5 ms·

My first machine was a 286, so they have a special place in my heart. It should be pointed out that the clock speed of an ATMega328P (original Arduino / Arduino

My first machine was a 286, so they have a special place in my heart. It should be pointed out that the clock speed of an ATMega328P (original Arduino / Arduino nano) is nearly exactly the same as an 80286. Two of the same could easily outcompute a 286! You can buy them for ~$3 at the moment... I've put plenty of AVRs on ethernet - and once, a 386, using an ancient DOS TCP/IP stack, but never a 286. Linux doesn't support it due to requiring an FPU. It'd be fun to design the electronics for an old-style computer, but IMHO it'd be far more amusing to do it from scratch with an original architecture than try recreating a third party motherboard using a similar layout scheme. Something with parallelism and auto-scaling would be quaint.

einr 2y ago
I've put plenty of AVRs on ethernet - and once, a 386, using an ancient DOS TCP/IP stack, but never a 286. Linux doesn't support it due to requiring an FPU. mTCP (http://brutmanlabs.org/mTCP/ http://brutmanlabs.org/mTCP/) is a currently-maintained TCP/IP stack for MS-DOS that will run on any PC, 8088 and up. You also need a packet driver for your NIC. 286 machines are easy; nearly any 16-bit ISA NIC will work. XT-class machines are slightly trickier because not all NICs will work in 8-bit ISA slots and some packet drivers use 186/286 instructions.
pantalaimon 2y ago
Linux runs fine without an FPU, it even has a software emulation. It’s the hardware task switching features that the 386 introduced that it requires (and the reason why Linus got one in the first place back in the day)
- rep_lodsb 2y ago
  Hardware task switching was introduced on the 80286. And no modern OS - including Linux, except for the earliest versions - uses it, because the way it is implemented, the mechanism will do a lot of unnecessary duplicate work (preserving "garbage" register contents from just before the kernel is about to return to user mode, and will consequently have to restore them itself). The only situation where it would be required to use this misfeature is for handling exceptions like stack overflow in the case that they can occur in kernel mode. A "Task State Segment" is really more like a pre-allocated stack frame anyway, the CPU puts active ones in a linked list and does not allow freely switching between them, only returning to the outer level. x86-64 does not have hardware task switching anymore, instead it is now possible to switch to another stack on an exception or interrupt, which is all that the TSS was used for in practice anyway.
  - actionfromafar 2y ago
    Linux started on 80386.
    - rep_lodsb 2y ago
      Where am I disagreeing with that? Hardware task switching already existed on the 16-bit 80286, and remained supported on newer generations until being finally removed in x86-64. Early versions of Linux did use it for a time, but doing it in software turns out to be faster, because the mechanism was badly designed for what a general-purpose kernel has to do.
      - actionfromafar 2y ago
        Sorry, I see now that your comment could be read both ways, and my misinterpretation was actually a bit far-fetched.
- lproven 2y ago
  > It’s the hardware task switching features that the 386 introduced that it requires Nope. 8086 and 80286 multitasked fine. It was memory management hardware that came in with the 80386. You need that for virtual memory.
fredoralive 2y ago
It's the lack of a "modern" paged memory management unit that's the main issue for mainline Linux on a 286 AFAIK. Nobody wants to deal with x86 segmented memory, having a nice flat addressing paged mode is one of the big wins of the 386. Although I believe Linux dropped support for 386 (and 486?) a while back so they could remove some special cases for such old chips.
- rep_lodsb 2y ago
  Yes, paging is what is necessary to run Linux. The main problem it solves is how to allocate physical memory without having to constantly move variably-sized segments around to get rid of fragmentation. But I'm kind of sad that all OSes converged on a flat address space for the sake of ~PoRtAbIlItY~. Monocultures are bad, and we currently have one, where everything is basically a UNIX clone written in C. 32-bit x86 allowed to use both segments (of any length up to 4 GiB) and paging at the same time. Before the NX bit was introduced, this was the way to have write-xor-execute permissions. And some security features of segmentation can't be replicated at all in a flat address space: with a separate stack segment (or one with the same flat base address but a different limit), it will be impossible to add the wrong offset to a pointer to stack and get access to data that is outside of it. IMHO, the iAPX 432 had some good ideas, and x86 should have evolved in that direction, adding a few more "extra" segment registers which code can use freely as pointers to isolated objects. Each would have two "limit" fields (for negative and positive offsets from the base), with one of these spaces used for storing pointers to other objects, which can only be manipulated through CPU microcode that guarantees memory safety. Instead they eliminated segments completely in 64-bit mode, except for FS/GS serving as an extra offset that gets added with no limit checking whatsoever.
  - AshamedCaptain 2y ago
    I do agree with what you say and also have nice memories of 8086 segmentation. I find it funny that we are continuously forced to add workaround on top of workarounds to the flat address space in order to avoid accidental memory errors. Segmentation had all that decades ago, and easier debugability to begin with. But it is clear to me that we are moving again towards something like that, even if enforced at a different level.
cesarb 2y ago
> Linux doesn't support it due to requiring an FPU. I have used Linux on a 386SX which did not have any FPU. Linux had x87 floating point emulation for a long time (AFAIK, that emulation has since been removed, together with the rest of the support for very old CPUs). The main reasons Linux doesn't support the 80286 are AFAIK that the 80286 is a 16-bit CPU, and Linux (other than the short-lived ELKS fork) has never supported 16-bit CPUs; and that the 80286 doesn't have paging (it has only segmentation), which would require running it as a "no-MMU" CPU.
- lproven 2y ago
  They meant MMU.
int_19h 2y ago
Linux doesn't support it, but https://en.wikipedia.org/wiki/Embeddable_Linux_Kernel_Subset https://en.wikipedia.org/wiki/Embeddable_Linux_Kernel_Subset does.
lproven 2y ago
> Linux doesn't support it due to requiring an FPU. This is incorrect. FPU means "floating point unit". Linux does not need this. Few OSes do at all. You meant "MMU", meaning memory management unit. This is what Linux uses to perform virtual memory handling, and that is why Linux can't run on an 80286: the '286 had no MMU (or FPU). The difference being that 99% of computers without an FPU could just have one added. This was never true of an MMU. Indeed some early Unix machines based on CPUs with no MMU used an entire 2nd CPU just for MMU duties.
burnt-resistor 2y ago
Incorrect. Clock speed is not a 1:1 correlation with cycle efficiency. There are plenty of "slow" single-cycle architectures with very good cycle efficiency, and there are many architectures like the older P4 that have very poor cycle efficiency to chase the MHz wars.

einr 2y ago

I've put plenty of AVRs on ethernet - and once, a 386, using an ancient DOS TCP/IP stack, but never a 286. Linux doesn't support it due to requiring an FPU. mTCP (http://brutmanlabs.org/mTCP/ http://brutmanlabs.org/mTCP/) is a currently-maintained TCP/IP stack for MS-DOS that will run on any PC, 8088 and up. You also need a packet driver for your NIC. 286 machines are easy; nearly any 16-bit ISA NIC will work. XT-class machines are slightly trickier because not all NICs will work in 8-bit ISA slots and some packet drivers use 186/286 instructions.

pantalaimon 2y ago

Linux runs fine without an FPU, it even has a software emulation. It’s the hardware task switching features that the 386 introduced that it requires (and the reason why Linus got one in the first place back in the day)

rep_lodsb 2y ago
Hardware task switching was introduced on the 80286. And no modern OS - including Linux, except for the earliest versions - uses it, because the way it is implemented, the mechanism will do a lot of unnecessary duplicate work (preserving "garbage" register contents from just before the kernel is about to return to user mode, and will consequently have to restore them itself). The only situation where it would be required to use this misfeature is for handling exceptions like stack overflow in the case that they can occur in kernel mode. A "Task State Segment" is really more like a pre-allocated stack frame anyway, the CPU puts active ones in a linked list and does not allow freely switching between them, only returning to the outer level. x86-64 does not have hardware task switching anymore, instead it is now possible to switch to another stack on an exception or interrupt, which is all that the TSS was used for in practice anyway.
- actionfromafar 2y ago
  Linux started on 80386.
  - rep_lodsb 2y ago
    Where am I disagreeing with that? Hardware task switching already existed on the 16-bit 80286, and remained supported on newer generations until being finally removed in x86-64. Early versions of Linux did use it for a time, but doing it in software turns out to be faster, because the mechanism was badly designed for what a general-purpose kernel has to do.
    - actionfromafar 2y ago
      Sorry, I see now that your comment could be read both ways, and my misinterpretation was actually a bit far-fetched.
lproven 2y ago
> It’s the hardware task switching features that the 386 introduced that it requires Nope. 8086 and 80286 multitasked fine. It was memory management hardware that came in with the 80386. You need that for virtual memory.

fredoralive 2y ago

It's the lack of a "modern" paged memory management unit that's the main issue for mainline Linux on a 286 AFAIK. Nobody wants to deal with x86 segmented memory, having a nice flat addressing paged mode is one of the big wins of the 386. Although I believe Linux dropped support for 386 (and 486?) a while back so they could remove some special cases for such old chips.

rep_lodsb 2y ago
Yes, paging is what is necessary to run Linux. The main problem it solves is how to allocate physical memory without having to constantly move variably-sized segments around to get rid of fragmentation. But I'm kind of sad that all OSes converged on a flat address space for the sake of ~PoRtAbIlItY~. Monocultures are bad, and we currently have one, where everything is basically a UNIX clone written in C. 32-bit x86 allowed to use both segments (of any length up to 4 GiB) and paging at the same time. Before the NX bit was introduced, this was the way to have write-xor-execute permissions. And some security features of segmentation can't be replicated at all in a flat address space: with a separate stack segment (or one with the same flat base address but a different limit), it will be impossible to add the wrong offset to a pointer to stack and get access to data that is outside of it. IMHO, the iAPX 432 had some good ideas, and x86 should have evolved in that direction, adding a few more "extra" segment registers which code can use freely as pointers to isolated objects. Each would have two "limit" fields (for negative and positive offsets from the base), with one of these spaces used for storing pointers to other objects, which can only be manipulated through CPU microcode that guarantees memory safety. Instead they eliminated segments completely in 64-bit mode, except for FS/GS serving as an extra offset that gets added with no limit checking whatsoever.
- AshamedCaptain 2y ago
  I do agree with what you say and also have nice memories of 8086 segmentation. I find it funny that we are continuously forced to add workaround on top of workarounds to the flat address space in order to avoid accidental memory errors. Segmentation had all that decades ago, and easier debugability to begin with. But it is clear to me that we are moving again towards something like that, even if enforced at a different level.

cesarb 2y ago

> Linux doesn't support it due to requiring an FPU. I have used Linux on a 386SX which did not have any FPU. Linux had x87 floating point emulation for a long time (AFAIK, that emulation has since been removed, together with the rest of the support for very old CPUs). The main reasons Linux doesn't support the 80286 are AFAIK that the 80286 is a 16-bit CPU, and Linux (other than the short-lived ELKS fork) has never supported 16-bit CPUs; and that the 80286 doesn't have paging (it has only segmentation), which would require running it as a "no-MMU" CPU.

lproven 2y ago
They meant MMU.

int_19h 2y ago

Linux doesn't support it, but https://en.wikipedia.org/wiki/Embeddable_Linux_Kernel_Subset https://en.wikipedia.org/wiki/Embeddable_Linux_Kernel_Subset does.

lproven 2y ago

> Linux doesn't support it due to requiring an FPU. This is incorrect. FPU means "floating point unit". Linux does not need this. Few OSes do at all. You meant "MMU", meaning memory management unit. This is what Linux uses to perform virtual memory handling, and that is why Linux can't run on an 80286: the '286 had no MMU (or FPU). The difference being that 99% of computers without an FPU could just have one added. This was never true of an MMU. Indeed some early Unix machines based on CPUs with no MMU used an entire 2nd CPU just for MMU duties.

burnt-resistor 2y ago

Incorrect. Clock speed is not a 1:1 correlation with cycle efficiency. There are plenty of "slow" single-cycle architectures with very good cycle efficiency, and there are many architectures like the older P4 that have very poor cycle efficiency to chase the MHz wars.