foam3 VS xv6-public

Thank you for sharing that video! Your foam project looks fascinating too: https://github.com/kgrgreer/foam3

I wrote a BASIC to JS compiler/transpiler that includes all of the programs from "BASIC Computer Games" as examples.

You can try it out in your browser at: https://codepen.io/kgr/full/yLQyLjR

Just select the game you want to to run from the top-left list box, then press the "Compile" button and you'll see the translated JS source in the right text-area. Then press the "Run" button to run it.

The source code for the compiler is available at: https://github.com/kgrgreer/foam3/tree/429f2fd2b4cef0e37996a...

Sort of related but an interesting idea is take it one step further with the Feature Oriented Active Modeler (FOAM) [1,2] paradigm and use code to model your whole system, which generates diagrams model, and runnable code in whatever language needed. The project is still young and it may not be practical today with the available tooling but it seems like a cool idea and project. It is influenced by the unix principle of “coding the perimeter not the area” which is essentially factoring your dev tasks into building NM capabilities, but instead of building NM things individually build N+M tools that can be composed into N*M capabilities [2].

So with FOAM the idea is if we want to maintain a model of our software, and build it as well, what if we can use one composable tool to generate both, rather than model everything and code it separately.

[1] https://github.com/kgrgreer/foam3

[2] https://foam-framework.github.io/foam/

[3] https://m.youtube.com/watch?v=3Ea3pkTCYx4

https://github.com/kgrgreer/foam3#videos

FOAM is a modelling framework that generates cross-language boilerplate for you, but it takes a much broader view of what constitutes boilerplate than most systems. Typically, it can generate between 95-98% of a working cross-language cross-tier system.

FOAM helps you create features for modelled data. Features include things like a Java/Javascript/Swift classes to hold your modelled data, code to marshall to/from JSON/XML/CSV/etc., various GUI Views, and support for storing your data in various databases or file formats. However, FOAM models are themselves modelled, meaning they're afforded all of the above benefits as well. This lets you apply the MVC technique of having multiple views work against the same underlying data-model concurrently (say a grid and a pie-chart in a spreadsheet), so that you can choose the best view or views for your current need. When treated this way, your code is no longer text (but it can be, if that's one of your views), and you can easily view and store it in many different ways and more easily programmatically manipulate it.

+1 for mini operating system.

Us, application developers, rely on many OS features: memory management, filesystem, etc. I'm sure eventually we'll ask "how such things are done behind the scene?"

That's why I tinker with xv6 (https://github.com/mit-pdos/xv6-public) during sparetime. Learning various process scheduling algorithms from textbook is a thing. Implementing it is another thing. I learn a lot. And it's definitely fun, even though there's almost zero chance the knowledge gained is relevant for my job (I'm a mobile app dev).

Recently I compiled xv6 using gcc 7.5.0 on Ubuntu 18 , everything is ok. But when I try to compile it using gcc 13.2.1 on latest Arch, it's failed: result

https://github.com/mit-pdos/xv6-public has under 10,000 lines of C and assembly including some user space programs.

xv6 was originally written for 32-bit x86; the RISC-V port is a relatively recent development. See e.g. https://github.com/mit-pdos/xv6-public for some of the earlier history.

rxv64 was written for a specific purpose: we had to ramp up professional engineers on both 64-bit x86_64 and kernel development in Rust; we were pointing them to the MIT materials, which at the time still focused on x86, but they were getting tripped up 32-bit-isms and the original PC peripherals (e.g., accessing the IDE disk via programmed IO). Interestingly, the non sequitur about C++ aside, porting to Rust exposed several bugs or omissions in the C original; fixes were contributed back to MIT and applied to the original (and survived into the RISC-V port).

Oh, by the way, the use of the term "SMP" predates Intel's usage by decades.

I'd define an arena as the pattern where the arena itself owns N objects. So you free the arena to free all objects.

My first job was at EA working on console games (PS2, GameCube, XBox, no OS or virtual memory on any of them), and while at the time I was too junior to touch the memory allocators themselves, we were definitely not malloc-ing and freeing all the time.

It was more like you load data for the level in one stage, which creates a ton of data structures, and then you enter a loop to draw every frame quickly. There were many global variables.

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Wikipedia calls it a region, zone, arena, area, or memory context, and that seems about right:

https://en.wikipedia.org/wiki/Region-based_memory_management

It describes history from 1967 (before C was invented!) and has some good examples from Apache ("pools") and Postgres ("memory contexts").

I also just looked at these codebases:

https://github.com/mit-pdos/xv6-public (based on code from the 70's)

https://github.com/id-Software/DOOM (1997)

I looked at allocproc() in xv6, and gives you an object from a fixed global array. A lot of C code in the 80's and 90's was essentially "kernel code" in that it didn't have an OS underneath it. Embedded systems didn't run on full-fledges OSes.

DOOM tends to use a lot of what I would call "pools" -- arrays of objects of a fixed size, and that's basically what I remember from EA.

Though in g_game.c, there is definitely an arena of size 0x20000 called "demobuffer". It's used with a bump allocator.

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So I'd say

- malloc / free of individual objects was NEVER what C code looked like (aside from toy code in college)

- arena allocators were used, but global vars and pools are also very common.

- arenas are more or less wash for memory safety. they help you in some ways, but hurt you in others.

The reason C programmers don't malloc/free all the time is for speed, not memory safety. Arenas are still unsafe.

When you free an arena, you have no guarantee there's nothing that points to it anymore.

Also, something that shouldn't be underestimated is that arena allocators break tools like ASAN, which use the malloc() free() interface. This was underscored to me by writing a garbage collector -- the custom allocator "broke" ASAN, and that was actually a problem:

https://www.oilshell.org/blog/2023/01/garbage-collector.html

If you want memory safety in your C code, you should be using ASAN (dynamically instrumented allocators) and good test coverage. Arenas don't help -- they can actually hurt. An arena is a trivial idea -- the problem is more if that usage pattern actually matches your application, and apps evolve over time.

Magma is proudly licensed under the MIT license, and uses code from Xv6 and Yagura.

My OS classes used xv6, a reimplementation of Unix Version 6 for a RISC-V architecture. Accompanying that was the OSTEP textbook.

Check out xv6 if you are only getting started with operating systems and want something simpler.

Here is kernel.ld.