Preface

In 2013, ARM released its 64-bit ARMv8 architecture, the first major change to the ARM architecture since ARMv7 in 2007, and the most fundamental and far reaching change since the original ARM architecture was created.

Development of the architecture has continued for some years. Early versions were being used before the Cortex-A Series Programmer’s Guide for ARMv7-A was first released. The first of the Programmer’s Guide series from ARM, it post-dated the introduction of the 32-bit ARMv7 architecture by some years. Almost immediately there were requests for a version to cover the ARMv8 architecture. It was intended from the outset that a guide to ARMv8 should be available as soon as possible.

This book was started when the first versions of the ARMv8 architecture were being tested and codified. As always, moving from a system that is known and understood to something new and unknown can present a number of problems. The engineers who supplied information for the present book are, by and large, the same engineers who supplied the information for the original Cortex-A Series Programmer’s Guide. This book has been made richer by their observations and insights as they use, and solve the problems presented by the new architecture.

The Programmer’s Guides are meant to complement, rather than replace, other ARM documentation available, such as the Technical Reference Manuals (TRMs) for the processors themselves, documentation for individual devices or boards or, most importantly, the ARM Architecture Reference Manual (the ARM ARM). They are intended to provide a gentle introduction to the ARM architecture, and cover all the main concepts that you need to know about, in an easy to read format, with examples of actual code in both C and assembly language, and with hints and tips for writing your own code. 

It might be argued that if you are an application developer, you do not need to know what goes on inside a processor. ARM Application processors can easily be regarded as black boxes which simply run your code when you say go. Instead, this book provides a single guide, bringing together information from a wide variety of sources, for those programmers who get the system to the point where application developers can run applications, such as those involved in ASIC verification, or those working on boot code and device drivers.

During bring-up of a new board or System-on-Chip (SoC), engineers may have to investigate issues with the hardware. Memory system behavior is among the most common places for these to manifest, for example, deadlocks where the processor cannot make forward progress because of memory system lock. Debugging these problems requires an understanding of the operation and effect of cache or MMU use. This is different from debugging a failing piece of code.

In a similar vein, system architects (usually hardware engineers) make choices early in the design about the implementation of DMA, frame buffers and other parts of the memory system where an understanding of data flow between agents in required. In this case it is difficult to make sensible decisions about it if you do not understand when a cache will help you and when it gets in the way, or how the OS will use the MMU. Similar considerations apply in many other places.

This is not an introductory level book, nor is it a purely technical description of the architecture and processors, which merely state the facts with little or no explanation of ‘how’ and ‘why’. ARM and all who have collaborated on this book hope it successfully navigates between the two extremes, while attempting to explain some of the more intricate aspects of the architecture.

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