gcc: Some Assembly Required

There was a time when you pretty much had to be an assembly language programmer to work with embedded systems. Yes, there have always been high-level languages available, but it took improvements in tools and processors for that to make sense for anything but the simplest projects. Today, though, high-quality compilers are readily available for a lot of languages and even an inexpensive CPU is likely to outperform even desktop computers that many of us have used.

So assembly language is dead, right? Not exactly. There are several reasons people still want to use assembly. First, sometimes you need to get every clock cycle of performance out of a chip. It can be the case that a smart compiler will often produce better code than a person will write off the cuff. However, a smart person who is looking at performance can usually find a way to beat a compiler’s generated code. Besides, people can make value trades of speed versus space, for example, or pick entirely different algorithms. All a compiler can do is convert your code over as cleverly as possible.

Besides that, some people just like to program in assembly. Morse code, bows and arrows, and steam engines are all archaic, but there are still people who enjoy mastering them anyway. If you fall into that category, you might just want to write everything in assembly (and that’s fine). Most people, though, would prefer to work with something at a higher level and then slip into assembly just for that critical pieces. For example, a program might spend 5% of its time reading data, 5% of its time writing data, and 90% of the time crunching data. You probably don’t need to recreate the reading and writing parts. They won’t go to zero, after all, and so even if you could cut them in half (and you probably can’t) you get a 2.5% boost for each one. That 90% section is the big target.

The Profiler

Sometimes it is obvious what’s taking time in your programs. When it isn’t, you can actually turn on profiling. If you are running GCC under Linux, for example, you can use the -pg option to have GCC add profiling instrumentation to your code automatically. When you compile the code with -pg, it doesn’t appear to do anything different. You run your program as usual. However, the program will now silently write a file named gmon.out during execution. This file contains execution statistics you can display using gprof (see partial output below). The function b_fact takes up 65.9% of CPU time.


If you don’t have a profiling option for your environment, you might have to resort to toggling I/O pins or writing to a serial port to get an idea of how long your code spends doing different functions. However you do it, though, it is important to figure it out so you don’t waste time optimizing code that doesn’t really affect overall performance (this is good advice, by the way, for any kind of optimization).


If you start with a C or C++ program, one thing you can do is ask the compiler to output assembly language for you. With GCC, use a file name like test.s with the -o option and then use -S to force assembly language output. The output isn’t great, but it is readable. You can also use the -ahl option to get assembly code mixed with source code in comments, which is useful.

You can use this trick with most, if not all, versions of GCC. Of course, the output will be a lot different, depending. A 32-bit Linux compiler, a 64-bit Linux compiler, a Raspberry Pi compiler, and an Arduino compiler are all going to have very different output. Also, you can’t always figure out how the compiler mangles your code, so that is another problem.

If you find a function or section of code you want to rewrite, you can still use GCC and just stick the assembly language inline. Exactly how that works depends on what platform you use, but in general, GCC will send a string inside asm() or __asm__() to the system assembler. There are rules about how to interact with the rest of the C program, too. Here’s a simple example from the a GCC HOWTO document (from a PC program):

__asm__ ("movl %eax, %ebx\n\t"
"movl $56, %esi\n\t"
"movl %ecx, $label(%edx,%ebx,$4)\n\t"
"movb %ah, (%ebx)");

You can also use extended assembly that lets you use placeholders for parts of the C code. You can read more about that in the HOWTO document. If you prefer Arduino, there’s a document for that, too. If you are on ARM (like a Raspberry Pi) you might prefer to start with this document.


You may never need to mix assembly language with C code. But if you do, it is good to know it is possible and maybe not even too difficult. You do need to find what parts of your program can benefit from the effort. Even if you aren’t using GCC, there is probably a way to mix assembly and your language, you just have to learn how. You also have to learn the particulars of your platform.

On the other hand, what if you want to write an entire program in assembly? That’s even more platform-specific, but we’ll look at that next time.

Hackaday Prize Entry: An Interface For The Headless Linux System

Connecting a headless Raspberry Pi to a wireless network can be quite a paradoxical situation. To connect it to the network, you need to open an SSH connection to configure the wireless port. But to do so, you need a network connection in the first place. Of course, you can still get command-line access using a USB-to-UART adapter or the Pi’s ethernet port – if present – but [Arsenijs] worked out a much more convenient solution for his Hackaday Prize entry: The pyLCI Linux Control Interface.

His solution is a software framework written in Python that uses a character display and buttons to make a simple hardware interface. This allows you to configure all important aspects of a Raspberry Pi – or any other Linux SBC – from a tidily organized click-and-scroll menu. [Arsenijs] implemented a whole bunch of useful tools: There’s a network tool to scan and connect to WiFi networks. A systemctl tool that lets you manage the services running on the system, which is especially helpful when you need to restart a stuck service. A partition tool helps with viewing and unmounting mass storage devices. He’s even planning to add a filesystem browser.

With his Open Source project, [Arsenjs] aims to shorten the development time for embedded projects by taking out the efforts of implementing the basic interface functions from scratch. Indeed, there are countless scenarios, where a basic display interface can be of great value. Given the great project documentation and the fact that this can work with virtually any Arduino or Raspberry Pi LCD-pushbutton-hat or shield, we’re sure this is going to be used a lot. Enjoy the video!

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Meter All The Phases: Three Phase Energy Meter With OpenWrt

Keeping track your overall electricity usage is a good thing, and it’s even better if you know where all the kilowatt-hours are going. [Anurag Chugh’s] house has the three phases coming from the electrical distribution box tidily organized: One for the lighting and fans, one for household appliances, and one for the hot water supply. To monitor and analyze the electrical fingerprint of his house, [Anurag] installed a 3 phase energy meter and hooked it up to the internet.

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BeagleBone Green, Now Wireless

Over the past few years, the BeagleBone ecosystem has grown from the original BeagleBone White, followed two years later by the BeagleBone Black. The Black was the killer board of the BeagleBone family, and for a time wasn’t available anywhere at any price. TI has been kind to the SoC used in the BeagleBone, leading to last year’s release of the BeagleBone Green, The robotics-focused BeagleBone Blue, and the very recent announcement of a BeagleBone on a chip. All these boards have about the same capabilities, targeted towards different use cases; the BeagleBone on a Chip is a single module that can be dropped into an Eagle schematic. The BeagleBone Green is meant to be the low-cost that plays nicely with Seeed Studio’s Grove connectors. They’re all variations on a theme, and until now, wireless hasn’t been a built-in option.

This weekend at Maker Faire, Seeed Studio is showing off their latest edition of the BeagleBone Green. It’s the BeagleBone Green Wireless, and includes 802.11 b/g/n, and Bluetooth 4.1 LE.

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This Teddy Bear Steals Your Ubuntu Secrets

Ubuntu just came out with the new long-term support version of their desktop Linux operating system. It’s got a few newish features, including incorporating the “snap” package management format. One of the claims about “snaps” is that they’re more secure — being installed read-only and essentially self-contained makes them harder to hack across applications. In principle.

[mjg59] took issue with their claims of increased cross-application security. And rather than just moan, he patched together an exploit that’s disguised as a lovable teddy bear. The central flaw is something like twenty years old now; X11 has no sense of permissions and any X11 application can listen in on the keyboard and mouse at any time, regardless of which application the user thinks they’re providing input to. This makes writing keylogging and command-insertion trojans effortless, which is just what [mjg59] did. You can download a harmless version of the demo at [mjg59]’s GitHub.

This flaw in X11 is well-known. In some sense, there’s nothing new here. It’s only in light of Ubuntu’s claim of cross-application security that it’s interesting to bring this up again.


And the teddy bear in question? Xteddy dates back from when it was cool to display a static image in a window on a workstation computer. It’s like a warmer, cuddlier version of Xeyes. Except it just sits there. Or, in [mjg59]’s version, it records your keystrokes and uploads your passwords to shady underground characters or TLAs.

We discussed Snappy Core for IoT devices previously, and we think it’s a step in the right direction towards building a system where all the moving parts are only loosely connected to each other, which makes upgrading part of your system possible without upgrading (or downgrading) the whole thing. It probably does enhance security when coupled with a newer display manager like Mir or Wayland. But as [mjg59] pointed out, “snaps” alone don’t patch up X11’s security holes.

Modifying a Raspberry Pi 2 To Fit In Small Places

Still can’t get your hands on a Pi Zero? We know. Why not de-solder a few headers from a Raspberry Pi 2 to fit in your next project instead? Using a pair of 2.5″ HDD enclosures, [nodenet] made a mini linux laptop using the Raspberry Pi 2 — It even has a touch screen, and features a 1000mAh battery!

All in all it cost him about $120 for all the components, but before you JrECiM0rush out to make your own, you will need pretty good skills with a soldering iron to successfully downsize your Raspberry Pi 2. The modifications require removing both USB ports, the Ethernet plug, the GPIO pins, the HDMI port, the A/V jack and the camera connectors.

He used a combination of a mini hacksaw, and a soldering iron to remove all the components — what you’re left with his a business card sized computer — but the real fun part is re-attaching all the components with leads.

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Arduino Comes To The Raspberry Pi, Linux ARM Devices

Arduino is the perfect introduction to microcontrollers and electronics. The recent trend of powerful, cheap, ARM-based single board Linux computers is the perfect introduction to computer science, programming, and general Linux wizardry. Until now, though, Arduino and these tiny ARM computers have been in two different worlds. Now, finally, there are nightly builds of Arduino IDE on the Raspberry Pi and other single board Linux computers.

The latest Arduino build for ARM Linux popped up on the arduino.cc downloads page early this week. This is the result of an incredible amount of work from dozens of open source developers across the Arduino project. Now, with just a simple download and typing ‘install’ into a terminal, the Arduino IDE is available on just about every single board Linux computer without having to build the IDE from source. Of course, Arduino has been available on the Raspberry Pi for a very long time with sudo apt-get install arduino, but this was an older version that cannot work with newer Arduino boards.

Is this distribution of the Arduino IDE the same you would find on OS X and Windows? Yep, everything is the same:

While this is really just arduino.cc improving their automated build process and putting a link up on their downloads page, it does make it exceptionally easy for anyone to set up a high school electronics lab. The Raspberry Pi is almost a disposable computing device, and combining it with Arduino makes for a great portable electronics lab.