The ARM architecture – yesterday, today, and tomorrow
The Special Bit
Another special feature in the instruction set is the S bit, which serves several purposes. For one, it helps to achieve more granular control of conditional execution. Normally, the processor changes the condition flags for each command; for example, if the result of a computation is 0, it sets the zero flag. In ARM, this only happens if the S bit is set, thus keeping the condition flags' status independent of current computations.
The S bit can also be used for controlling the processor modes and their specific registers. Setting the S bit thus allows access to the user mode registers. If a program with the S bit set writes to the program counter, the processor automatically switches to the previous mode. In combination with a load multiple, a programmer can thus implement a very elegant approach for returning from an interrupt.
Neon, Thumb, Jazelle
The ARM design supports easy extensibility by adding up to 16 coprocessors, which are controlled by special ARM coprocessor instructions, thus supporting, for example, floating-point computations. If no coprocessor responds to a statement of this kind, an exception allows a simple emulation in software. Other notable and commonly used extensions are a memory management unit and a media processing unit by the name of Neon.
Besides the actual ARM instruction set, most ARM processors, support up to three other instruction sets: First, there are now two versions of Thumb mode, allowing a higher code density through the use of 16-bit instructions.
Whereas the first version of Thumb accessed only half of the registers and had to switch back to the ARM instruction set to handle exceptions, its successor, Thumb 2, allows 16- and 32-bit instructions and waives most of the restrictions, thus still allowing performance comparable to ARM mode despite the higher code density. Additionally, the Jazelle instruction set also provides hardware acceleration for Java bytecode; however, ARM has reduced the scope of support in recent versions [4].
Multicore
When the clock speed is increased, energy consumption grows faster than computing speed. Thus, energy efficiency decreases as clock speed increases; this is a problem for mobile devices in particular. The use of multiple cores partly solves the problem because they achieve the same number of computations per unit of time at a lower clock speed, thus improving energy efficiency. Multicore CPUs are thus interesting for ARM architecture.
A variety of solutions come from both ARM itself – for example, the MP core with up to four cores – and by architecture licensees. Two of the biggest challenges in the design of multicore CPUs are cache coherence and interrupt distribution. ARM only offers one IP block for the ARM11 that contains the cores, the cache coherency logic, and interrupt distribution.
Starting with the Cortex A9, ARM has sold these components as separate IP blocks to allow SoC designers more freedom of design. For holders of an architecture license, this freedom is even greater, but the details are generally sparsely documented, or not at all, publicly.
Buy this article as PDF
(incl. VAT)
Buy Linux Magazine
Subscribe to our Linux Newsletters
Find Linux and Open Source Jobs
Subscribe to our ADMIN Newsletters
Support Our Work
Linux Magazine content is made possible with support from readers like you. Please consider contributing when you’ve found an article to be beneficial.
News
-
Canonical Bumps LTS Support to 12 years
If you're worried that your Ubuntu LTS release won't be supported long enough to last, Canonical has a surprise for you in the form of 12 years of security coverage.
-
Fedora 40 Beta Released Soon
With the official release of Fedora 40 coming in April, it's almost time to download the beta and see what's new.
-
New Pentesting Distribution to Compete with Kali Linux
SnoopGod is now available for your testing needs
-
Juno Computers Launches Another Linux Laptop
If you're looking for a powerhouse laptop that runs Ubuntu, the Juno Computers Neptune 17 v6 should be on your radar.
-
ZorinOS 17.1 Released, Includes Improved Windows App Support
If you need or desire to run Windows applications on Linux, there's one distribution intent on making that easier for you and its new release further improves that feature.
-
Linux Market Share Surpasses 4% for the First Time
Look out Windows and macOS, Linux is on the rise and has even topped ChromeOS to become the fourth most widely used OS around the globe.
-
KDE’s Plasma 6 Officially Available
KDE’s Plasma 6.0 "Megarelease" has happened, and it's brimming with new features, polish, and performance.
-
Latest Version of Tails Unleashed
Tails 6.0 is based on Debian 12 and includes GNOME 43.
-
KDE Announces New Slimbook V with Plenty of Power and KDE’s Plasma 6
If you're a fan of KDE Plasma, you'll be thrilled to hear they've announced a new Slimbook with an AMD CPU and the latest version of KDE Plasma desktop.
-
Monthly Sponsorship Includes Early Access to elementary OS 8
If you want to get a glimpse of what's in the pipeline for elementary OS 8, just set up a monthly sponsorship to help fund its continued existence.