CS:APP Chapter 4
Computer Architecture
Pipelined
Implementation
Part I
Randal E. Bryant
Carnegie Mellon University
http://csapp.cs.cmu.edu
CS:APP2e
Overview
General Principles of Pipelining


Goal
Difficulties
Creating a Pipelined Y86 Processor



–2–
Rearranging SEQ
Inserting pipeline registers
Problems with data and control hazards
CS:APP2e
Real-World Pipelines: Car Washes
Sequential
Parallel
Pipelined
Idea



–3–
Divide process into
independent stages
Move objects through stages
in sequence
At any given times, multiple
objects being processed
CS:APP2e
Computational Example
300 ps
20 ps
Combinational
logic
R
e
g
Delay = 320 ps
Throughput = 3.12 GIPS
Clock
System



–4–
Computation requires total of 300 picoseconds
Additional 20 picoseconds to save result in register
Must have clock cycle of at least 320 ps
CS:APP2e
3-Way Pipelined Version
100 ps
20 ps
100 ps
20 ps
100 ps
Comb.
logic
A
R
e
g
Comb.
logic
B
R
e
g
Comb.
logic
C
20 ps
R
Delay = 360 ps
e
Throughput = 8.33 GIPS
g
Clock
System


Divide combinational logic into 3 blocks of 100 ps each
Can begin new operation as soon as previous one passes
through stage A.
 Begin new operation every 120 ps

Overall latency increases
 360 ps from start to finish
–5–
CS:APP2e
Pipeline Diagrams
Unpipelined
OP1
OP2
OP3

Time
Cannot start new operation until previous one completes
3-Way Pipelined
OP1
OP2
A
B
C
A
B
C
A
B
OP3
C
Time

–6–
Up to 3 operations in process simultaneously
CS:APP2e
Operating a Pipeline
239
241 300 359
Clock
OP1
A
OP2
B
C
A
B
C
A
B
OP3
0
120
240
360
C
480
640
Time
100 ps
20 ps
100 ps
20 ps
100 ps
20 ps
Comb.
logic
A
R
e
g
Comb.
logic
B
R
e
g
Comb.
logic
C
R
e
g
Clock
–7–
CS:APP2e
Limitations: Nonuniform Delays
50 ps
20 ps
150 ps
20 ps
100 ps
Comb.
logic
R
e
g
Comb.
logic
B
R
e
g
Comb.
logic
C
A
OP1
OP2
A
B
OP3
B
A
R
Delay = 510 ps
e
Throughput = 5.88 GIPS
g
Clock
C
A
20 ps
C
B
C
Time



–8–
Throughput limited by slowest stage
Other stages sit idle for much of the time
Challenging to partition system into balanced stages
CS:APP2e
Limitations: Register Overhead
50 ps 20 ps 50 ps 20 ps 50 ps 20 ps 50 ps 20 ps 50 ps 20 ps 50 ps 20 ps
Comb.
logic
R
e
g
Comb.
logic
R
e
g
Comb.
logic
R
e
g
Clock


 3-stage pipeline:
 6-stage pipeline:
–9–
R
e
g
Comb.
logic
R
e
g
Comb.
logic
R
e
g
Delay = 420 ps, Throughput = 14.29 GIPS
As try to deepen pipeline, overhead of loading registers
becomes more significant
Percentage of clock cycle spent loading register:
 1-stage pipeline:

Comb.
logic
6.25%
16.67%
28.57%
High speeds of modern processor designs obtained through
very deep pipelining
CS:APP2e
Data Dependencies
Combinational
logic
R
e
g
Clock
OP1
OP2
OP3
Time
System

– 10 –
Each operation depends on result from preceding one
CS:APP2e
Data Hazards
Comb.
logic
A
OP1
OP2
R
e
g
A
Comb.
logic
B
R
e
g
Comb.
logic
C
Clock
B
C
A
B
C
A
B
C
A
B
OP3
OP4
R
e
g
C
Time


– 11 –
Result does not feed back around in time for next operation
Pipelining has changed behavior of system
CS:APP2e
Data Dependencies in Processors

1
irmovl $50, %eax
2
addl %eax ,
3
mrmovl 100( %ebx ),
%ebx
%edx
Result from one instruction used as operand for another
 Read-after-write (RAW) dependency


Very common in actual programs
Must make sure our pipeline handles these properly
 Get correct results
 Minimize performance impact
– 12 –
CS:APP2e
SEQ Hardware


– 13 –
Stages occur in sequence
One operation in process
at a time
CS:APP2e
SEQ+ Hardware


Still sequential
implementation
Reorder PC stage to put at
beginning
PC Stage


Task is to select PC for
current instruction
Based on results
computed by previous
instruction
Processor State
– 14 –

PC is no longer stored in
register

But, can determine PC
based on other stored
information
CS:APP2e
Adding Pipeline Registers
newPC
PC
valE, valM
Write back
valM
Data
Data
memory
memory
Memory
Addr, Data
valE
CC
CC
Execute
ALU
ALU
Cnd
aluA, aluB
valA, valB
srcA, srcB
dstA, dstB
Decode
A
B
Register
Register M
file
file
E
icode ifun
rA , rB
valC
valP
,
Fetch
Instruction
Instruction
memory
memory
PC
PC
increment
increment
PC
– 15 –
CS:APP2e
Pipeline Stages
Fetch



Select current PC
Read instruction
Compute incremented PC
Decode

Read program registers
Execute

Operate ALU
Memory

Read or write data memory
Write Back

– 16 –
Update register file
CS:APP2e
PIPE- Hardware

Pipeline registers hold
intermediate values
from instruction
execution
Forward (Upward) Paths


Values passed from one
stage to next
Cannot jump past
stages
 e.g., valC passes
through decode
– 17 –
CS:APP2e
Signal Naming Conventions
S_Field

Value of Field held in stage S pipeline
register
s_Field

– 18 –
Value of Field computed in stage S
CS:APP2e
Feedback Paths
Predicted PC

Guess value of next PC
Branch information


Jump taken/not-taken
Fall-through or target
address
Return point

Read from memory
Register updates

– 19 –
To register file write
ports
CS:APP2e
Predicting the
PC

Start fetch of new instruction after current one has completed
fetch stage
 Not enough time to reliably determine next instruction

Guess which instruction will follow
 Recover if prediction was incorrect
– 20 –
CS:APP2e
Our Prediction Strategy
Instructions that Don’t Transfer Control


Predict next PC to be valP
Always reliable
Call and Unconditional Jumps


Predict next PC to be valC (destination)
Always reliable
Conditional Jumps


Predict next PC to be valC (destination)
Only correct if branch is taken
 Typically right 60% of time
Return Instruction

– 21 –
Don’t try to predict
CS:APP2e
Recovering
from PC
Misprediction

Mispredicted Jump
 Will see branch condition flag once instruction reaches memory
stage
 Can get fall-through PC from valA (value M_valA)

Return Instruction
 Will get return PC when ret reaches write-back stage (W_valM)
– 22 –
CS:APP2e
Pipeline Demonstration
irmovl
$1,%eax
#I1
irmovl
$2,%ecx
#I2
irmovl
$3,%edx
#I3
irmovl
$4,%ebx
#I4
halt
#I5
1
2
3
4
5
6
7
8
F
D
E
M
W
F
D
E
M
W
F
D
E
M
W
F
D
E
M
W
F
D
E
M
9
W
Cycle 5
File: demo-basic.ys
W
I1
M
I2
E
I3
D
I4
F
I5
– 23 –
CS:APP2e
Data Dependencies: 3 Nop’s
# demo-h3.ys
1
2
3
4
5
0x000: irmovl $10,%edx
F
D
E
M
W
F
D
E
M
W
F
D
E
M
W
F
D
E
M
W
F
D
E
M
W
F
D
E
M
W
F
D
E
M
0x006: irmovl
$3,%eax
0x00c: nop
0x00d: nop
0x00e: nop
0x00f: addl %edx,%eax
0x011: halt
6
7
8
9
10
Cycle 6
W
R[ %eax] f 3
Cycle 7
D
– 24 –
valA f R[ %edx] = 10
valB f R[ %eax
]=3
CS:APP2e
11
W
Data Dependencies: 2 Nop’s
# demo-h2.ys
1
2
3
4
5
0x000: irmovl $10,%edx
F
D
E
M
W
F
D
E
M
W
F
D
E
M
W
F
D
E
M
W
F
D
E
M
W
F
D
E
M
0x006: irmovl
$3,%eax
0x00c: nop
0x00d: nop
0x00e: addl %edx,%eax
0x010: halt
6
7
8
9
10
W
Cycle 6
W
R[ %eax] f 3
•
•
•
D
valA f R[ %edx] = 10
valB f R[ %eax] = 0
– 25 –
Error
CS:APP2e
Data Dependencies: 1 Nop
# demo-h1.ys
1
2
3
4
5
0x000: irmovl $10,%edx
F
D
E
M
W
F
D
E
M
W
F
D
E
M
W
F
D
E
M
W
F
D
E
M
0x006: irmovl
$3,%eax
0x00c: nop
0x00d: addl %edx,%eax
0x00f: halt
6
7
8
9
W
Cycle 5
W
R[ %edx] f 10
M
M_valE = 3
M_dstE = %eax
•
•
•
D
– 26 –
valA f R[ %edx] = 0
valB f R[ %eax] = 0
Error
CS:APP2e
Data Dependencies: No Nop
# demo-h0.ys
1
2
3
4
5
0x000: irmovl $10,%edx
F
D
E
M
W
F
D
E
M
W
F
D
E
M
W
F
D
E
M
0x006: irmovl
$3,%eax
0x00c: addl %edx,%eax
0x00e: halt
6
7
8
W
Cycle 4
M
M_valE = 10
M_dstE = %edx
E
e_valE f 0 + 3 = 3
E_dstE = %eax
D
valA f R[ %edx] = 0
valB f R[ %eax] = 0
– 27 –
Error
CS:APP2e
Branch Misprediction Example
demo-j.ys
0x000:
xorl %eax,%eax
0x002:
jne t
0x007:
irmovl $1, %eax
0x00d:
nop
0x00e:
nop
0x00f:
nop
0x010:
halt
0x011: t: irmovl $3, %edx
0x017:
irmovl $4, %ecx
0x01d:
irmovl $5, %edx

– 28 –
# Not taken
# Fall through
# Target (Should not execute)
# Should not execute
# Should not execute
Should only execute first 8 instructions
CS:APP2e
Branch Misprediction Trace
# demo-j
0x000:
xorl %eax,%eax
0x002:
jne t # Not taken
1
2
3
4
5
F
D
E
M
W
F
D
E
M
W
F
D
E
M
W
F
D
E
M
W
F
D
E
M
0x011: t: irmovl $3, %edx # Target
0x017:
irmovl $4, %ecx # Target+1
0x007:
irmovl $1, %eax # Fall Through
6
7
8
9
W
Cycle 5
M

Incorrectly execute two
instructions at branch target
M_Cnd =0
M_valA = 0x007
E
valE f 3
dstE = %edx
D
valC = 4
dstE = %ecx
F
– 29 –
valC f 1
rB f %eax
CS:APP2e
demo-ret.ys
Return Example
0x000:
0x006:
0x007:
0x008:
0x009:
0x00e:
0x014:
0x020:
0x020:
0x021:
0x022:
0x023:
0x024:
0x02a:
0x030:
0x036:
0x100:
0x100:

– 30 –
irmovl Stack,%esp
nop
nop
nop
call p
irmovl $5,%esi
halt
.pos 0x20
p: nop
nop
nop
ret
irmovl $1,%eax
irmovl $2,%ecx
irmovl $3,%edx
irmovl $4,%ebx
.pos 0x100
Stack:
# Initialize stack pointer
# Avoid hazard on %esp
# Procedure call
# Return point
# procedure
#
#
#
#
Should
Should
Should
Should
not
not
not
not
be
be
be
be
executed
executed
executed
executed
# Stack: Stack pointer
Require lots of nops to avoid data hazards
CS:APP2e
Incorrect Return Example
# demo-ret
0x023:

ret
0x024:
D
irmovl $1,%eax # Oops! F
0x02a:
irmovl $2,%ecx # Oops!
0x030:
irmovl $3,%edx # Oops!
0x00e:
irmovl $5,%esi # Return
Incorrectly execute 3
instructions following ret
F
E
D
F
M
E
D
F
W
M
E
D
F
W
M
E
D
W
M
E
W
M
W
W
valM = 0x0e
M
valE = 1
dstE = %eax
E
valE f 2
dstE = %ecx
D
valC = 3
dstE = %edx
F
– 31 –
valC f 5
rB f %esi
CS:APP2e
Pipeline Summary
Concept


Break instruction execution into 5 stages
Run instructions through in pipelined mode
Limitations


Can’t handle dependencies between instructions when
instructions follow too closely
Data dependencies
 One instruction writes register, later one reads it

Control dependency
 Instruction sets PC in way that pipeline did not predict correctly
 Mispredicted branch and return
Fixing the Pipeline

– 32 –
We’ll do that next time
CS:APP2e