Removing i486 from Linux: Molnar's patch

# Removing i486 Support from the Linux Kernel: Molnar's Patch and Historical Context

Ingo Molnar, the x86 architecture maintainer in the Linux kernel, has prepared a patch to completely remove the CONFIG_M486, CONFIG_M486SX, and CONFIG_MELAN options. This will exclude support for 486DX, 486SX, and AMD ELAN processors from modern kernel builds.

Technical Consequences of Removal

i486 support requires emulating key instructions: CX8 (cmpxchg8b for 64-bit atomic operations) and TSC (Time Stamp Counter for the task scheduler). This code adds kernel complexity, complicates debugging, and periodically triggers bugs.

Removing these layers will cut the codebase by 14,104 lines. In particular:

• The CX8 and TSC emulators will go.
• The math-emu library for FPU on 486SX will disappear.

This optimizes the scheduler and locking subsystems, easing the load on developers.

Note: No major distribution ships recent kernels with M486=y enabled. Current Linux branches on 32-bit i486 hardware are barely used.

Linus Torvalds' Position

Linus Torvalds has raised this issue multiple times. In 2022, he pointed out that emulating cmpxchg8b on i486 is pointless: the instruction exists on Pentium and all x86-64 CPUs. Torvalds suggested using LTS branches for legacy systems.

Earlier, in 2012, i386 support was dropped. Thirteen years ago, Torvalds proposed removing math-emu, but the community held off due to 486SX clones still in production. The consensus has since shifted.

Compatibility with Embedded Systems

Specialized chips like Intel Quark (i486-based) support CX8 and TSC natively—the patch won't touch them. Original i486s run on outdated distributions and old kernels.

Modern 32-bit distributions use X86_PAE, which requires CX8. Pure 32-bit x86 support is minimal.

• Benefits of Removal:

1. Code reduction by over 14k lines.

2. Simplified scheduler and locks.

3. Fewer emulation-related bugs.

4. Focus on relevant architectures.

Key Points

• Molnar's patch removes CONFIG_M486*, CX8/TSC emulators, and math-emu—minus 14,104 lines of code.
• i486s aren't used with modern kernels; distributions have shifted to PAE or 64-bit.
• Torvalds backs it: cmpxchg8b emulation is obsolete—use LTS for legacy.
• Embedded Quarks are unaffected—they have native CX8/TSC.

These changes will make the kernel cleaner for mid- and senior-level developers working on x86.

— Editorial Team