mezantrop/i486SX_soft_FPU: Bring back FPU emulation for i486SX CPU on NetBSD

2025-04-27 17:04:00
github.com

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

x87 FPU Emulated Instructions

🧠 Control & Initialization

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]`

📤 Store from FPU Stack

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]`

➕➖✖️➗ Arithmetic Operations

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`

🔁 Stack Manipulation & Modification

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`

📈 Logarithmic and Special Math Operations

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`

$ cd /usr/src/sys/arch/i386/conf/
$ vi GENERIC_TINY_486SX# or GENERIC_PS2TINY_486SX or create your own kernel configuration with "options MATH_EMULATE"
$ config GENERIC_TINY_486SX
$ cd ../compile/GENERIC_TINY_486SX
$ make depend
$ make

# If everythig good, install the new kernel under root:
# mv /netbsd /netbsd.old
# mv netbsd /

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.

Source Link

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