assembly language

29 Articles

It Isn’t WebAssembly, But It Is Assembly In Your Browser

April 11, 2023 by 18 Comments

You might think assembly language on a PC is passe. After all, we have a host of efficient high-level languages and plenty of resources. But there are times you want to use assembly for some reason. Even if you don’t, the art of writing assembly language is very satisfying for some people — like an intricate logic puzzle. Getting your assembly language fix on a microcontroller is usually pretty simple, but on a PC there are a lot of hoops to jump. So why not use your browser? That’s the point of this snazzy 8086 assembler and emulator that runs in your browser. Actually, it is not native to the browser, but thanks to WebAssembly, it works fine there, too.

No need to set up strange operating system environments or link to an executable file format. Just write some code, watch it run, and examine all the resulting registers. You can do things using BIOS interrupts, though, so if you want to write to the screen or whatnot, you can do that, too.

The emulation isn’t very fast, but if you are single-stepping or watching, that’s not a bad thing. It does mean you may want to adjust your timing loops, though. We didn’t test our theory, but we expect this is only real mode 8086 emulation because we don’t see any protected mode registers. That’s not a problem, though. For a learning tool, you’d probably want to stick with real mode, anyway. The GitHub page has many examples, ranging from a sort to factorials. Just the kind of programs you want for learning about the language.

Why not learn on any of a number of other simulated processors? The 8086 architecture is still dominant, and even though x86_64 isn’t exactly the same, there is a lot of commonalities. Besides, you have to pretend to be an 8086, at least through part of the boot sequence.

If you’d rather compile “real” programs, it isn’t that hard. There are some excellent tutorials available, too.

Assembly Language 80’s Minicomputer Style

March 6, 2023 by 19 Comments

In the days before computers usually used off-the-shelf CPU chips, people who needed a CPU often used something called “bitslice.” The idea was to have a building block chip that needed some surrounding logic and could cascade with other identical building block chips to form a CPU of any bit width that could do whatever you wanted to do. It was still harder than using a CPU chip, but not as hard as rolling your own CPU from scratch. [Usagi Electric] has a Centurion, which is a 1980s-vintage minicomputer based on a bitslice processor. He wanted to use it to write assembly language programs targeting the same system (or an identical one). You can see the video below.

Truthfully, unless you have a Centurion yourself, the details of this are probably not interesting. But if you have wondered what it was like to code on an old machine like this, you’ll enjoy the video. Even so, the process isn’t quite authentic since he uses a more modern editor written for the Centurion. Most editors from those days were more like CP/M ed or DOS edlin, which were painful, indeed.

The target program is a hard drive test, so part of it isn’t just knowing assembly but understanding how to interface with the machine. That was pretty common, too. You didn’t have a lot of help from canned routines in those days. For example, it was common to read an entire block from a hard drive, tape, or drum and have to figure out what part of it you were actually interested in instead of, say, opening a file and reading a stream of characters.

If nothing else, fast forward over to the 25-minute mark and see what a hard drive from that era looked like. Guess how much storage was on that monster? If you guessed more than 10 MB, you probably didn’t live through the 1980s. We won’t even guess what the price tag was, but you can bet it was spendy.

If you think entering programs like this is painful, try a front panel. That made paper tape seem like a great thing.

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ARM Programming By Example

October 11, 2022 by 40 Comments

The ARM processor is popping up everywhere. From Raspberry Pis, to phones, to Blue Pill Arduino-like boards, you don’t have to go far to find an ARM processor these days. If you program in C, you probably don’t care much or even think about it. But do you know ARM assembly language? Well, if you look at it one way, it can’t be too hard. The CPU only has about 30 distinct operations — that’s why it is called RISC. Of course, sometimes fewer instructions actually make things more difficult. But you can get a great starting tutorial with the 21 programs on the ARM Assembly by Example website.

You need a 32-bit ARMv6 or better — so Raspberry Pi will work here. The compiler, of course, is gcc and all the associated tools. if you have the right hardware, there are sections on using the floating point unit and the NEON co-processor, too.

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Building MS-DOS From Scratch Like It’s 1983

December 20, 2021 by 52 Comments

Building a complete operating system by compiling its source code is not something for the faint-hearted; a modern Linux or BSD distribution contains thousands of packages with millions of lines of code, all of which need to be processed in the right order and the result stored in the proper place. For all but the most hardcore Gentoo devotees, it’s way easier to get pre-compiled binaries, but obviously someone must have run the entire compilation process at some point.

What’s true for modern OSes also holds for ancient software such as MS-DOS. When Microsoft released the source code for several DOS versions a couple of years ago, many people pored over the code to look for weird comments and undocumented features, but few actually tried to compile the whole package. But [Michal Necasek] over at the OS/2 Museum didn’t shy away from that challenge, and documented the entirely-not-straightforward process of compiling DOS 2.11 from source.

The first problem was figuring out which version had been made available: although the Computer History Museum labelled the package simply as “MS-DOS 2.0”, it actually contained a mix of OEM binaries from version 2.0, source code from version 2.11 and some other stuff left from the development process. The OEM binaries are mostly finished executables, but also contain basic source code for some system components, allowing computer manufacturers to tailor those components to their specific hardware platform.

Compiling the source code was not trivial either. [Michal] was determined to use period-correct tools and examined the behaviour of about a dozen versions of MASM, the assembler likely to have been used by Microsoft in the early 1980s. As it turned out, version 1.25 from 1983 produced code that most closely matched the object code found in existing binaries, and even then some pieces of source code required slight modifications to build correctly. [Michal]’s blog post also goes into extensive detail on the subtle differences between Microsoft-style and IBM-style DOS, which go deeper than just the names of system files (MSDOS.SYS versus IBMDOS.COM).

The end result of this exercise is a modified DOS 2.11 source package that actually compiles to a working set of binaries, unlike the original. And although this does not generate any new code, since binaries of DOS 2.11 have long been available, it does provide a fascinating look into software development practices in an age when even the basic components of the PC platform were not fully standardized. And don’t forget that even today some people still like to develop new DOS software.

Oddball X86 Instructions

February 25, 2021 by 66 Comments

David Letterman made the top ten list famous. [Creel] has a top ten that should appeal to many Hackaday readers: the top 10 craziest x86 assembly language instructions. You have to admit that the percentage of assembly language programmers is decreasing every year, so this isn’t going to have mass appeal, but if you are interested in assembly or CPU architecture, this is a fun way to kill 15 minutes.

Some would say that all x86 instructions are crazy, especially if you are accustomed to reduced instruction set computers. The x86, like other non-RISC processors, has everything but the kitchen sink. Some of these instructions might help you get that last 10 nanoseconds shaved off a time-critical loop.

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Boot Sector Pong As A Crash Course In Assembly

September 28, 2020 by 13 Comments

Have you ever wanted to develop a playable game small enough to fit into a disk’s 512 byte boot sector? How about watching somebody develop a program in assembly for nearly two hours? If you answered yes to either of those questions, or ideally both of them, you’re going to love this project from [Queso Fuego].

Whether you just want to check out the public domain source code or watch along as he literally starts from a blank file and codes every line for your viewing pleasure, chances are good that you’ll pick up a trick or two from this project. For example, he explains how all of the “graphics” in the game are done in 80 x 25 text mode simply by setting the background color of character cells without printing any text to them.

We really like the presentation in the video after the break, which was recorded over the course of multiple days, judging by the changing light levels in the background. As he types out each line of code, he explains what its function is and gives any background information necessary to explain how it will fit into the larger program. If you’ve ever wondered if you had what it takes to program in ASM, watching this video is a great way to decide.

[Queso Fuego] mentions that this project, and his research into this sort of low-level programming, came about due to the social distancing boredom that many of us are feeling. While we’re certainly not advocating for him to kept locked in his home permanently, with projects like this, you’ve got to admit it seems like a win for the rest of us.

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Learn PDP-11 Assembly For Fun But Probably No Profit

April 7, 2020 by 43 Comments

Learning a new skill is fun, especially one that could land you a new job. We don’t think you’ll find too much demand for PDP-11 assembly language programmers, but if it still interests you, check out [ChibiAkumas’s] video that starts a series on that subject for “absolute beginners.”

The PDP-11 is a venerable computer, but you can still find simulators ranging from SIMH to browser-based virtual devices with front panels. If you want real hardware, there is a PDP-11 on a chip that is still around (or you can score the real chips, sometimes) and there are some nice hardware simulations, too.

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