ARMアーキテクチャについて
ARMアーキテクチャは、毎年出荷数十億ARMベースのデバイスで、今日世界で最も人気のあるプロセッサアーキテクチャの一つです。
A、RおよびM.：3つのアーキテクチャのプロファイルがあります。
| - プロフィール（アプリケーション） |
R-プロファイル（リアルタイム） |
M-プロファイル（マイクロコントローラ） |
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まだいくつかの基本機能を共有しながら、これらの3つのプロファイルは、ARMアーキテクチャが異なる使用例ニーズに合わせて調整することを可能にします。
注意： アームのCortexは、ARMのプロセッサIPの提供のために使用しているブランド名です。パートナーは、ARMアーキテクチャを使用して、他のプロセッサのブランドを提供します。
これは、1つのエンドデバイスは、複数のARMプロセッサを使用すること、及びこれらのプロセッサは、異なるアーキテクチャプロファイルを実装することがよくあります。たとえば、次の図では、現代のスマートフォンで見つけるかもしれないものを示しています。
この例ではスマートフォンは、AndroidのようなリッチOSを実行して、Aプロファイルのプロセッサを有しています。電話も接続性を提供するために、セルラーモデムが必要になります。モデムのこのタイプは、一般にRプロファイルARMプロセッサに基づいています。携帯電話はまた、システムの電力管理などの操作を処理する、電話でいくつかのMプロファイルのプロセッサを含みます。
このガイドでは、我々は唯一のA-プロファイルのアーキテクチャとその最新バージョン、Armv8-Aを見ていきます。
注： また、上記の図は、のSecurCore、追加のセキュリティ機能を備えたMプロファイルのプロセッサを示しています。SecurCoreプロセッサは、一般的に、スマートカードに使用されています。
Architecture and micro-architecture
Architecture does not tell you how a processor is built and actually works. The build and design of a processor is referred to as micro-architecture. Micro-architecture tells you how a particular processor works.
Micro-architecture includes things like:
- Pipeline length and layout.
- Number and sizes of caches.
- Cycle counts for individual instructions.
- Which optional features are implemented.
For example, Cortex-A53 and Cortex-A72 are both implementations of the Armv8-A architecture. This means that they have the same architecture, but they have very different micro-architectures, as shown in the following image:
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| Target |
Optimized for power efficiency | Optimized for performance |
| Pipeline | 8 stages In-order |
15+ stages Out-of-order |
| Caches | L1 I cache: 8KB - 64KB L1 D cache: 8KB - 64KB L2 cache: optional, up to 2MB |
L1 I cache: 48KB fixed L1 D cache: 48KB fixed L2 cache: mandatory, up to 2MB |
Software that is architecturally-compliant can run on either the Cortex-A53 or Cortex-A72 without modification, because they both implement the same architecture.
Development of the Arm architecture
The Arm architecture is developed over time and each version builds on what came before.
You will commonly see the architecture referred to as something like:
Armv8-A
This means Version 8 of the architecture, for A-Profile.
Or, in short form:
v8-A
This figure shows the development of the Arm architecture from version 5 to version 8, with the new features that were added each time. In this guide, we will only look at Armv8-A.
Armv8-A was a major milestone for Arm. Up to and including Armv7-A/R, the Arm architecture was a 32-bit architecture. Armv8-A is a 64-bit architecture, although it still supports 32-bit execution to provide backwards compatibility for legacy software (for example, v7, v6, and v5).
We will not discuss all the features listed on the diagram here, but we will introduce them in later topics.
Other Arm architectures
The Arm architecture is the best known Arm specification, but it is not the only one. Arm has similar specifications for many of the components that make up a modern System-on-Chip (SoC). This diagram provides some examples:
Generic Interrupt Controller
The Generic Interrupt Controller (GIC) specification is a standardized interrupt controller for use with Armv7-A/R and Armv8-A/R.
System Memory Management Unit
A System Memory Management Unit (SMMU or sometimes IOMMU) provides translation services to non-processor masters.
Generic Timer
The Generic Timer provides a common reference system count to all the processors in the system. These provide timer functionality, which is used for things like the operating system’s scheduler tick. The Generic Timer is part of the Arm architecture, but the system counter is a system component.
Server Base System Architecture and Trusted Base System Architecture
The Server Base System Architecture (SBSA) and Trusted Base System Architecture (TBSA) provide system design guidelines for SoC developers.
Advanced Microcontroller Bus Architecture
The Advanced Microcontroller Bus Architecture (AMBA) family of bus protocols control how components in an Arm-based system are connected, and the protocols on those connections.
Understanding Arm documentation
Arm provides a lot of documentation to developers. We will explain where to find documentation and other information for developing on Arm.
Where is the documentation?
The Arm developer website - This is where you can download the Arm architecture and processor manuals.
The Arm community is where you can ask development questions, and find articles and blogs on specific topics from Arm experts.
Which document describes what?
- Each Arm Architecture Reference Manual (Arm ARM) describes an architecture specifications. An Arm ARM is relevant to any implementation of that architecture.
- Each Arm Cortex processor has a Technical Reference Manual (TRM). The TRM describes the features specific to that processor. In general, the TRMs will not repeat any information given in the Arm ARMs.
- Each Arm Cortex processor also has a Configuration or Integration Manual (CIM). The CIM describes how to integrate the processor into a system. Generally, this information is only relevant to SoC designers.
Note: The CIMs are only available to IP licensees. The TRMs are available to download from developer.arm.com without a license.
So, what does this mean for me?
If you are looking for information on a particular processor, you might need to refer to several different documents. Here we can see the different documents you might need to use with a Cortex-A75 processor. 
Cortex-A75 implements ARMv8.2-A, a GICv4 CPU interface and AMBA bus interfaces, so you would need to refer to separate documents for each element. Plus, you would need to refer to the documents detailing the micro-architecture.
If you are working with an existing SoC, you will also use documentation from the SoC’s manufacturer. This documentation is typically referred to as a datasheet. The datasheet gives information specific to that SoC.
What information will I find in each document?
| Architecture | Micro-architecture | SoC Datasheet | ||||
| Arm ARM |
GIC specifications | AMBA specifications | TRM | CIM | ||
| Instruction set | X | |||||
| Instruction cycle timings | X | |||||
| Architectural registers | X | X | ||||
| Processor specific registers | X | |||||
| Memory model | X | |||||
| Exception model | X
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| Support for optional features | X
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X (some might be synthesis choice) |
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| Size of caches/TLBs | X | |||||
| Power management | X | |||||
| Bus ports | X | X | ||||
| All legal bus transactions | X | |||||
| Bus transactions generated by processor | X | |||||
| Memory map | X | |||||
| Peripherals | X | |||||
| Pin-out of SoC | X | |||||
Differences between reference manuals and user guides
The documents we have looked at so far, Arm ARMs, TRMs and CIMs, are reference manuals. This means that they do not provide guidance on how to use the processor. For example, the Arm ARM does not have a section on how to turn on an MMU.
This structure is deliberate, and is intended to keep a clear divide between the technical detail of what the architecture requires, which is found in reference manuals, and documents that provide more general guidance, such as this guide. Some general guidance documents will introduce concepts, and others provide instructions for you to follow.