ARM Community: Software Enablement - ARM Community

Finding one's way through references to ARM processors is not always obvious.
This article is the first of a series on ARM fundamentals that will introduce various topics to help you get more familiar with the ARM architecture. It aims at helping you to better understand ARM processors, starting with explaining how they are named, and then showing how knowing your processor matters by introducing a few of their recent features.

If you are curious about what is in your pretty electronic device or are a developer willing to understand how to start getting the best out of your processor, you may find some useful information here. The second part of the article may be technically a bit more challenging than the first, but don't worry! The few code samples are only concrete examples used to illustrate the explanations. The specific details are not necessary to understand the global picture.

The first step is to understand how ARM processors are referenced: it certainly sounds nice, but what is this "dual Cortex-A9, based on ARMv7" in your super-phone?

Web applications like Gmail, Google Maps and Google Docs use JavaScript extensively, and the user experience is greatly improved on systems with fast, efficient JavaScript engines. In 2008, this motivated Google to create the V8 JavaScript engine project.

V8 is now, on modern benchmarks, the fastest JavaScript engine available. Rather than interpreting JavaScript as the old engines used to do, V8 uses a Just-In-Time compiler to produce and execute native instructions tailored to the processor on which it is running. The generated instructions are cached, avoiding the overhead of repeated code generation, and deleted when no longer needed.

V8 is now the core technology used in a number of important applications. It is the JavaScript engine used in Google's super-f...

Every practical general-purpose computing architecture has a mechanism of conditionally executing some code. Such mechanisms are used to implement the if construct in C, for example, in addition to several other cases that are less obvious.

ARM, like many other architectures, implements conditional execution using a set of flags which store state information about a previous operation. I intend, in this post, to shed some light on the operation of these flags. Of course, the Architecture Reference Manual is the definitive source of information, so if you need to know about a specific corner-case that I do not cover here, that is where you need to look.

Consider a simple fragment of C code:

A compiler might implement that structure as follows:

The last two instructions are of particular interest. The cmp (compare) instruction compares r4 with 0, and the bne instruction is simply a b (branch) instruction that executes if the result of the cmp instruction was "not equal". The code works because cmp sets some global f...