The ARM Microprocessor: A Little British Success Story Michelle Nabavian V Microprocessors Professor Robert Dewar Spring 2002

2 The ARM Chip Meant to be “MIPS for the masses” RISC load/store design Simple originally with a short 3-stage pipeline operating in either big- or little-endian mode Leading provider of 16/32-bit embedded RISC microprocessor solutions High performance Low cost Power efficient Established as standard in wireless communications Dominates mobile telephone market today

3 ARM: Then and Now Original ARM ARM1, 2 and 3 32-bit CPU 26-bit addressing ARM today ARMv5 instruction set and architecture is being developed and widely used This slide set mostly concentrates on the ARM10 family of processors Completely 32 bits Though it has 26 bit modes for compatibility with previous models

4 The Instruction Set It is important to increase chip’s capabilities through efficient code Strengths Dense code unlike other load/store processors Shortcomings Low clock rate Relatively short pipeline 3-6 stages depending on the model

5 Code Efficiency and Density Density achieved through Thumb 16-bit instruction set Instruction predication Improved branch prediction A barrel shifter Other extensions include DSP instruction set Jazelle technology FOR MORE INFO...

6 The Thumb Instruction Set 16-bit version of the ARM A low cost solution for code density Provides memory savings of up to 35% over equivalent 32-bit code Recodes a subset of ARM instructions into 16 bits Decoded to 32-bit instructions with no penalty 32-bit code can be mixed with 16-bit code when the full instruction set is needed Retains access to full 32-bit address space

7 Instruction Predication Instruction predication introduced in microprocessors by ARM All instructions are predicated using a 4-bit condition code Predicate bits suggest whether the current instruction should or should not be executed Memory Disambiguation: Propagation One bit in each instruction indicates whether the condition codes should be set Prevents some intervening instructions from changing condition codes Eliminates many branches and speeds execution

8 Branch Prediction The prefetch unit Responsible for grabbing instructions from the memory system that are processed as required by the integer core A prefetch buffer holds up to three instructions and allows for accurate branch prediction Provides branch target addresses to later stages of the pipeline When a branch is predicted as not taken, it can be removed entirely from the instruction stream The target address of the branch is still calculated in case the prediction is incorrect

9 More About Branches Branch folding Branch removal from instruction stream based on a prediction Substitution with predicted next instruction Condition codes of branch are “folded” into next predicted instruction Branch itself takes zero cycles Note: The branch prediction mechanism is static (uses no history information) Conditional forward branches are predicted as not taken while conditional backward branches are predicted as taken Mispredicted branches have a 3-cycle penalty

10 The Barrel Shifter Operates on the second operand of most ALU (Arithmetic Logic Unit) operations Allows shifts to be combined with most operations as well as index registers for addressing Can combine two or more instructions into one Used for decoding and scaling operations

11 DSP and Jazelle Extensions DSP instruction set A set of arithmetic instructions for DSP applications For systems that require flexibility of a microcontroller as well as data-processing capabilities of a DSP (Digital Signal Processor) Instruction set offers 16-bit and 32-bit arithmetic capabilities in addition to the existing capabilities of the CPU Jazelle technology Enables direct execution of Java byte-code Provides developers freedom to run Java code alongside other applications on a single chip Offers higher performance and reduced power consumption

12 VFP Coprocessor Adds full vector floating point operations to the ARM core Accompanied by bit registers which can be loaded, stored and operated on for vector-vector or vector-scalar operations Tools like Matlab can be used to derive the application code Offers increased performance for imaging applications Scaling and 2D/3D transforms Font generation Digital filters

13 Power-Saving Modes ARM10 family includes two new power- saving modes for lower power consumption, an important feature for low power embedded applications NAP: power down core (preserves state in caches) SLEEP: power down entire chip (state must be saved in memory) Importance of a low-power processor seen as demand for performance skyrockets due to high-security multimedia applications Low power processors offer fast and inexpensive way to reduce consumption

14 Dominating the Mobile Market FOR MORE INFO ARM-architecture CPU cores account for a 70% share of the mobile telephone market Palm and Microsoft plan to use the CPU core for their newest PDAs (Personal Digital Assistants) simply because they know it will be the most popular and abundantly used in the future

15 A Stable Architecture Minimal variations to the architecture and a stable instruction set Strictly managed instruction set, only steadily expanded to meet embedded applications needs, and all the time maintaining downward compatibility Made it possible for many OS and development tools to support ARM CPUs Ideal for cell phones and digital home appliances Simple maintenance for large number of models

16 Attracting Licensees Convinced integrated circuit (IC) manufacturers to adopt the core Internal bus disclosure Permitting licensees to develop a wide variety of IC chips using the same CPU core Cooperative activities with semiconductor firms Architecture licenses with Intel and Motorola Licensees gain design and manufacturing rights Intel’s StrongARM and newer Xscale have additional Intel coprocessor instructions only used by Intel Next generation ARM architecture will be jointly developed

17 An Expanding Foundry Program Program builds a 3-way partnership between ARM, an approved silicon foundry and an original equipment manufacturer ARM provides design kits for its microprocessor cores, leaving the manufacturing to foundry partners Increases access to ARM technology Assures availability of design and manufacturing resources Lowers development costs and accelerates time-to- market

18 Targeting New Markets Growing markets include: Networking Consumer entertainment Storage Imaging Automotive Security Industrial Control Success is mostly due to a reputation of being low power and high performance

19 The Future Future processors based on the ARMv6 architecture Co-developed with Intel and Texas Instrument Design targeted towards new markets and next generation applications Maintenance of backwards compatibility is important As well as the slogan of higher levels of performance and maintained power efficiency A continuing expansion of foundry program, licenses, and partnerships Ensures success introduction of new innovations in the architecture