cpu

123 Articles

Circuit diagram of linear-feedback shift register.

Can We Replace A Program Counter With A Linear-Feedback Shift Register? Yes We Can!

May 31, 2025 by 32 Comments

Today we heard from [Richard James Howe] about his new CPU. This new 16-bit CPU is implemented in VHDL for an FPGA.

The really cool thing about this CPU is that it eschews the typical program counter (PC) and replaces it with a linear-feedback shift register (LFSR). Apparently an LFSR can be implemented in hardware with fewer transistors than are required by an adder.

Usually the program counter in your CPU increments by one, each time indicating the location of the next instruction to fetch and execute. When you replace your program counter with an LFSR it still does the same thing, indicating the next instruction to fetch and execute, but now those instructions are scattered pseudo-randomly throughout your address space!

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The Transputer In Your Browser

April 4, 2025 by 11 Comments

We remember when the transputer first appeared. Everyone “knew” that it was going to take over everything. Of course, it didn’t. But [Oscar Toledo G.] gives us a taste of what life could have been like with a JavaScript emulator for the transputer, you can try in your browser.

If you don’t recall, the transputer was a groundbreaking CPU architecture made for parallel processing. Instead of giant, powerful CPUs, the transputer had many simple CPUs and a way to chain them all together. Sounds great, but didn’t quite make it. However, you can see the transputer’s influence on CPUs even today.

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A SNES CPU Replacement Via FPGA

March 31, 2025 by 18 Comments

Let’s say you had a SNES with a busted CPU. What would you do? Your SNES would be through! That is, unless, you had a replacement based on an FPGA. [leonllr] has been developing just such a thing.

The project was spawned out of necessity. [leonllr] had purchased a SNES which was struck down with a dead CPU—in particular, a defective S-CPU revision A. A search for replacements only found expensive examples, and ones that were most likely stripped from working machines. A better solution was necessary.

Hence, a project to build a replacement version of the chip using the ICE40HX8K FPGA. Available for less than $20 USD, it’s affordable, available, and has enough logic cells to do the job. It’s not just a theoretical or paper build, either. [leonllr] has developed a practical installation method to hook the ICE40HX8K up to real hardware, which uses two flex PCBs to go from the FPGA mainboard to the SNES motherboard itself. As for the IP on the FPGA, the core of the CPU itself sprung from the SNESTANG project, which previously recreated the Super Nintendo on Sipeed Tang FPGA boards. As it stands, boards are routed, and production is the next step.

It’s nice to see classic hardware resurrected by any means necessary. Even if you can’t get a whole bare metal SNES, you might be able to use half of one with a little help from an FPGA. We’ve seen similar work on other platforms, too. Meanwhile, if you’re working to recreate Nintendo 64 graphics chips in your own basement, or something equally weird, don’t hesitate to let us know!

A New 8-bit CPU For C

February 21, 2025 by 60 Comments

It is easy to port C compilers to architectures that look like old minicomputers or bigger CPUs. However, as the authors of the Small Device C Compiler (SDCC) found, pushing C into a typical 8-bit CPU is challenging. Lessons learned from SDCC inspired a new 8-bit architecture, F8. This isn’t just a theoretical architecture. You can find an example Verilog implementation in the SDDC project and on GitHub. The name choice may turn out to be unfortunate as there was an F8 CPU from Fairchild back in the 1970s that apparently few people remember.

In the video from FOSDEM 2025, [Phillip Krause] provides a nice overview of the how and why of F8. While it might seem odd to create a new 8-bit CPU when you can get bigger CPUs for pennies, you have to consider that 8-bit machines are more than enough for many jobs, and if you can squeeze one into an FPGA, it might be a good choice as opposed to having to get a bigger FPGA to hold your design and a 32-bit CPU.

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Homebrew CPU Gets A Beautiful Rotating Cube Demo

February 19, 2025 by 10 Comments

[James Sharman] designed and built his own 8-bit computer from scratch using TTL logic chips, including a VGA adapter, and you can watch it run a glorious rotating cube demo in the video below.

The rotating cube is the product of roughly 3,500 lines of custom assembly code and looks fantastic, running at 30 frames per second with shading effects from multiple light sources. Great results considering the computing power of his system is roughly on par with vintage 8-bit home computers, and the graphics capabilities are limited. [James]’s computer uses a tile map instead of a frame buffer, so getting 3D content rendered was a challenge.

The video is about 20 seconds of demo followed by a detailed technical discussion on how exactly one implements everything required for a 3D cube, from basic math to optimization. If a deep dive into that sort of thing is up your alley, give it a watch!

We’ve featured [James]’ fascinating work on his homebrew computer before. Here’s more detail on his custom VGA adapter, and his best shot at making it (kinda) run DOOM.

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Going Minimal: 64×4, The Fun In Functional Computing

January 22, 2025 by 38 Comments

If you’ve ever wondered what makes a computer tick, the Minimal 64×4 by [Slu4] is bound to grab your attention. It’s not a modern powerhouse, but a thoughtfully crafted throwback to the essence of computing. With just 61 logic ICs, VGA output, PS/2 input, and SSD storage, this DIY wonder packs four times the processing power of a Commodore 64.

What sets [Slu4]’s efforts apart is his refusal to follow the beaten track of CPU development. He imposes strict complexity limits on his designs, sticking to an ultra-minimalist Von Neumann architecture. His journey began with the ‘Minimal Ur-CPU’, a logic-chip-based computer that could crunch numbers but little else. Next came the ‘Minimal 64’, featuring VGA graphics and Space Invaders-level performance. The latest ‘Minimal 64×4’ takes it further, adding incredible speed while keeping the design so simple it’s almost ridiculous. It’s computing stripped to its rawest form—no fancy sound, no dazzling graphics, just raw resourcefulness.

For enthusiasts of retro-tech and DIY builds, this project is a treasure trove. From text editors to starfield simulations to Sokoban, [Slu4] proves you don’t need complexity to make magic. Continue reading “Going Minimal: 64×4, The Fun In Functional Computing” →

Regular (Expression) Chess

January 7, 2025 by 17 Comments

[Nicholas Carlini] found some extra time on his hands over the holiday, so he decide to do something with “entirely no purpose.” The result: 84,688 regular expressions that can play chess using a 2-ply minmax strategy. No kidding. We think we can do some heavy-duty regular expressions, but this is a whole other level.

As you might expect, the code to play is extremely simple as it just runs the board through series of regular expressions that implement the game logic. Of course, that doesn’t count the thousands of strings containing the regular expressions.

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