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

None of your assumptions hold for mainframes. For starters, I don't know of a mainframe which uses IEEE 754: IBM uses base 16 floating point, and both of the Unisys mainframes use base 8. The Unisys machines are a bit special in many other respects: Bo has mentioned the 2200 architecture, but the MPS architecture is even stranger: 48 bit tagged words. (Whether the word is a pointer or not depends on a bit in the word.) And the numeric representations are designed so that there is no real distinction between floating point and integral arithmetic: the floating point is base 8; it doesn't require normalization, and unlike every other floating point I've seen, it puts the decimal to the right of the mantissa, rather than the left, and uses signed magnitude for the exponent (in addition to the mantissa). With the results that an integral floating point value has (or can have) exactly the same bit representation as a signed magnitude integer. And there are no floating point arithmetic instructions: if the exponents of the two values are both 0, the instruction does integral arithmetic, otherwise, it does floating point arithmetic. (A continuation of the tagging philosophy in the architecture.) Which means that while int may occupy 48 bits, 8 of them must be 0, or the value won't be treated as an integer.