Mayor of Architecture x86, Ingo Mollnar, has initiated discussions regarding a set of patches for i486 processors and the initial 586 processors in the Linux kernel. The proposal aims to only support X86 processors that are equipped with the CX8 (CMPXCHG8B) and TSC registers, which were introduced in Pentium processors.
To maintain support for the i486 CPUs in the kernel, the code for emulating the cx8 and tsc operations needs to be retained. However, this code adds complexity to the kernel and can lead to issues that require developers’ time to analyze. By discontinuing support for i486, 14104 lines of code can be removed from the kernel, simplifying certain functions by eliminating the need for emulating CX8 and TSC, and allowing removal of the Math-EMU library used for emulating FPU.
Prior to Ingo’s patch proposal, Linus Torvalds questioned the necessity of continuing i486 support during a discussion about emulation issues related to CX8. Linus supports the removal of i486 support, citing the removal of i386 processor support from the kernel in 2012. He believes it is time to stop allocating resources to solve problems stemming from i486 processors. A similar proposal by Linus in October 2022 did not progress.
In current Linux kernels, i486 systems are rarely utilized. Specialized options for 486 processors found in embedded systems, such as Intel Quark, already support CX8 and TSC, so their functionality will not be affected. Older original i486 CPUs are typically used with outdated distributions that provide older versions of the Linux kernel. Modern Linux distributions have transitioned away from supporting 32-bit X86 systems and now require CX8 support when utilizing the X86_PAE option.