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8086 Segmented Memory was a good idea

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Re: 8086 Segmented Memory was a good idea

#71

Post by juancn »

gdwatson wrote: post #24
st_goliath wrote: post #11

> 8086 Segmented Memory Was a Good Idea. Yet the article goes about the most ass backward way of explaining 8086 segments and constructs a convoluted mental picture of dividing memory into overlapping chunks. It's really, really simple: segments on the 8086/88 are 64k sliding windows into an 1M address space. You can move them around at 16 byte granularity. You need more than 64k for code + data? No problem, the CPU…

It was a clever hack for porting existing code. But it doesn’t scale at all – you’ve just described adding four registers to a register-starved architecture in order to solve the issue for one CPU generation or so.

Yeah, but it was probably the right call at the time.

Backward compatibility was a breath of fresh air at a time were code needed constant porting and rewriting. No two machines were alike.

It's one of the reasons the PC became so popular.

Re: 8086 Segmented Memory was a good idea

#72

Post by AnimalMuppet »

AshamedCaptain wrote: post #59
st_goliath wrote: post #11

> 8086 Segmented Memory Was a Good Idea. Yet the article goes about the most ass backward way of explaining 8086 segments and constructs a convoluted mental picture of dividing memory into overlapping chunks. It's really, really simple: segments on the 8086/88 are 64k sliding windows into an 1M address space. You can move them around at 16 byte granularity. You need more than 64k for code + data? No problem, the CPU…

I think you are missing the entire point of the article (which I kinda agree with), and just repeating the popular wisdom. In the era a machine with "object addressing" sounded like a perfectly valid futuristic design (what a Lisp machine strived to be; I guess today you would call it tagged memory of some kind). The 8086 is not that, but the original design would have allowed to evolve it into something like that. T…

But with only four segment registers, you couldn't treat it as that - not unless you only had four objects. So treating it as a sliding window was all you actually could do with it.

Re: 8086 Segmented Memory was a good idea

#73

Post by alerighi »

wewewedxfgdf wrote: post #19

I seem to recall at the time that flat memory was self evidently a better idea. It's not like people were sitting around going "gee I can't think of any better way to do memory addressing that this" until some genius suggests "how about flat?!?!?" Everyone knew flat was best but were stuck with 8086 crap.

Except than later we returned to a sort-of segmented memory. That is of course paging, our programs allocates pages of memory that are a fixed size (4096 bytes) and are arranged in memory or in swap space how the OS decides.

We just have the illusion of a "flat" memory model, but it's not really flat, the CPU and the operating system does an important job in translating our flat memory model in something that is not flat at all. All that address translation work could have been avoided if we accepted to not have a flat memory model and be aware that our memory is divided in pages.

Basically we are doing in hardware the job of managing a non flat memory space that the programmer, or well, the compiler (or these days you would say the AI agent) could probably to better because it knows how to allocate things to avoid being them on page boundaries, and all of this to give the illusion to the programmer that it's working with a flat memory (except when it does something wrong and gets a segmentation fault, that, as the name suggests, is an hint that at the end the memory is not really flat).

Re: 8086 Segmented Memory was a good idea

#74

Post by bluGill »

rob74 wrote: post #25
gdwatson wrote: post #24

Earlier quoted context omitted.

It was a clever hack for porting existing code. But it doesn’t scale at all – you’ve just described adding four registers to a register-starved architecture in order to solve the issue for one CPU generation or so.

Plus all this pointer juggling would have been more or less ok (or not ok, but doable) when programming in assembly, but for a compiler it would have been a recipe for disaster...

I'm sure a modern compiler would have no problem with it, but in 1980 optimizer technology wasn't there yet. Modern compilers use more memory that engineers would dare dream of in 1980.

By having no problem I mean we know enough about writing an optimizer to write such a thing. I don't think any compiler does, just that they could.

Re: 8086 Segmented Memory was a good idea

#75

Post by NoGravitas »

PunchyHamster wrote: post #38

Author comes from some weird assumption that software is some annoying byproduct of making hardware, rather than a fact that the hardware is made to run software and making it easier is a goal. It was just a hack . Hack to delay migration to 32 bit architecture. Effective one, but hack nonetheless

I think it's more like marking a transition in how we thought about software.

When I was learning C, we did things at a reasonably low level. I was learning data structures, and building things like binary trees out of things like structs, and the structs were fixed-sized memory blocks holding pointers to regions of memory which were either more structs or data fields. All reasonable stuff. But we weren't writing for a particular machine. We were writing for the idea of a machine, and part of that idea was that the machine had a flat memory model. This really struck me when I compiled my homework (parse some data into a tree) on the departmental SunOS server, and it worked fine, and then took it home and compiled it with Borland C for DOS on my 386 and it segfaulted on the same data. That was when I learned to hate segmented memory, but looking back, it seems to me that I learned the wrong lesson.

I learned to write software for a lowish-level model of an idealized computer. The generation before me was always writing software for a specific computer, consisting of a specific set of hardware. The software was always the goal, but the nature of the task was defined by the hardware. Things like memory segmentation were facts about the hardware, and the available hardware varied widely at the time in a way modern hardware doesn't, really, except maybe in the embedded space.

Re: 8086 Segmented Memory was a good idea

#76

Post by bluGill »

alerighi wrote: post #73
wewewedxfgdf wrote: post #19

I seem to recall at the time that flat memory was self evidently a better idea. It's not like people were sitting around going "gee I can't think of any better way to do memory addressing that this" until some genius suggests "how about flat?!?!?" Everyone knew flat was best but were stuck with 8086 crap.

Except than later we returned to a sort-of segmented memory. That is of course paging, our programs allocates pages of memory that are a fixed size (4096 bytes) and are arranged in memory or in swap space how the OS decides. We just have the illusion of a "flat" memory model, but it's not really flat, the CPU and the operating system does an important job in translating our flat memory model in something that is not…

In turn though, the thinking needed to handle non-flat memory is a complexity that most programmers cannot handle - and even those who could probably should spend their brain power on the complex parts of their program not managing memory. Best to leave that hard part to a few experts instead of make everyone understand it.

The above is very similar to the argument that you should use a garbage collected langauge.

Re: 8086 Segmented Memory was a good idea

#77

Post by sumtechguy »

mschaef wrote: post #37
RagnarD wrote: post #35

I had to use it to do image processing on a 256MB image buffer back in the 1980s in assembly language. It was absolutely hideous. Give me a flat 32 bit memory address space any day (e.g. MC68000 around the same time.)

> I had to use it to do image processing on a 256MB image buffer back in the 1980s in assembly language.... Give me a flat 32 bit memory address space any day (e.g. MC68000 around the same time.) Huh? There were no segmented x86 machines capable of addressing 256MB of RAM, aside from the 386 (maybe). If you had a 386 and the $130K of memory your statement implies, you probably also could afford a Unix (or something e…

Probably talking about swapping it in from some external datastore. These days you would open the file and dump it into a single buffer and rip across it, and not even really stress about it. Even 256 meg of hard drive. That would have been impressively expensive in the 80s.

Back then you had to chunk it out and fiddle with the offsets. Even then you still would have had to manage loading out the next chunk.

If my memory is right 1MB of memory in the early 90s was like 200-300 per meg. Would have to dig up a computer shopper and look.

Re: 8086 Segmented Memory was a good idea

#78

Post by phkahler »

wewewedxfgdf wrote: post #19

I seem to recall at the time that flat memory was self evidently a better idea. It's not like people were sitting around going "gee I can't think of any better way to do memory addressing that this" until some genius suggests "how about flat?!?!?" Everyone knew flat was best but were stuck with 8086 crap.

I was drooling over the MC68000. I had a pre-release reference manual from 1979 and it was the most awesome chip around. Atari and Commodore and Apple used it after their 6502 systems. Arcade games used it after their 6502 and 6809 days. The only reason x86 became popular was because IBM put it in the PC - not for any technical reasons.

Re: 8086 Segmented Memory was a good idea

#79

Post by AnimalMuppet »

mschaef wrote: post #55
senfiaj wrote: post #51

Earlier quoted context omitted.

>> 8086 can't address 2MB of memory This was just for illustration, not claiming that actual 8086 does this. >> Raymond Chen talks a bit about how it worked in Windows 3.x here: https://devblogs.microsoft.com/oldnewthing/20171113-00/?p=97 ... And this is the problem, it was very painful just to walk through a 200 KB buffer. This required compiler/runtime tricks, different selector increments in real vs protected mode…

> And this is the problem, it was very painful just to walk through a 200 KB buffer. This required compiler/runtime tricks, different selector increments in real vs protected mode, and special pointer types. Most of that could be (and often was) hidden by the tooling. If you needed to bypass it, you could, but you didn't need to. That's not very different from today... there's a lot of hidden magic that can be bypass…

OK, but we spent a decade having to worry about this garbage, until the tooling finally caught up. You always had to keep it in the back of your mind if you wrote PC software. Even if you just ran PCs, you had to worry about the compatibility between your OS, your "DOS extender", and your programs.

There were literally millions of man-hours wasted on segment registers. A kludge that helped Intel conquer the world, but what a filthy, disgusting architecture, and what a waste of everybody's time and brain power.

Re: 8086 Segmented Memory was a good idea

#80

Post by waynecochran »

I blame 8086 segmented memory and the rest of its horrid architecture on why no one liked programming in assembly language. There were other elegant RISC machines with flat memory models and large general register sets that were a complete joy to program. Memory paging allowed you to do everything you needed to do that segmented memory provided and left the programmer unbothered for the most part.
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