Linux-Kernel: Drop 486 CPU Support
Linux Kernel Considers Dropping Support for Ancient x86 Processors
A prominent Linux kernel developer, ingo Molnar, is advocating for the removal of support for older x86 processors, specifically those of the 486 generation and some early 586 models. Molnar refers to existing support as “compatibility adhesive” and believes its removal would benefit the kernel.
The focus is on CPUs lacking registers for a Time Stamp Counter (TSC) and those without the CMPXCHG8B command (CX8 bit in the CPUID). These features were absent in processors predating Intel’s Pentium (P5), released in 1993. While other manufacturers produced processors labeled “586” before the Pentium, thay differed significantly in architecture. Intel transitioned away from numerical naming conventions due to trademark limitations, opting for the “Pentium” brand.
Molnar stated on a kernel mailing list that the Linux kernel currently includes emulations to enable functionality from modern 64-bit processors on these CPUs, which are now over 30 years old. He argues that maintaining and testing this compatibility is disproportionately burdensome and prone to errors. Molnar also referenced similar sentiments recently expressed by Linus Torvalds.
Potential for Significant Code Reduction
Molnar estimates that removing support for these older CPUs could result in over 14,000 deletions from the Linux kernel. He notes that a significant portion of the support resides in the ”Math-Emu” library.Eliminating 486 support would require changes to approximately 80 files across the kernel, with 33 files affected even without considering Math-Emu.
The proposed changes would only affect the current Linux kernel and future development. Molnar’s proposal does not impact older kernel versions, ensuring continued support for these historical processors.He added that many current Linux distributions already require 64-bit processors, rendering them incompatible with 486 and early Pentium systems.
Linux kernel Considers Dropping Support for Ancient x86 Processors: A Q&A
Are Linux developers considering dropping support for older x86 processors?
Yes, a prominent Linux kernel developer, Ingo Molnar, is proposing the removal of support for older x86 processors, specifically the 486 generation and some early 586 models.
Why is this proposal being made?
Molnar refers to the existing support as “compatibility adhesive” and believes its removal would benefit the kernel. He argues that maintaining and testing compatibility for these older processors is disproportionately burdensome and prone to errors.
What specific processors are targeted by this proposal?
The focus is on CPUs lacking the Time Stamp Counter (TSC) register and those without the CMPXCHG8B command (CX8 bit in the CPUID). These features were absent in processors predating Intel’s Pentium (P5), which was released in 1993.
Why are early “586” processors included in this consideration?
While other manufacturers produced processors labeled “586” before the Pentium, thay differed substantially in architecture. Intel transitioned away from numerical naming conventions due to trademark limitations, opting for the “Pentium” brand.
What are the potential benefits of removing support for these old processors?
Code Reduction: Removing support coudl result in over 14,000 deletions from the Linux kernel, according to Molnar.
Reduced Maintenance Burden: Less code to maintain and test translates to fewer potential bugs and a more efficient development process.
Where does this support reside within the kernel?
A notable portion of the support resides in the “Math-emu” library.
How many files would be affected by removing 486 support?
Eliminating 486 support would require changes to approximately 80 files across the kernel. Even without considering the Math-Emu library, 33 files would still be affected.
Will this proposal affect older versions of the Linux kernel?
No. The proposed changes would only affect the current Linux kernel and future development. Older kernel versions will continue to support these past processors.
Why is this even possible?
The Linux kernel includes emulations to enable functionality from modern 64-bit processors on these older cpus, which are over 30 years old.
Are older processors still widely used?
Many current Linux distributions already require 64-bit processors, rendering them incompatible with 486 and early Pentium systems.
What specific features are missing from these older CPUs that are causing compatibility issues?
The primary focus is on CPUs lacking the following:
Registers for a Time Stamp Counter (TSC)
The CMPXCHG8B command (CX8 bit in the CPUID)
Who else supports this idea?
Linus Torvalds, the creator of Linux, has expressed similar sentiments.
What are the key differences between the 486, early 586 (before Pentium), and the Pentium?
The transition to the pentium brought significant architectural changes, including the introduction of features like the Time Stamp Counter (TSC) and the CMPXCHG8B command (CX8 bit). These features are crucial for modern operating systems and performance optimization. The 486 and early 586 processors lacked these features, leading to the need for emulation in the Linux kernel to support modern features.
Here is a table summarizing the key differences:
| Processor | Release Date | Key Features (Notable Absences) | Impact of Modern Linux Support |
|---|---|---|---|
| 486 | Early 1990s | Lacks features like TSC and CMPXCHG8B | Requires extensive emulation, increasing kernel size and complexity |
| Early 586 (Pre-Pentium) | Early to Mid 1990s | Varied architectures, frequently enough lacking TSC and CMPXCHG8B | Creates compatibility challenges, adding to the need for emulation |
| Pentium (P5) | 1993 | Introduces TSC and other modern features | better support for modern features which leads to more efficient modern operating systems |