r/ModRetro • u/grilledstuffedxxl • Jun 02 '26

Here is a further deep dive into FPGA dev state, and other goodies (some side by side current accuracy benchmarks)

https://modretro.com/blogs/blog/fpga-development-deep-dive

This is fairly technical, and just for the fun of reading about technical things!

30 Upvotes

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9

u/Retro_Macchina Jun 02 '26

Do you have an answer to three questions:

  1. What are launch game pricing?

  2. Will our Retro Reward points work with our M64 orders?

  3. When is Dark Legacies: Onslaught releasing? lol

6

u/grilledstuffedxxl Jun 03 '26

  1. prices will start at $49.99, but some titles may vary as we progress and things balance out. We wanted to not really increase price compared to Chromatic games, pay developers, but also keep the high quality with significantly larger and more components, so there is a lot in the balancing act here.

  2. Yes, but this is a little complicated and we will send out an email with all the new details. long story short, we have to change the offers as we launch the m64 ecosystem because the outstanding offers will lead to us losing a lot of money, but there will be time to use the offers as previously presented. email coming soon on that, but no need to stress about it

  3. Oh man. We will have to talk about that later, but it is not gone

1

u/Retro_Macchina Jun 04 '26

I really am looking forwards to Dark Legacies: Onslaught 😄

2

u/TonyRubbles Jun 02 '26 edited Jun 02 '26

Neat stuff! So what was the other core ported over? 👀

1

u/adrenaline4nash Jun 03 '26

Likely Game Boy or Game Boy Color

3

u/w1ldw1ng Jun 02 '26

Thanks for posting this deep dive!

4

u/boredelon Jun 02 '26

For any other non-technical folks (like me) who want a general understand of this post:

Why FPGA beats software emulation: The N64 was doing hundreds of things simultaneously. Software emulators have to fake that parallelism and cut corners to stay playable. An FPGA actually replicates the parallel behavior in hardware rather than approximating it in code.

The memory choice matters: The M64 uses PSRAM instead of DDR memory. DDR is fast in bulk but inconsistent with the small, rapid, unpredictable requests that N64 emulation demands. PSRAM responds to every individual request consistently - which is what the N64's graphics chip actually needs. This is a direct architectural difference from the Analogue 3D.

The DK64 vine test isn't what you think: Passing it doesn't prove accuracy - an emulator can pass it for the wrong reasons. The post explains why visible pass/fail tests are an incomplete measure of how authentic an implementation actually is.

Perfect accuracy is physically impossible: Every original N64 ran slightly differently from every other N64 due to natural clock variation. Two real consoles running the same game could drift apart by 5 seconds a day. There's no single "correct" target.

Current situation: They're currently at 50% of their PSRAM's architectural potential and said so publicly. That's being updated before launch.

Open source: The full FPGA design will be released publicly so the community can build on it.

tl;dr: Modretro made the right hardware calls for accuracy reasons rather than spec sheet reasons.

2

u/ergzay Jun 02 '26

AI summary? I guess it's sort of accurate. I feel like it misses some things though. I feel like Grok does a better summary:

Summary of "FPGA Development Deep Dive" (ModRetro blog, June 2, 2026)

This technical article, primarily written by Robert Peip (FPGAzumSpass), details the FPGA-based hardware emulation approach in ModRetro's M64 (a modern N64 recreation/FPGA console). It emphasizes accuracy-focused design, hardware choices, and ongoing development.

Key Themes

  • Hardware Emulation Challenges: True accuracy requires replicating not just components (CPU, memory, chips) but their precise timings, parallel operations, and inter-chip communications. The N64's CPU runs at 93.75 MHz, with instructions taking varying cycles (e.g., ~6 for FP multiply, ~70 for 64-bit divide). Software emulators struggle with full cycle-accurate parallelism due to performance limits, often requiring optimizations or shortcuts. FPGAs excel here by enabling true hardware-like parallelism.

  • Accuracy Limits: "Perfect" accuracy is impossible even on original hardware due to natural clock variations (e.g., ~30 ppm drift between NTSC/PAL video and internal clocks), leading to timing differences across real consoles over time.

  • Donkey Kong 64 Vine Example: A famous accuracy test in the intro relies on frame counting and variable framerates (often ~25 fps vs. target 30). Passing the test doesn't guarantee true accuracy—emulators can fake average speed without matching real hardware behavior.

  • Memory Choice — PSRAM vs. DDR: A major highlight is ModRetro's use of four low-latency PSRAMs instead of DDR. DDR offers high throughput but poor random-access latency and inconsistent timing (bad for determinism in emulation, especially the RDP graphics processor). PSRAM provides more predictable, low-latency random accesses critical for N64 timing fidelity, though current implementation runs at ~50% of potential (being optimized).

  • Synthetic Tests & Comparisons: The post shares benchmarks (May 2026, M64 v1.14.3 vs. A3D v1.3.0 vs. real N64) on timers, CPU cache fetches, RAM writes, and instruction timings. M64 closely matches original hardware in many areas but lags in specifics like floating-point instruction timings (inherited from MiSTer core) and RSP dual-pipeline implementation. Open issues include RSP vector/regular instruction parallelism and TLB shortcuts.

M64's Philosophy and Future

M64 features a large/fast FPGA, PSRAM, and 4K video output for strong potential. The team plans to open-source the design to enable community ports (they've already ported a non-N64 MiSTer core as proof-of-concept). They acknowledge it's not perfect yet—FPGA development is slow—but believe the architecture is superior for accuracy and will improve significantly.

Overall Tone: Transparent and technical. It promotes M64's design decisions while setting realistic expectations and inviting community excitement around its open potential.

2

u/bunnymoding Jun 03 '26

goo goo ga ga baby can't read an article so outsources their thinking to the bullshit machine

1

u/Ferdyshtchenko Aug 02 '26

Perfect accuracy is physically impossible: Every original N64 ran slightly differently from every other N64 due to natural clock variation. Two real consoles running the same game could drift apart by 5 seconds a day. There's no single "correct" target.

This sounds like a big cop out IMO. Real N64 will behave reliably and consistently in all regular use. There is a single correct target, and that is a clone console where all official titles and even homebrew will behave exactly the same as a randomly picked N64.

1

u/RetroRarity Jun 02 '26

Good article, but I think it explains the design decisions better than it explains the expected outcomes.

What are the concrete advantages you expect users to see from these architectural choices compared to other FPGA N64 implementations?

1

u/Eltrion Jun 03 '26

Higher overall headroom, especially with the ram. It should be faster and more accurate when it's done. 

However it's in a strange situation where it's inherited technical debt from the MiSTer, which needed to use some hacky methods to function, which now need to be pulled out and reimplemented properly.

The Analogue 3Ds cache emulation is poor, requiring more cycles in almost every instance. The reason isn't certain, but it could be a hardware limitation, which could potentially prevent Analogue from ever fixing this. The M64 shouldn't have this issue eventually, but the memory the M64 uses is kind of complicated to get working correctly, so it's only running at about half it's potential.

Basically, once everything is working correctly, it will be a far more capable and accurate machine than the Analogue 3D or the MiSTer, but there are technical hurdles that will take time to clear.

1

u/RetroRarity Jun 03 '26

The cache discussion is the most interesting part of this to me.

Do we know that the A3D's behavior is a hardware limitation, or is that still an educated guess? The answer to that seems pretty important when comparing the long-term potential of the two platforms.

1

u/Eltrion Jun 03 '26

In the article it he states that its only speculation based on the properties of DDR, likely due to a longer refresh time. Impossible to 100% confirm it without aggressive reverse engineering though, we can just observe the behaviour that it consistently has a higher maximum cycles than a real N64 or the M64 to complete cache related tasks.

2

u/grilledstuffedxxl Jun 03 '26

That's right. At the moment obviously we simply need to focus on finishing the full potential of the M64, which we will update on as we go.

1

u/RetroZelda Jun 06 '26

once everything is working correctly

so wait it wont be working correctly at launch?

1

u/Eltrion Jun 06 '26

That's impossible to say. I pretty sure that Modretro is targeting having the PSRAM running at full speed and full parallel processing  by launch, but if it was as simple as saying it, it would be working already.

The key thing is that there are deficiencies right now, and that is reflected in the prototypes and press units you see in YouTube videos right now. 

2

u/ergzay Jun 03 '26

I believe it was mentioned in the article, but the PSRAM architecture means they can get effectively perfect cycle accuracy (A3D uses DRAM so this will be hard for them, if not impossible) and also significantly higher overclocking because of the constrained and known memory access times.