Bootstrapping An MSDOS Assembler With Batch Files

You have a clean MSDOS system, and you need to write some software for it. What do you do? You could use debug, of course. But there are no labels so while you can get machine code from mnemonics, you’ll still need to figure out the addresses on your own. That wasn’t good enough for [mniip], who created an assembler using mostly batch files. There are a few .COM files and it looks as if the first time you use debug to create those, but there’s also source you can assemble on subsequent builds with the assembler.

Why? We aren’t entirely sure. But it is definitely a hack. The technique sort of reminded us of our own universal cross assembler — sort of.

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Creating Black Holes: Division By Zero In Practice

Dividing by zero — the fundamental no-can-do of arithmetic. It is somewhat surrounded by mystery, and is a constant source for internet humor, whether it involves exploding microcontrollers, the collapse of the universe, or crashing your own world by having Siri tell you that you have no friends.

It’s also one of the few things gcc will warn you about by default, which caused a rather vivid discussion with interesting insights when I recently wrote about compiler warnings. And if you’re running a modern operating system, it might even send you a signal that something’s gone wrong and let you handle it in your code. Dividing by zero is more than theoretical, and serves as a great introduction to signals, so let’s have a closer look at it.

Chances are, the first time you heard about division itself back in elementary school, it was taught that dividing by zero is strictly forbidden — and obviously you didn’t want your teacher call the cops on you, so you obeyed and refrained from it. But as with many other things in life, the older you get, the less restrictive they become, and dividing by zero eventually turned from forbidden into simply being impossible and yielding an undefined result.

And indeed, if a = b/0, it would mean in reverse that a×0 = b. If b itself was zero, the equation would be true for every single number there is, making it impossible to define a concrete value for a. And if b was any other value, no single value multiplied by zero could result in anything non-zero. Once we move into the realms of calculus, we will learn that infinity appears to be the answer, but that’s in the end just replacing one abstract, mind-boggling concept with another one. And it won’t answer one question: how does all this play out in a processor? Continue reading “Creating Black Holes: Division By Zero In Practice”

Single Board Computer Plays Nice with NVIDIA GPU

It’s about convenience when it comes to single board computers. The trade-off of raw compute power for size means the bulk of them end up being ARM based, but there are a few exceptions like the x86 based Udoo Ultra. The embedded Intel 405 GPU on the Udoo Ultra is better than most in the category, but that won’t begin to play much of anything outside of a browser window. Not satisfied with “standard” [Matteo] put together his build combining an Udoo x86 Ultra with a NVIDIA 1060 GPU. It seems ridiculous to have an expansion card almost three times longer than the entire computer its attached to, but since when did being ridiculous stop anyone in the pursuit of a few more polygons?

M.2 adapter board trim comparison
M.2 to PCIe adapter board (Top) Trimmed adapter board (Bottom)

Since the Udoo Ultra doesn’t feature a PCIe slot [Matteo] slotted in a M.2 to PCIe adapter board. There are two PCIe lines accessible by the Udoo Ultra’s M.2 port although trimming the adapter board was required in order to fit. The PCIe female slot was cut open to allow the 1060 GPU to slide in. All of the throughput of the 1060 GPU wouldn’t be utilized given the Udoo Ultra’s limitations anyway.

Windows 10 was the OS chosen for the machine so that all those NVIDIA drivers could be installed, and there’s also the added benefit of being able to sneak in a little Trackmania Turbo too. So to accompany the build, [Matteo] created a graphics comparison video to show the remarkable improvement over the embedded graphics chip. You can see the Time Spy benchmark results in the video below.

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Learn ARM Assembly with the Raspberry Pi

We live in a time when you don’t have to know assembly language to successfully work with embedded computers. The typical processor these days has resources that would shame early PCs and some of the larger ones are getting close to what was a powerful desktop machine only a few years ago. Even so, there are some cases where you really want to use assembly language. Maybe you need more speed. Or maybe you need very precise control over timing. Maybe you just like the challenge. [Robert G. Plantz] from Sonoma State University has an excellent book online titled “Introduction to Computer Organization: ARM Assembly Langauge Using the Raspberry Pi.” If you are interested in serious ARM assembly language, you really need to check out this book.

If you are more interested in x86-64 assembly and Linux [Plantz] has you covered there, too. Both books are free to read on the Internet, and you can pick up a printed version of the Linux book for a small payment if you want.

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Calm Down: It’s Only Assembly Language

Based on [Ben Jojo’s] title — x86 Assembly Doesn’t have to be Scary — we assume that normal programmers fear assembly. Most hackers don’t mind it, but we also don’t often have an excuse to program assembly for desktop computers.

In fact, the post is really well suited for the typical hacker because it focuses the on real mode of an x86 processor after it boots. What makes this tutorial a little more interesting than the usual lecture is that it has interactive areas, where a VM runs your code in the browser after assembling with NASM.

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PC-XT Emulator On ESP8266

Do you remember the simpler times when you had a DOS command line, a handful of commands, and you talked to the hardware through a few BIOS and DOS interrupts? Okay, maybe it was a little limited, but nostalgia doesn’t care. Now [mcuhacker] is working on bringing some of those memories back by getting a PC-XT emulator running on an ESP8266.

For the x86 CPU emulator, he ported Fake86 which is written in C, and created an Arduino IDE environment for it. The MS-DOS 3.3 bootdisk image is stored in flash and is accessed as the A: drive. There’s no keyboard yet but he has 640×200 CGA working with 80×25 characters on a 3.5″ TFT display with the help of a low pass filter circuit. In the video below he shows it booting to the point where it asks for the date.

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Learn to Reverse Engineer x86_64 Binaries

Opening up things, see how they work, and make them do what you want are just the basic needs of the average hacker. In some cases, a screwdriver and multimeter will do the job, but in other cases a binary blob of random software is all we have to work with. Trying to understand an unknown binary executable is an exciting way to discover a system’s internal functionality.

While the basic principles of software reverse engineering are universal across most platforms, the details can naturally vary for different architectures. In the case of the x86 architecture, [Leo Tindall] felt that most tutorials on the subject focus mostly on 32-bit and not so much on the 64-bit specifics. Determined to change that, [Leo] ended up with an extensive introduction tutorial for reverse engineering x86_64 binaries starting at the very basics, then gradually moving forward using crackme examples. Covering simple string analysis and digging through disassembled binaries to circumvent fictional security, the tutorial later introduces the Radare2 framework.

All example source code is provided in the accompanying GitHub repository, although it is advised to avoid looking at them to keep it more interesting and challenging. And in case you are looking for more challenges later on, or generally prefer a closer connection to the hardware, these MSP430 based capture the flag online challenges might be worth to look at next.