Adding Portals To Quake

July 6, 2023 by Matthew Carlson 14 Comments

For those who have played Quake extensively, adding portals seems unnecessary, as teleporters are already a core part of the game mechanics. What [Matthew Earl] accomplishes is more of the Portal style of portal by rendering what is on the other side of the portal with a seamless teleportation transition.

Of course, Quake is an old game with a software renderer. Just throwing another camera into the scene, rendering to another texture, and then mapping that texture to the scene isn’t an option. Quake uses an edge rasterizer and generates spans along scanlines that track where edges intersect the current scanline. Rather than making expensive per-pixel comparisons, [Matt] stashes the portal spans and renders them in a second render, so even with multiple portals, only a single screen’s worth of pixels are rendered.

However, this technique has no near clipping plane, which means objects can appear in the portal that don’t make any sense as they are in front of the portal’s viewpoint. Luckily, Quake has an ingenious method for polygon occlusion: the BSP. While [Matt] is manually checking polygons, the BSP is the perfect tool for bisecting a room along a plane. It’s an incredible hack, and we’re excited to see Quake expand into a puzzle game. [Matt] dives into greater detail on how the software renderer works in another video that’s well worth a watch.

Perhaps the most incredible aspect of this technique is that it could run on original hardware. If you want to bring a little more Quake to life, why not get the Quake light flicker in your house? Video after the break.

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Freq Out With LTSpice

July 3, 2023 by Al Williams 4 Comments

We always enjoy [FesZ’s] videos, and his latest about FREQ function in LTSpice is no exception. In fact, LTSpice doesn’t document it, but it is part of the underlying Spice system. So, of course, you can figure it out or just watch the video below. The FREQ keyword allows you to change component attributes in a frequency-depended way.

Of course, capacitors and inductors are frequency dependent by design. But the FREQ technique allows you to adjust things like voltage sources or resistance in arbitrary ways. By default, you must specify the frequency response data in decibels, which isn’t always convenient. However, [FesZ] shows you how to use other methods to express them using modifiers to the command.

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Microsoft BASIC For The Dragon 64 Recovered

June 29, 2023 by Lewin Day 17 Comments

There are a great many pieces of software of yesteryear that are no longer readily accessible. It’s now possible to cross Microsoft BASIC for the Dragon 64 off that list, with the source code now posted for all to enjoy on GitHub.

The repository concerns the Microsoft 16K BASIC Interpreter as built for the Motorola 6809, as used in the Dragon 64 computer. This is also known as BASIC-69 or Extended Color Basic.

Hilariously, the source code was recovered from 340 pages of fan-fold tractor paper stored in four bundles. The output of a Motorola assembler was printed back in 1983 at Dragon Data’s R&D facility in Wales, and was recently recovered after being stored in an attic for much of the last four decades. The paper was carefully scanned at the 2022 Dragon Meetup, before passing the resulting images through OCR software. The output was then manually corrected and the source code was complete for both the 32K and 64K mode ROMs. There are some differences between the scanned source and what Microsoft shipped, which is outlined in the repository.

We’ve seen other heroic retrocomputer recovery efforts before, too, like the work to save the Polish CROOK OS. If you’ve been working on similar feats, be sure to let us know.

A Browser Approach To Parsing

June 26, 2023 by Al Williams 3 Comments

There are few rites of programmer passage as iconic as writing your first parser. You might want to interpret or compile a scripting language, or you might want to accept natural-language-like commands. You need a parser. [Varunramesh] wants to show you parser combinators, a technique used to make practical parsers. But the demonstration using interactive code cells in the web page is nearly as interesting as the technique.

Historically, you parse tokens, and this technique can do that too, but it can also operate directly on character streams if you prefer. The idea is related to recursive descent parsing, where you attempt to parse certain things, and if those things fail, you try again.

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Exploring The Anatomy Of A Linux Kernel Exploit

June 25, 2023 by Bryan Cockfield 21 Comments

A lot of talk and discussion happens anytime a hardware manufacturer releases a new line of faster, more powerful, or more efficient computers. It’s easy to see better and better specifications and assume that’s where all the progress is made. But without improved software and algorithms, often the full potential of the hardware can’t be realized. That’s the reason for the creation of io_uring, an improved system call interface in the Linux kernel. It’s also where [chompie] went to look for exploits.

The reason for looking here, in a part of the kernel [chompie] had only recently learned about, was twofold. First, because it’s a place where user space applications interact with the kernel, and second because it’s relatively new and that means more opportunities to find bugs. The exploit involves taking advantage of a complicated asynchronous buffer system, specifically at a location where the code confuses a memory location being used by the kernel with one which is supposed to be used for user space.

To actually get this to work as an exploit, though, a much more involved process is needed to make sure the manipulation of these memory addresses results in something actually useful, but it is eventually used to gain local privilege escalation. More about it can be found in this bug report as well. Thanks to the fact that Linux is open-source, this bug can quickly be fixed and the patch rolled out to prevent malicious attackers from exploiting it. Open-source software has plenty of other benefits besides being inherently more secure, though.

A 32-Bit RISC-V CPU Core In 600 Lines Of C

June 24, 2023 by Donald Papp 29 Comments

If you have ever wanted to implement a RISC-V CPU core in about 600 lines of C, you’re in luck! [mnurzia]’s rv project does exactly that, providing a simple two-function API.

Technically, it’s a user-level RV32IMC implementation in ANSI C. There are many different possible flavors of RISC-V, and in this case is a 32-bit base integer instruction set (RV32I), with multiplication and division extension (M), and compressed instruction set extension (C).

There’s a full instruction list and examples of use on the GitHub repository. As for readers wondering what something like RISC-V emulation might be good for, it happens to be the not-so-secret sauce to running Linux on an RP2040.

Robodog Goes Free Thanks To Unofficial SDK

June 24, 2023 by Donald Papp 8 Comments

What’s better than a pretty nice legged robot? One with an alternate SDK version that opens up expensive features, of course. The author didn’t like that the original SDK only came as pre-compiled binaries restricted to the most expensive models, so rolled up their sleeves and started writing a new one.

The manufacturer’s SDK limits access to programmatic functions, but that needn’t stop *you*.

There are a number of commercially-available robotic quadrupeds that can trace their heritage back to the MIT Mini Cheetah design, and one of them is the Unitree Go1 series which sports a distinctive X-shaped sensor cluster on its “face”. The basic models are affordable (as far as robots go, anyway) but Unitree claims only the high-priced EDU model can be controlled via the SDK. Happily, the Free Dog SDK provides a way to do exactly that.

The SDK is a work in progress, but fully usable and allows the user to send various high level and low level commands to the Go1 robots. High level examples include things like telling the robot to perform pushups, turn 90 degrees, or walk. Low level commands are things like specifying exact positions or torque levels for individual limbs. With the new SDK, doing those things programmatically is only a Python script away.

Know any other robots that might be based on the same system? This SDK might work on them, too.