Show HN: I486SX_soft_FPU – Software FPU Emulator for NetBSD 10 on 486SX

This retro-computing project restores support for x87 floating-point unit (FPU) emulation in the NetBSD kernel, targeting legacy 486SX-class processors without hardware FPUs. It brings back the original MATH_EMULATE option into NetBSD 10.x and beyond, as well as reverts and reworks the changes introduced in commit dfe83e0, which removed FPU emulation support from the kernel.

This project is a work in progress and may contain bugs or incomplete functionality. Use at your own risk. The author is not responsible for any issues caused by its use.

• fyl2x seems to work properly on FPU, but when executed from libc, log functions bring incorrect results
• Some operations have hard-to-detect issues with precision

Instruction Status Description Opcode Example

fninit	✅ OK	Initialize FPU	9B DB E3	fninit

Instruction Status Description Opcode Example

fld	✅ OK	Load floating-point value	D9 /0	fld st(1)
fldt	✅ OK	Load 80-bit extended precision	DB /5	fldt [mem]
filds	✅ OK	Load integer (short)	DB /0	filds [mem]
fildl	✅ OK	Load long integer	DB /A	fildl [mem]

Instruction Status Description Opcode Example

fstps	✅ OK	Store and pop single precision	D9 /3	fstps [mem]
fstpt	✅ OK	Store 80-bit extended precision & pop	DB /7	fstpt [mem]

Instruction Status Description Opcode Example

fadd	✅ OK	Add floating-point values	D8 /0	fadd st(1), st
faddl	✅ OK	Add long double from memory	DA /0	faddl [mem]
fsub	✅ OK	Subtract floating-point values	D8 /4	fsub st(1), st
fsubp	✅ OK	Subtract with pop	DE /5	fsubp st(1), st(0)
fmul	✅ OK	Multiply floating-point values	D8 /1	fmul st(1), st
fdiv	✅ OK	Divide floating-point values	D8 /6	fdiv st(1), st
fdivp	✅ OK	Divide with pop	DE /7	fdivp st(1), st(0)
fscale	✅ OK	Scale ST(0) by ST(1)	D9 FD	fscale

Instruction Status Description Opcode Example

fcom	✅ OK	Compare floating-point values	D8 /2	fcom st(1)
fucom	✅ OK	Unordered compare ST(0) with ST(i)	DD E0+i	fucom st(1)
fucomp	✅ OK	Unordered compare and pop	DD E8+i	fucomp st(1)
ftst	✅ OK	Compare ST(0) with 0.0	D9 E4	ftst

Instruction Status Description Opcode Example

fxch	✅ OK	Exchange ST(0) with ST(i)	D9 C8+i	fxch st(1)
fchs	✅ OK	Change sign of ST(0)	D9 E0	fchs
fabs	✅ OK	Absolute value of ST(0)	D9 E1	fabs
frndint	✅ OK	Round ST(0) to integer	D9 FC	frndint

Instruction Status Description Opcode Example

fyl2x	⏳ TBD	Compute y × log₂(x) (ST(1) × log₂(ST(0))) and pop	D9 F1	fyl2x
fyl2xp1	❌ N/A	Compute y × log₂(x+1) and pop	D9 F9	fyl2xp1
fxtract	❌ N/A	Extract: ST(0) → ST = significand, ST+1 = exponent	D9 F4	fxtract
f2xm1	✅ OK	Compute 2^x - 1 for ST(0)	D9 F0	f2xm1
fpatan	❌ N/A	Compute arctangent(ST(1)/ST(0)) and pop	D9 F3	fpatan
fsqrt	❌ N/A	Compute square root of ST(0)	D9 FA	fsqrt

• Read Chapter 32. Obtaining the sources
• Read Chapter 34. Compiling the kernel
• Add the repository contents under /src/sys/arch on NetBSD 10.x machine, then run:

Check Releases for the compiled netbsd kernel with FPU-emulation as well, as the drive image

If you have an idea, a question, or have found a problem, feel free to open an issue or contact me directly: Mikhail Zakharov.

My changes to the original code by Linus Torvalds, William "Bill" Jolitz, and NetBSD are licensed under the BSD-2-Clause license.