faster

+ Insn latency doesn’t matter anyway … insn throughput matters

• Key: exploit inter-insn parallelism

insn0.fetch, dec, exec

insn1.fetch, dec, exec

insn0.dec insn0.fetch

insn1.dec insn1.fetch

insn0.exec

insn1.exec

64

Pipelining

• Pipelining: important performance technique • Improves insn throughput rather than insn latency

• Exploits parallelism at insn-stage level to do so

• Begin with multi-cycle design

• When insn advances from stage 1 to 2, next insn enters stage 1

• Individual insns take same number of stages

+ But insns enter and leave at a much faster rate

• Physically breaks “atomic” VN loop ... but must maintain illusion

• Revisit at end of semester (hopefully)

insn0.dec insn0.fetch

insn1.dec insn1.fetch

insn0.exec

insn1.exec

insn0.dec insn0.fetch

insn1.dec insn1.fetch

insn0.exec

insn1.exec

65

Summary

• Datapaths: • Single Cycle

• What do we need?

• Control

• How control is implemented

• Multi-cycle

• Faster clock (yay!)

• Worse CPI (boooo)

• Performance:

• IPC

• Performance / Watt

• CPU Performance Equation

• Pipelining

• Teaser for later!

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