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Transcript of ARM_RG
7/27/2019 ARM_RG
http://slidepdf.com/reader/full/armrg 1/30
1
Lessons from the ARM Architecture
Richard Grisenthwaite
Lead Architect and Fellow
ARM
7/27/2019 ARM_RG
http://slidepdf.com/reader/full/armrg 2/30
2
ARM Processor Applications
7/27/2019 ARM_RG
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Overview
Introduction to the ARM architecture
Definition of “Architecture”
History & evolution
Key points of the basic architecture
Examples of ARM implementations
Something to the micro-architects interested
My Lessons on Architecture design …or what I wish I’d known 15 years ago
7/27/2019 ARM_RG
http://slidepdf.com/reader/full/armrg 4/30
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Definition of “ Architecture”
The Architecture is the contract between the Hardware and the Software Confers rights and responsibilities to both the Hardware and the Software
MUCH more than just the instruction set
The architecture distinguishes between:
Architected behaviors:
Must be obeyed
May be just the limits of behavior rather than specific behaviors
Implementation specific behaviors – that expose the micro-architecture
Certain areas are declared implementation specific. E.g.: Power-down
Cache and TLB Lockdown
Details of the Performance Monitors
Code obeying the architected behaviors is portable across implementations Reliance on implementation specific behaviors gives no such guarantee
Architecture is different from Micro-architecture
What vs How
7/27/2019 ARM_RG
http://slidepdf.com/reader/full/armrg 5/30
5
History
ARM has quite a lot of history
First ARM core (ARM1) ran code in April 1985…
3 stage pipeline very simple RISC-style processor
Original processor was designed for the Acorn Microcomputer Replacing a 6502-based design
ARM Ltd formed in 1990 as an “Intellectual Property” company
Taking the 3 stage pipeline as the main building block
This 3 stage pipeline evolved into the ARM7TDMI Still the mainstay of ARM’s volume
Code compatibility with ARM7TDMI remains very important
Especially at the applications level
The ARM architecture has features which derive from ARM1
Strong “applications level” compatibility focus in the ARM products
7/27/2019 ARM_RG
http://slidepdf.com/reader/full/armrg 6/30
6
Evolution of the ARM Architecture
Original ARM architecture:
32-bit RISC architecture focussed on core instruction set
16 Registers - 1 being the Program counter – generally accessible
Conditional execution on all instructions
Load/Store Multiple operations - Good for Code Density
Shifts available on data processing and address generation
Original architecture had 26-bit address space
Augmented by a 32-bit address space early in the evolution
Thumb instruction set was the next big step
ARMv4T architecture (ARM7TDMI)
Introduced a 16-bit instruction set alongside the 32-bit instruction set Different execution states for different instruction sets
Switching ISA as part of a branch or exception
Not a full instruction set – ARM still essential
ARMv4 architecture was still focused on the Core instruction set only
7/27/2019 ARM_RG
http://slidepdf.com/reader/full/armrg 7/30
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Evolution of the Architecture (2)
ARMv5TEJ (ARM926EJ-S) introduced: Better interworking between ARM and Thumb
Bottom bit of the address used to determine the ISA
DSP-focussed additional instructions
Jazelle-DBX for Java byte code interpretation in hardware
Some architecting of the virtual memory system
ARMv6K (ARM1136JF-S) introduced:
Media processing – SIMD within the integer datapath
Enhanced exception handling
Overhaul of the memory system architecture to be fully architected
Supported only 1 level of cache
ARMv7 rolled in a number of substantive changes:
Thumb-2* - variable length instruction set TrustZone*
Jazelle-RCT
Neon
7/27/2019 ARM_RG
http://slidepdf.com/reader/full/armrg 8/30
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Extensions to ARMv7
MPE – Multiprocessing Extensions
Added Cache and TLB Maintenance Broadcast for efficient MP
VE - Virtualization Extensions
Adds hardware support for virtualization:
2 stages of translation in the memory system
New mode and privilege level for holding an Hypervisor
With associated traps on many system relevant instructions
Support for interrupt virtualization
Combines with a System MMU
LPAE – Large Physical Address Extensions
Adds ability to address up to 40-bits of physical address space
7/27/2019 ARM_RG
http://slidepdf.com/reader/full/armrg 9/30
9
VFP – ARM’s Floating-point solution
VFP – “Vector Floating-point”
Vector functionality has been deprecated in favour of Neon
Described as a “coprocessor”
Originally a tightly-coupled coprocessor Executed instructions from ARM instruction stream via dedicated
interface
Now more tightly integrated into the CPU
Single and Double precision floating-point Fully IEEE compliant
Until VFPv3, implementations required support code for denorms
Alternative Flush to Zero handling of denorms also supported
Recent VFP versions:
VFPv3 – adding more DP registers (32 DP registers)
VFPv4 – adds Fused MAC and Half-precision support (IEEE754-2008)
7/27/2019 ARM_RG
http://slidepdf.com/reader/full/armrg 10/30
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ARM Architecture versions and products
Key architecture revisions and products:
ARMv1-ARMv3: largely lost in the mists of time
ARMv4T: ARM7TDMI – first Thumb processor
ARMv5TEJ(+VFPv2): ARM926EJ-S
ARMv6K(+VFPv2): ARM1136JF-S, ARM1176JFZ-S, ARM11MPCore – first Multiprocessing Core
ARMv7-A+VFPv3 Cortex-A8
ARMv7-A+MPE+VFPv3: Cortex-A5, Cortex-A9
ARMv7-A+MPE+VE+LPAE+VFPv4Cortex-A15
ARMv7-R : Cortex-R4, Cortex-R5
ARMv6-M Cortex–M0
ARMv7-M: Cortex-M3, Cortex-M4
7/27/2019 ARM_RG
http://slidepdf.com/reader/full/armrg 11/30
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ARMv7TDMI
Simple 3 stage pipeline
Fetch, Decode, Execute
Multiple cycles in execute stage
for Loads/Stores
Simple core
“Roll your own memory system”
7/27/2019 ARM_RG
http://slidepdf.com/reader/full/armrg 12/30
12
InstructionFetch
FETCH DECODE EXECUTE MEMORY WRITEBACK
ARM926EJ-S
5 stage pipeline single issue core Fetch, Decode, Execute, Memory, Writeback
Most common instructions take 1 cycle in each pipeline stage
Split Instruction/Data Level1 caches Virtually tagged
MMU – hardware page table walk based
Java DecodeStack
Management
Register Write
Java DecodeRegister
Read
Sum/Accumulate& Saturation
Memory Access
ComputePartial Products
Shift + ALU
Thumb Decode
ARM Decode
Register Decode
Register Decode
Register Read
Register Read
InstructionStream
7/27/2019 ARM_RG
http://slidepdf.com/reader/full/armrg 13/30
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ARM1176JZF-S
8 stage pipeline single issue
Split Instruction/Data Level1 caches Physically tagged
Two cycle memory latency
MMU – hardware page table walk based Hardware branch prediction
ALU and MAC PipelineI-Cache Access
+Dynamic
Branch Prediction
PF1 PF2Decode
+
StaticB
P
RStack
DEInstr Issue
+
Register
Read
ISS SH ALU SATWB
ex
MAC1 MAC2 MAC3
LSU Pipeline
LS
addDC1 DC2
WB
LS
7/27/2019 ARM_RG
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Cortex-A8
Dual Issue, in-order
10 stage pipeline (+ Neon Engine)
2 levels of cache – L1 I/D split, L2 unified
Aggressive Branch Prediction
NEON
Load and store
data queue
NEON
Instruction
Decode
Instruction Execute and Load/Store
E1 E3 E4 M1E2 M2 M3 N1 N6N2 N3 N4 N5E5
LS pipe 0 or 1
Instruction
Fetch
F1 F2F0 D1 D2 D3 D4
Instruction Decode
L3 memory system
BIU pipeline
L2 Data ArrayL2 Tag Array
L1 L2 L3 L4 L5 L6 L8
L1 data cache miss
L1 instruction cache miss
Branch mispredict penalty
NEON store data
Integer register writeback
NEON register writeback
Replay penalty
D0 E0
L9L7Embedded Trace Macrocell
T10T3T0 T4 T5 T6 T7 T8 T9T2T1 T11
M0
T13T12
MUL pipe 0
ALU pipe 0
ALU pipe 0
Integer ALU pipe
Integer MUL pipe
Integer shift pipe
Non-IEEE FP ADD pipe
Non-IEEE FP MUL pipe
IEEE FP engine
LS permute pipeL2 data
External trace port
L1 data
7/27/2019 ARM_RG
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15
Cortex-A9
Dual Issue, out-of-order core
MP capable – delivered as clusters of 1 to 4 CPUs
MESI based coherency scheme across L1 data caches
Shared L2 cache (PL310)
Integrated interrupt controller
7/27/2019 ARM_RG
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2.5Ghz in 28 HP process
12 stage in-order, 3-12 stage OoO pipeline
3.5 DMIPS/Mhz ~ 8750 DMIPS @ 2.5GHz
ARMv7A with 40-bit PA
Dynamic repartitioning Virtualization
Fast state save and restore
Move execution between cores/clusters
128-bit AMBA 4 ACE bus
Supports system coherency
ECC on L1 and L2 caches
2.5Ghz in 28 HP process
12 stage in-order, 3-12 stage OoO pipeline
3.5 DMIPS/Mhz ~ 8750 DMIPS @ 2.5GHz
ARMv7A with 40-bit PA
Dynamic repartitioning Virtualization
Fast state save and restore
Move execution between cores/clusters
128-bit AMBA 4 ACE bus
Supports system coherency
ECC on L1 and L2 caches
Cortex-A15 – Just Announced - Core Detail
F e t c h
F e t c h
D e c o d
e
D e c o d e
R e n a m
e
R e n a m
e
Simple Cluster 0Simple Cluster 0
Simple Cluster 1Simple Cluster 1
Multiply AccumulateMultiply Accumulate
12 Stage In-order pipeline
(Fetch, 3 Instruct ion
Decode, Rename)
ComplexComplex
ComplexComplex
Load & Store 0Load & Store 0
Load & Store 1Load & Store 1
3-12 Stage
out-of-order
pipeline
(capable of 8
issue)
Eagle Pipeline
64-bit/128bit AMBA4 in terface
7/27/2019 ARM_RG
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Just some basic lessons from experience
Architecture is part art, part science There is no one right way to solve any particular problem
….Though there are plenty of wrong ways
Decision involves balancing many different criteria
Some quantitative, some more subjective
Weighting those criteria is inherently subjective
Inevitably architectures have fingerprints of their architects
Hennessy & Patterson quantitative approach is excellent
But it is only a framework for analysis – a great toolkit
Computer science – we experiment using benchmarks/apps …but the set of benchmarks/killer applications is not constant
Engineering is all about technical compromise, and balancing factors
The art of Good Enough
7/27/2019 ARM_RG
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First Lesson – It’s all about Compatibility
Customers absolutely expect compatibility
Customers buy your roadmap, not just your products
The software they write is just expected to work
Trying to obsolete features is a long-term task
Issues from the real world:
Nobody actually really knows what their code uses
…and they’ve often lost the sources/knowledge
People don’t use features as the architect intended Bright software engineers come up with clever solutions
The clever solutions find inconvenient truths
Compatibility is with what the hardware actually does
Not with how you wanted it to be used
There is a thing called “de facto architecture”
7/27/2019 ARM_RG
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Second lesson – orthogonality
Classic computer texts tell you orthogonality is good
Beware false orthogonalities
ARM architecture R15 being the program counter
Orthogonality says you can do lots of wacky things using the PC
On a simple implementation, the apparent orthogonality is cheap
ARM architecture has “shifts with all data processing”
Orthogonality from original ARM1 pipeline
But the behaviour has to be maintained into the future
Not all useful control configurations come in powers of 2
Fear the words “It just falls out of the design”
True for today’s microarchitecture – but what about the next 20 years?
Try to only architect the functionality you think will actually be useful
Avoid less useful functionality that emerges from the micro-architecture
7/27/2019 ARM_RG
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20
Third Lesson – Microarchitecture led features
Few successful architectures started as architectures
Code runs on implementations, not on architectures
People buy implementations, not architectures
…IP licensing notwithstanding
Most architectures have “micro-architecture led features”
“it just fell out”
Optimisations based on first target micro-architecture
MIPS – delayed branch slots
ARM – the PC offset of 2 instructions
Made the first implementation cheaper/easier than the pure approach
…but becomes more expensive on subsequent implementations
Surprisingly difficult trade-off
Short-term/long-term balance
Meeting a real need sustainably vs overburdening early implementations
7/27/2019 ARM_RG
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Fourth Lesson: New Features
Successful architectures get pulled by market forces
Success in a particular market adds features for that market
Different points of success pull successively over time
Solutions don’t necessarily fit together perfectly
Unsuccessful architectures don’t get same pressures
…which is probably why they appear so clean!
Be very careful adding new features:
Easy to add, difficult to remove
Especially for user code
“Trap and Emulate” is an illusion of compatibility
Performance differential is too great for most applications
7/27/2019 ARM_RG
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Lessons on New Features
If a feature requires a combination of hardware and specific software…
…be afraid – development timescales are different
Be very afraid of language specific features All new languages appear to be different…
….but very rarely are
New features rarely stay in the application space you expect….
…Or want – architects are depressingly powerless
Customers will exploit whatever they find
So worry about the general applicability of that feature
Assume it has to go everywhere in the roadmap
Avoid solving this years problem in next year’s architecture
... Next year’s problem may be very different
Point solutions often become warts – all architectures have them
If the feature has a shelf-life, plan for obsolence
Example: Jazelle-DBX in the ARM architecture
7/27/2019 ARM_RG
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Thoughts on instruction set design
What is the difference between an instruction and a micro-op? RISC principles said they were the same
Very few PURE RISC architectures exist today
ARM doesn’t pretend to be “hard-core” RISC
…I’ll claim some RISC credentials
Choice of Micro-ops is micro-architecture dependent
An architecture should be micro-architecturally independent
Therefore mapping of instructions to micro-ops is inherently “risky”
Splitting instructions easier than fusing instructions
If an instruction can plausibly be done in one block, might be right to express it Even if some micro-architectures are forced to split the instruction
But, remember legacy lasts for a very long time
Imagine explaining the instructions in 5 years time
Avoid having instructions that provide 2 ways to do much the same thing
Everyone will ask you which is better - lots of times…
If it feels a bit clunky when you first design it…
…. it won’t improve over time
7/27/2019 ARM_RG
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Final point – Architecture is not enough
Not Enough to ensure perfect “ write once, run everywhere”
Customers expectations of compatibility go beyond architected behaviour
People don’t write always code to the architecture
…and they certainly can’t easily test it to the architecture
ARM is developing tools to help address this
Architecture Envelope Models – a suite of badly behaved legal implementations
The architecture defines whether they are software bugs or hardware incompatibilities
…allows you to assign blame (and fix the problem consistently)
Beware significant performance anomalies between architectural compliant cores
If I buy a faster core, I want it to go reliably faster…without recompiling
Multi-processing introduce huge scope for functional differences from timing
Especially in badly synchronised code
Concurrency errors
BUT THE ARCHITECTURE IS A STRATEGIC ASSET
7/27/2019 ARM_RG
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History – the Passage of Time
7/27/2019 ARM_RG
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Microprocessor Forum 1992
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Count the Architectures (11)
ARMMIPS29K
PA
NVAX
N32x16
88xxx
Alpha
x86
i960
SPARC
x86
x86 x86
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The Survivors – At Most 2
ARMMIPS
x86
SPARC
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What Changed?
Some business/market effects
Some simple scale of economy effects
Some technology effects
Adoption and Ecosystem effects
It wasn’t all technology – not all of the disappearing
architectures were bad
Not all survivors are good!
Not all good science succeeds in the market!
Never forget “Good enough”
7/27/2019 ARM_RG
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30
Thank you
Questions