> ...Intel could've made ARM32/64-mode segments a reality...
While I myself admire this particular breed of masochism, the direction that Intel currently wants to take is apparently quite the opposite.
In May last year, they proposed X86S[1][2][3] which tosses out 16-bit support completely, along with 32 bit kernel mode (i.e. the CPU boots directly into 64 bit mode, 32 bit code is only supported in ring 3).
The proposal trims a lot of historical baggage, including fancy segmentation/TSS shenanigans, privilege rings 1 & 2, I/O port access from ring 3, non-flat memory models, etc... limiting the CPU to 64 bit kernel mode, and 64 or 32 bit x86 user mode. With the requirement for 64 bit kernel mode, it effectively also removes un-paged memory access.
[1] https://en.wikipedia.org/wiki/X86-64#X86S
[2] https://www.intel.com/content/www/us/en/developer/articles/t...
It's not just Intel. It's clear that ARM is also going in the same direction, by allowing newer cores to be 64-bit (AArch64) only, dropping compatibility with the older 32-bit ARM ISA (actually three ISAs: traditional 32-bit ARM, Thumb, and Thumb2), and IIRC some manufacturers of ARM-based chips are already doing that.
If anyone actually has a serious need to use ancient 16 bit software, emulators like 86Box work very well. Software that old doesn’t really need performance faster than, say, a Pentium 90, which 86Box has no trouble achieving on my M1 (ARM) MacBook.
You can also use winevdm[1] on modern 64 bit Windows operating systems. I have this in production use for a niche case where someone can’t give up a particular 16 bit app, and I didn’t want to tangle with a VM for them.
The technical details of making sure a modern CPU still functions exactly like an 80386, which in turn made sure it functioned like an 80286, when you fire up a 16 bit task on, say, 32-bit Windows 10 (or 64-bit with something like winevdm[1]) sound like a nightmare for a microcode engineer or QA tester.
...and thus it didn't get used, meaning Intel didn't make it faster, and so the vicious cycle continued.
Hardware task switching could've made software simpler and more forward-compatible.
Of course they eventually reinvented most of it with the virtualisation extensions anyway.
During that time, Linux had a limit on the number of processes, which was due the maximum number of TSS entries that fit in the x86 GDT.
Eventually the Linux kernel was changed to the more versatile context-switch method it uses today. Among other things, this change was important for thread performance, as thread context switches can skip the TLB flush. Same for kernel mode tasks. Software task switching also greatly increased the number of processes and threads that can be launched, from about 8000 (across all CPU cores) to millions.
I'd argue the break started with the Pentium Pro, at that point things shifted architecturally.
Some CPUs (I cannot remember which) built in an A20 gate to their CPUs to improve performance.
The P6 was a complete implementation of the 80286 and 80386, Virtual 8086 mode, TSS, and all - you could boot DOS or an 80286 operating system on a P6 without any problems, although the design was not optimised to improve performance of 16-bit software. This was enough of a problem that they rolled back that design by the Celeron era because there were still a lot of people using 16-bit apps.
All of that is a tiny amount of die area relative to the whole CPU. After all, a 386 has only 275k transistors.
X86S is Stupid. Intel apparently forgot what made them worth choosing over competitors like ARM and now RISC-V. Non-compatible x86 makes little sense.
...and if they want to include the virtualisation extension, they still need to include that backwards-compatible functionality.
Notice that only 32bit kernel/R-0 is removed, but not usermode/R-3 so even when reducing this your 64bit Windows will still run clean 32bit software built for Win95 from the 90s.
Even today you need to run a virtualized 32bit OS to run old 16bit software (the negative part is if you still run a virtualized 32bit OS then it'll need to be emulated instead of HW virtualized if the virtualization solutions allowed that).
People (myself and others I know) choose ARM chips because they don't absolutely mandate the purchase of sanctioned chipsets/other supporting components you don't have access to, impossible-to-obtain specs, etc.