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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 se

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.
- PaulHoule 2y ago
  I did not mind programming the 16-bit x86 model in assembly. You could do a lot of things such as use segment register pointers (16 byte aligned) for "large" data structures which could themselves be addressed with ordinary 16 bit pointers and chained together to make "extra-large" structures. Compilers like Turbo Pascal and Microsoft C all gave you a choice of which memory model you wanted to use, often you wrote programs where a 64k code and 64k data space is all you need.
  - rep_lodsb 2y ago
    Turbo Pascal actually didn't: IIRC versions 3.x and lower used the "small" model (they were also available for CP/M on Z80, where 64K was the entire address space anyway), then newer ones exclusively the "large" one. TP forcing use of large pointers for everything, and its lack of even trivial peephole optimizations, probably contributed to the myth that C was somehow inherently more efficient.
    - burnt-resistor 2y ago
      You misremembered. [BT]P7 includes for compatibility: tiny (.COM), small, compact, medium, large, and large w/ overlays (which can use EMS when available). There are DPMI clients for [BT]P7 that make it possible to switch to protected mode and use more memory. It definitely generated offset-only pointers in the tiny, small, and medium memory models because that's how they referred to data with the common DS segment. I did plenty of memory and instruction-level debugging in [BT] Profiler/Debugger and inspected plenty of pointers where they lived in RAM.
      - rep_lodsb 2y ago
        What command line option or compiler directive can I use to create a .COM program? How do I declare a near pointer variable? http://bitsavers.trailing-edge.com/pdf/borland/turbo_pascal/Turbo_Pascal_Version_7.0_Language_Guide_1992.pdf http://bitsavers.trailing-edge.com/pdf/borland/turbo_pascal/... Page 221 (232 in the PDF): "A Pointer type is stored as two words"
burnt-resistor 2y ago
Correction: Protected user-supervisor mode separation, and virtual memory with page faulting are what are essential. NX helps enforce W^X but it's not a deal-breaker functionally even if it's very useful. Who cares about address space organization? (k|)ASLR, PIC & PIE means this is a detail not worth your time. This kind of standardization is vital for security, debugging, and profiling. The 386 has LDT that generally isn't used, and there's only one TS that is updated rather than multiple ones. FS and GS tend to be used for thread-local storage.

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.

PaulHoule 2y ago
I did not mind programming the 16-bit x86 model in assembly. You could do a lot of things such as use segment register pointers (16 byte aligned) for "large" data structures which could themselves be addressed with ordinary 16 bit pointers and chained together to make "extra-large" structures. Compilers like Turbo Pascal and Microsoft C all gave you a choice of which memory model you wanted to use, often you wrote programs where a 64k code and 64k data space is all you need.
- rep_lodsb 2y ago
  Turbo Pascal actually didn't: IIRC versions 3.x and lower used the "small" model (they were also available for CP/M on Z80, where 64K was the entire address space anyway), then newer ones exclusively the "large" one. TP forcing use of large pointers for everything, and its lack of even trivial peephole optimizations, probably contributed to the myth that C was somehow inherently more efficient.
  - burnt-resistor 2y ago
    You misremembered. [BT]P7 includes for compatibility: tiny (.COM), small, compact, medium, large, and large w/ overlays (which can use EMS when available). There are DPMI clients for [BT]P7 that make it possible to switch to protected mode and use more memory. It definitely generated offset-only pointers in the tiny, small, and medium memory models because that's how they referred to data with the common DS segment. I did plenty of memory and instruction-level debugging in [BT] Profiler/Debugger and inspected plenty of pointers where they lived in RAM.
    - rep_lodsb 2y ago
      What command line option or compiler directive can I use to create a .COM program? How do I declare a near pointer variable? http://bitsavers.trailing-edge.com/pdf/borland/turbo_pascal/Turbo_Pascal_Version_7.0_Language_Guide_1992.pdf http://bitsavers.trailing-edge.com/pdf/borland/turbo_pascal/... Page 221 (232 in the PDF): "A Pointer type is stored as two words"

burnt-resistor 2y ago

Correction: Protected user-supervisor mode separation, and virtual memory with page faulting are what are essential. NX helps enforce W^X but it's not a deal-breaker functionally even if it's very useful. Who cares about address space organization? (k|)ASLR, PIC & PIE means this is a detail not worth your time. This kind of standardization is vital for security, debugging, and profiling. The 386 has LDT that generally isn't used, and there's only one TS that is updated rather than multiple ones. FS and GS tend to be used for thread-local storage.