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Is this really from 2012? Examples are in 1978 "K&R1" C:

  int *p;
    body ...
No, memory is not one big array, sorry. Objects have boundaries even though there are valid addresses below their base and above their last byte. Not everything that looks sensible from a machine language point of view is well-defined.

Comment below article:

> Marin Todinov said...

> Dear Programming Guru

> You are an absolute legend, ive been programming for 4 years and i have a masters in computer science, your explanation of pointers has helped me increase my efficiency in recursive functions and made a map in my breain of how these basic fundamental structers.

Lol, what the heck? Troll or astro-turfed? Where can you get a CS Master's Degree and on only four years of programming?

> No, memory is not one big array, sorry.

To the extent the operations are well-defined, both the compiler and the OS conspire to make this look true; if there's no OS, the compiler typically works harder to make it work, because the alternative would make programs too difficult to port to or away from the system in question.

The "big array of bytes, each with its own unique address" mental model is a useful lie which most programmers who know better don't pry into most of the time. Going beyond that would involve knowing about system-specific things that C is explicitly designed to abstract away, to make programs more portable.

So, no, the struct hack isn't valid C, but you can make huge arrays and, within those arrays, simple increments and decrements do work reliably, because the standard says so and the OS and the compiler will together contain enough code to make it work if they're any good at all.

As a CS undergrad having only really learned the "everything is a big array" style, can you provide more materials on the reality of memory in a system? I'm very curious how that works. What would you recommend I search for to learn more about this topic?

There are a few things you should now: first, the actual physical memory of your computer is pretty much like a big array: if you were writing code in assembly and were to run it with no operating system, that's exactly what you'd get.

However, with operating systems and multiple programs running at the same time, memory is no longer contiguous: instead, programs can request "pages" (blocks of memory). This is (more or less) what `malloc` does, if you've come across it. That's the key difference: in a modern operating system, you can't expect memory to be one big array, since your program might have requested more than one page of memory. In that sense, it's more like a collection of smaller arrays.

We have to do it this way so we can have memory protection (similar to file permissions - a program can decide if other programs can read one of their pages, write to it, etc) and swapping (i.e writing unused pages to non-volatile storage , like a hard drive, to free memory).

Not only that, but the linearity of physical ram is a fiction as well: in nearly all systems these days ram is made up of multiple memory modules (the MM in SIMM/DIMM), and to my knowledge, the OS is free to stitch them together in any way it sees fit.

(All of this is to say nothing of NUMA.)

However, one of the responsibilities of the OS is to hide all that messy detail from the bare-metal programmer or compiler writer and provide a simple(r) abstraction over the hardware. Thus, "(physical) memory is a big array".

On PC, BIOS configures memory controller in such way as to hide boundaries of memory modules from OS. Resulting address space still contains some holes and stuff that is not physical memory, but OS gets map of this from BIOS. Originally (on systems with 36pin SIMMs) this wasn't the case and you had to match memory modules such that they produce continuous block of addresses.

In essence the situation is pretty much same as for user space program: you get big address space and list of memory regions that are mapped and usable.

Sounds like a job for CS:APP http://csapp.cs.cmu.edu/ . (I haven't got through it myself yet.)

> What would you recommend I search for to learn more about this topic?

Very simple:


L1 cache.

L2 cache.


Somewhat more complex:

Virtual memory.

Memory pages.

Other posts have more information, but that should get you going.

> Lol, what the heck? Troll or astro-turfed? Where can you get a CS Master's Degree and on only four years of programming?

My experience (from interviewing people with various educational backgrounds) is that a lot of people who have a Masters in CS have very very little practical experience actually programming. People who go into the field to ascend the ivory tower often just don't do a lot of it, really.

Which is, I think, not really very different from a lot of fields. There's a distinct academic track to a lot of fields.

No, memory is not one big array, sorry.

No, no, memory is in fact one big array of bytes. Everything else is just really nice syntactic sugar over that fact.

Now, it may well be that attempts to access that memory result in page faults or weird interrupts or IO behavior or what have you, but the computer really does only see a big array.

> but the computer really does only see a big array.

Define "the computer" in this context.

Certainly not the x86 chip itself - it sees memory as a series of caches (L1, L2, L3) and eventually the memory bus, which it manages through various lookup tables (TLB etc.) more closely resembling a series of hash tables on steroids than an array - and that's ignoring per-processor caches on multiproc systems and all the invalidation logic that needs to occur as a result.

What about processes? One flat memory space! ...except when you communicate with another process, say by sharing memory. Then you realize you can't share your 'indicies' without associating them to other indicies, because even if the physical memory is the same, each process has their own 'array' for indexing into that memory (and yours doesn't even contain everything in theirs.) That's at least 68 arrays on my computer at the time of writing this, not one.

The kernel's the one managing this mess of arrays, pinning pages needed for interrupt handlers and software TLB support (for not even it is addressing pure physical memory most of the time?)

I guess you could argue that because your chip supports DMA, you can do all your array indexing through that to get to your 'one true' physical memory addressing scheme, label that as what your computer 'really sees', and ignore the 99.99% of instructions executing and making up the bulk of your computation, which have nothing to do with that addressing scheme, but that seems a bit disingenuous.

From C and assembly, that's very much how memory is access--compared with the objects notion mention in the GP.

The fact that certain accesses may cause memory layout to change or other strange things is something better left to the computer engineers. :)

No, that's untrue. I find that the thinking that it is so is generally derived from C attempting to impose it as part of its spec. Any architecture that uses bank switching, for example, is very much not a "big array of bytes". Or go try to write to byte 0x382 of your modern graphics card's VRAM, will you?

Message 1956 (8 left): Thu Jan 25 1990 2:44am

(that's the first line of the 3rd paragraph, maybe a dozen whole lines before the part you quoted...)

>No, memory is not one big array, sorry.

Yet that fact is not very relevant for his explanation, sorry.

>Lol, what the heck? Troll or astro-turfed? Where can you get a CS Master's Degree and on only four years of programming?

In lots of places.

In some cases a "CS Master degree" can mean that you took 4 years of Economics or Physics and took a CS postgraduate course afterwards -- not continuous BSc and CS education.

Other countries require 3 years for the BSc and 1 year for a master's degree.

It says 1990 in the original message.

> Lol, what the heck? Troll or astro-turfed? Where can you get a CS Master's Degree and on only four years of programming?

... and still don't totally grasp pointers apparently.

But seriously, what's so confusing about pointers?

> what's so confusing about pointers?

as with any constructor, you can make it confusing. just do pointer arithmetic with different types and you get yourself a confusing mess.

> Lol, what the heck? Troll or astro-turfed? Where can you get a CS Master's Degree and on only four years of programming?

Introduced to computer science / programming halfway through undergrad 2 years ago. Going to finish up a masters next fall. (3 and some change years after I started)

Just for interest: I got my masters in CS in four years, and this is pretty typical in the UK.

Bachelors + masters in 4 years? Wow, that's pretty fast.

(I think that's the parent comment's question, if you have a bachelors + masters in CS, you'd have had to code for 6 years or so).

I don't have a bachelors - I left the equivalent of US high school and started on a four year masters degree and now I'm doing a PhD. As I say it's not uncommon in the UK - it's not a special accelerated or course or anything.

Cool, that's new information!

At least in Japan, it's possible. I have a couple friends who are doing a master's in CS (taking 2 years) and did their bachelor's in something completely unrelated, so their programming experience will only be 2-3 years on graduation.

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