Exotic architectures the standards committees care about

Question

I know that the C and C++ standards leave many aspects of the language implementation-defined just because if there is an architecture with other characteristics, it would b

Answer 1

I'm fairly sure that VAX systems are still in use. They don't support IEEE floating-point; they use their own formats. Alpha supports both VAX and IEEE floating-point formats.

Cray vector machines, like the T90, also have their own floating-point format, though newer Cray systems use IEEE. (The T90 I used was decommissioned some years ago; I don't know whether any are still in active use.)

The T90 also had/has some interesting representations for pointers and integers. A native address can only point to a 64-bit word. The C and C++ compilers had CHAR_BIT==8 (necessary because it ran Unicos, a flavor of Unix, and had to interoperate with other systems), but a native address could only point to a 64-bit word. All byte-level operations were synthesized by the compiler, and a void* or char* stored a byte offset in the high-order 3 bits of the word. And I think some integer types had padding bits.

IBM mainframes are another example.

On the other hand, these particular systems needn't necessarily preclude changes to the language standard. Cray didn't show any particular interest in upgrading its C compiler to C99; presumably the same thing applied to the C++ compiler. It might be reasonable to tighten the requirements for hosted implementations, such as requiring CHAR_BIT==8, IEEE format floating-point if not the full semantics, and 2's-complement without padding bits for signed integers. Old systems could continue to support earlier language standards (C90 didn't die when C99 came out), and the requirements could be looser for freestanding implementations (embedded systems) such as DSPs.

On the other other hand, there might be good reasons for future systems to do things that would be considered exotic today.