Dreamcast Homebrew Gets Boost From SD Card Cache

While it might have been a commercial failure compared to contemporary consoles, the Sega Dreamcast still enjoys an active homebrew scene more than twenty years after its release. Partly it’s due to the fact that you can burn playable Dreamcast discs on standard CD-Rs, but fans of the system will also point out that the machine was clearly ahead of its time in many respects, affording it a bit of extra goodwill in the community.

That same community happens to be buzzing right now with news that well-known Dreamcast hacker [Ian Micheal] has figured out how to cache data to an SD card via the console’s serial port. At roughly 600 KB/s the interface is too slow to use it as swap space for expanding the system’s paltry 16 MB of memory, but it’s more than fast enough to load game assets which otherwise would have had to be loaded into RAM.

A third-party Dreamcast SD adapter.

In the video below, [Ian] shows off his new technique with a port of DOOM running at 640×480. He’s already seeing an improvement to framerates, and thinks further optimizations should allow for a solid 30 FPS, but that’s not really the most exciting part. With the ability to load an essentially unlimited amount of data from the SD card while the game is running, this opens the possibility of running mods which wouldn’t have been possible otherwise. It should also allow for niceties like saving screenshots or game progress to the SD card for easy retrieval.

[Ian] says he’ll be bringing the same technique to his Dreamcast ports of Quake and Hexen in the near future, and plans on posting some code to GitHub that demonstrates reading and writing to FAT32 cards so other developers can get in on the fun. The downside is that you obviously need to have an SD card adapter plugged into your console to make use of this technique, which not everyone will have. Luckily they’re fairly cheap right now, but we wouldn’t be surprised if the prices start climbing. If you don’t have one already, now’s probably the time to get one.

To be clear, this technique is completely separate from replacing the Dreamcast’s optical drive with an SD card, which itself is a very popular modification that’s helped keep Sega’s last home console kicking far longer than anyone could have imagined.

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Sushi Roll Helps Inspect Your CPU Internals

[Gamozolabs’] post about Sushi Roll — a research kernel for monitoring Intel CPU internals — is pretty long. While we were disappointed at the end that the kernel’s source is not exactly available due to “sensitive features”, we were so impressed with the description of the modern x86 architecture and some of the work done with Sushi Roll, that we just had to post it. If the post gets you wanting to actually try some of this, you can check out another [Gamozolabs] creation, Orange Slice.

While you probably know that a modern Intel CPU bears little resemblance to the old 8086 processor it emulates, it is surprising, sometimes, to realize just how far it has gone. The very first thing the CPU does is to break your instruction up into microoperations. The execution engine uses some sophisticated techniques for register renaming and scheduling that allow you to run instructions out of order and to run more than one instruction per clock cycle.

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Caching In On Program Performance

Most of us have a pretty simple model of how a computer works. The CPU fetches instructions and data from memory, executes them, and writes data back to memory. That model is a good enough abstraction for most of what we do, but it hasn’t really been true for a long time on anything but the simplest computers. A modern computer’s memory subsystem is much more complex and often is the key to unlocking real performance. [Pdziepak] has a great post about how to take practical advantage of modern caching to improve high-performance code.

If you go back to 1956, [Tom Kilburn’s] Atlas computer introduced virtual memory based on the work of a doctoral thesis by [Fritz-Rudolf Güntsch]. The idea is that a small amount of high-speed memory holds pieces of a larger memory device like a memory drum, tape, or disk. If a program accesses a piece of memory that is not in the high-speed memory, the system reads from the mass storage device, after possibly making room by writing some part of working memory back out to the mass storage device.

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Spectre And Meltdown: How Cache Works

The year so far has been filled with news of Spectre and Meltdown. These exploits take advantage of features like speculative execution, and memory access timing. What they have in common is the fact that all modern processors use cache to access memory faster. We’ve all heard of cache, but what exactly is it, and how does it allow our computers to run faster?

In the simplest terms, cache is a fast memory. Computers have two storage systems: primary storage (RAM) and secondary storage (Hard Disk, SSD). From the processor’s point of view, loading data or instructions from RAM is slow — the CPU has to wait and do nothing for 100 cycles or more while the data is loaded. Loading from disk is even slower; millions of cycles are wasted. Cache is a small amount of very fast memory which is used to hold commonly accessed data and instructions. This means the processor only has to wait for the cache to be loaded once. After that, the data is accessible with no waiting.

A common (though aging) analogy for cache uses books to represent data: If you needed a specific book to look up an important piece of information, you would first check the books on your desk (cache memory). If your book isn’t there, you’d then go to the books on your shelves (RAM). If that search turned up empty, you’d head over to the local library (Hard Drive) and check out the book. Once back home, you would keep the book on your desk for quick reference — not immediately return it to the library shelves. This is how cache reading works.

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Spectre And Meltdown: Attackers Always Have The Advantage

While the whole industry is scrambling on Spectre, Meltdown focused most of the spotlight on Intel and there is no shortage of outrage in Internet comments. Like many great discoveries, this one is obvious with the power of hindsight. So much so that the spectrum of reactions have spanned an extreme range. From “It’s so obvious, Intel engineers must be idiots” to “It’s so obvious, Intel engineers must have known! They kept it from us in a conspiracy with the NSA!”

We won’t try to sway those who choose to believe in a conspiracy that’s simultaneously secret and obvious to everyone. However, as evidence of non-obviousness, some very smart people got remarkably close to the Meltdown effect last summer, without getting it all the way. [Trammel Hudson] did some digging and found a paper from the early 1990s (PDF) that warns of the dangers of fetching info into the cache that might cross priviledge boundaries, but it wasn’t weaponized until recently. In short, these are old vulnerabilities, but exploiting them was hard enough that it took twenty years to do it.

Building a new CPU is the work of a large team over several years. But they weren’t all working on the same thing for all that time. Any single feature would have been the work of a small team of engineers over a period of months. During development they fixed many problems we’ll never see. But at the end of the day, they are only human. They can be 99.9% perfect and that won’t be good enough, because once hardware is released into the world: it is open season on that 0.1% the team missed.

The odds are stacked in the attacker’s favor. The team on defense has a handful of people working a few months to protect against all known and yet-to-be discovered attacks. It is a tough match against the attackers coming afterwards: there are a lot more of them, they’re continually refining the state of the art, they have twenty years to work on a problem if they need to, and they only need to find a single flaw to win. In that light, exploits like Spectre and Meltdown will probably always be with us.

Let’s look at some factors that paved the way to Intel’s current embarrassing situation.

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PCI I-RAM Working Without A PCI Slot

[Gnif] had a recent hard drive failure in his home server. When rebuilding his RAID array, he decided to update to the ZFS file system. While researching ZFS, [Gnif] learned that the file system allows for a small USB cache disk to greatly improve his disk performance. Since USB is rather slow, [Gnif] had an idea to try to use an old i-RAM PCI card instead.

The problem was that he didn’t have any free PCI slots left in his home server. It didn’t take long for [Gnif] to realize that the PCI card was only using the PCI slot for power. All of the data transfer is actually done via a SATA cable. [Gnif] decided that he could likely get by without an actual PCI slot with just a bit of hacking.

[Gnif] desoldered a PCI socket from an old faulty motherboard, losing half of the pins in the process. Luckily, the pins he needed still remained. [Gnif] knew that DDR memory can be very power-hungry. This meant that he couldn’t only solder one wire for each of the 3v, 5v, 12v, and ground pins. He had to connect all of them in order to share the current load. All in all, this ended up being about 20 pins. He later tested the current draw and found it reached as high as 1.2 amps, confirming his earlier decision. Finally, the reset pin needed to be pulled to 3.3V in order to make the disk accessible.

All of the wires from his adapter were run to Molex connectors. This allows [Gnif] to power the device from a computer power supply. All of the connections were covered in hot glue to prevent them from wriggling lose.

Building Your Own Fusion Drive

We missed the original announcement, but Apple unveiled more than just the iPad Mini at their last event. They’ve got a new storage system called Fusion Drive which is supposed to combine the access speeds of solid state with the storage density of platter drives. When you look just under the surface what you’re really seeing is a disc drive with grossly enlarged cache in the form of an SSD drive. How about moving from the 64 MB or so of cache seen on many large hard drives today to something like 64GB?

Well you don’t have to wait for Apple to do it. [Patrick Stein] gave it a shot using command line tools to combine an SSD with a physical drive. Sure, it’s not an all-in-one solution, but it is a pretty good proof. The linchpin that will really make it possible is a low-level driver that can handle the caching on the SDD, while ensuring that the data eventually makes it to the platter for long-term storage.

[via Engadget]