MiSTer z486 brings faster 486 PC performance and x87 support to FPGA

MiSTer’s PC side has taken a sizeable step forward with the latest z486 release. The update replaces the previous z386x CPU and z386 MiSTer naming with z486, reflecting how far the processor design has moved beyond its earlier target. The headline addition is experimental x87 floating-point support. That might sound like a small technical detail, but it changes what the core can realistically run. Floating-point-heavy software now has much more to work with, and performance has improved enough to make titles such as Quake genuinely useful as test cases rather than just proof that the software boots. There’s also adjustable CPU speed, faster floating-point execution and a reorganised internal design. Taken together, this is much more than a rename.

From z386 to z486

The new name makes sense once you look at the processor itself. z486 is an 80486-class pipelined FPGA CPU written in SystemVerilog, with a design aimed at delivering practical 486-level performance on MiSTer hardware.

It isn’t trying to copy one exact Intel chip gate by gate. Instead, the goal is closer to matching the kind of speed and behaviour you’d expect from a fast 486-era PC while staying within MiSTer’s FPGA limits. That approach seems to be paying off.

The project currently puts performance at roughly the level of a 486DX2-66 system. In Doom timedemo testing, z486 reaches 29.1 frames per second at maximum detail. The ao486 core, running on the same MiSTer setup, manages 21.0 frames per second. That’s a noticeable gap.

The CPU gets there by using hardwired logic for common instructions while handling more complicated x86 operations through microcode. It’s a practical design choice. Fast instructions stay fast, while the processor doesn’t need huge amounts of FPGA logic to handle every possible operation in dedicated hardware.

x87 support changes what the core can do

The biggest change is the addition of experimental x87 floating-point support. It’s still incomplete. That matters. But the implementation already covers enough of the x87 instruction set to run software that leans heavily on floating-point maths, including Quake. That’s where the numbers get interesting.

Quake 1.0.6 now reaches around 6.6 frames per second. The earlier floating-point implementation produced roughly 2.7 frames per second in the same test. So this isn’t a minor optimisation. It’s more than twice the previous frame rate.

Games from the early DOS period often cared mainly about integer CPU speed, but software became more demanding as the 1990s moved on. Engines such as Quake’s put far more pressure on floating-point hardware. A fast integer core alone can’t solve that problem. Adding a usable x87 implementation gives z486 a much better shot at handling that later generation of software.

CPU speed can now be turned down

Faster isn’t always better on an old PC. Some DOS software assumes a certain processor speed, and running it on something much quicker can cause timing problems, broken gameplay or behaviour the original programmers never expected.

The new z486 release deals with that by adding adjustable CPU speed. It works with the existing ao486 sysctl.exe utility, so users don’t need a completely new control tool just for this core. CPU speed can be changed from software, which makes it easier to match the processor to whatever program is running.

One example is the 0MHz Lemmings 1 disk image. It can run correctly in high-performance mode when the CPU is set to 30 MHz. That kind of control matters more than a headline benchmark. A fast FPGA CPU is useful, but a fast CPU that can slow down when software needs it is far more flexible.

Memory and graphics stay familiar

The rest of the system remains fairly straightforward. z486 uses MiSTer SDRAM as system memory, with support for 16, 32, 64 and 128 MB configurations. An SDRAM module is required.

Graphics support includes standard VGA along with ET4000-compatible SVGA modes. That keeps the surrounding PC environment familiar while most of the development work focuses on the processor itself.

The CPU’s internal structure has also been cleaned up. Functional units now have clearer ownership, and the accompanying architecture documentation has been updated. You won’t see that change while playing a game, but it should make the processor easier to work on as missing features and compatibility issues get addressed.

Installation needs more than a core file

There is one practical detail worth knowing. Because the project has moved from Z386 to Z486, MiSTer now treats it as a different system. You can’t simply drop in the new RBF and carry on exactly as before.

The release requires an updated MiSTer binary. The z486 RBF goes into the Computer directory, while ROMs and disk images now use a Z486 games directory. Anyone upgrading from the old z386 setup will therefore need to move or copy existing disk images into the new location.

Once that’s done, the 13 August release brings together the new z486 identity, experimental x87 support, much faster floating-point performance, adjustable CPU speed and a processor that now sits firmly in 486-class territory.

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