What memory address spaces are there?

Question

What forms of memory address spaces have been used?

Today, a large flat virtual address space is common. Historically, more complicated address spaces have been used

Answer 1

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.