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.
