What memory address spaces are there?

Just about anything you can imagine has probably been used. The first major division is between byte addressing (all modern architectures) and word addressing (pre-IBM 360/PDP-11, but I think modern Unisys mainframes are still word addressed). In word addressing, char* and void* would often be bigger than an int*; even if they were not bigger, the "byte selector" would be in the high order bits, which were required to be 0, or would be ignored for anything other than bytes. (On a PDP-10, for example, if p was a char*, (int)p < (int)(p+1) would often be false, even though int and char* had the same size.)

Among byte addressed machines, the major variants are segmented and non-segmented architectures. Both are still wide spread today, although in the case of Intel 32bit (a segmented architecture with 48 bit addresses), some of the more widely used OSs (Windows and Linux) artificially restrict user processes to a single segment, simulating a flat addressing.

Although I've no recent experience, I would expect even more variety in embedded processors. In particular, in the past, it was frequent for embedded processors to use a Harvard architecture, where code and data were in independent address spaces (so that a function pointer and a data pointer, cast to a large enough integral type, could compare equal).

I would say you are asking the wrong question, except as historical curiosity.

Even if your system happens to use a flat address space -- indeed, even if every system from now until the end of time uses a flat address space -- you still cannot treat pointers as integers.

The C and C++ standards leave all sorts of pointer arithmetic "undefined". That can impact you right now, on any system, because compilers will assume you avoid undefined behavior and optimize accordingly.

For a concrete example, three months ago a very interesting bug turned up in Valgrind:

https://sourceforge.net/p/valgrind/mailman/message/29730736/

(Click "View entire thread", then search for "undefined behavior".)

Basically, Valgrind was using less-than and greater-than on pointers to try to determine if an automatic variable was within a certain range. Because comparisons between pointers in different aggregates is "undefined", Clang simply optimized away all of the comparisons to return a constant true (or false; I forget).

This bug itself spawned an interesting StackOverflow question.

So while the original pointer arithmetic definitions may have catered to real machines, and that might be interesting for its own sake, it is actually irrelevant to programming today. What is relevant today is that you simply cannot assume that pointers behave like integers, period, regardless of the system you happen to be using. "Undefined behavior" does not mean "something funny happens"; it means the compiler can assume you do not engage in it. When you do, you introduce a contradiction into the compiler's reasoning; and from a contradiction, anything follows... It only depends on how smart your compiler is.

And they get smarter all the time.

There are various forms of bank-switched memory.

I worked on an embedded system that had 128 KB of total memory: 64KB of RAM and 64KB of EPROM. Pointers were only 16-bit, so a pointer into the RAM could have the same value of a pointer in the EPROM, even though they referred to different memory locations.

The compiler kept track of the type of the pointer so that it could generate the instruction(s) to select the correct bank before dereferencing a pointer.

You could argue that this was like segment + offset, and at the hardware level, it essentially was. But the segment (or more correctly, the bank) was implicit from the pointer's type and not stored as the value of a pointer. If you inspected a pointer in the debugger, you'd just see a 16-bit value. To know whether it was an offset into the RAM or the ROM, you had to know the type.

For example, Foo * could only be in RAM and const Bar * could only be in ROM. If you had to copy a Bar into RAM, the copy would actually be a different type. (It wasn't as simple as const/non-const: Everything in ROM was const, but not all consts were in ROM.)

This was all in C, and I know we used non-standard extensions to make this work. I suspect a 100% compliant C compiler probably couldn't cope with this.