C compilers and non-ASCII systems

Yes and no. They must be implemented in a way that looks like binary as far as the semantics visible to the abstract C machine are concerned. Whether the actual underlying storage mechanism is two-state or not is beyond the scope of the standard. (This is sometimes called the "as if" rule: everything else can be thrown out the window as long as the compiler and run-time (if applicable) arrange that it look as if the spec were being followed as far as C code can tell.) So, you can use decimal if you want, but you'll have to go through gyrations to make things like & and << and + operate _as if_ everything were actually binary. (This would probably be difficult, wasteful of storage, or both, especially considering the requirement that every object be viewable as an array of chars without loss of information. It would not be impossible.) 6.2.6.1 [#3] Values stored in unsigned bit-fields and objects of type unsigned char shall be represented using a pure binary notation. [#4] Values stored in non-bit-field objects of any other object type consist of n?CHAR_BIT bits, where n is the size of an object of that type, in bytes. ... Yes. There are three allowed representations for signed integral types: two's complement, one's complement, and sign/magnitude. 6.2.6.2 [#2] For signed integer types, the bits of the object representation shall be divided into three groups: value bits, padding bits, and the sign bit. There need not be any padding bits; there shall be exactly one sign bit. ... ... If the sign bit is one, the value shall be modified in one of the following ways: -- the corresponding value with sign bit 0 is negated (sign and magnitude); -- the sign bit has the value -(2N) (two's complement); -- the sign bit has the value -(2N-1) (one's complement). Which of these applies is implementation-defined, as is whether the value with sign bit 1 and all value bits zero (for the first two), or with sign bit and all value bits 1 (for one's complement), is a trap representation or a normal value. In the case of sign and magnitude and one's complement, if this representation is a normal value it is called a negative zero. A "trap representation" is an arrangement of bits that does not represent a value of the relevant type, and is not necessarily operable on even to the extent of storing it: 6.2.6.1 [#5] Certain object representations need not represent a value of the object type. If the stored value of an object has such a representation and is read by an lvalue expression that does not have character type, the behavior is undefined. If such a representation is produced by a side effect that modifies all or any part of the object by an lvalue expression that does not have character type, the behavior is undefined. Such a representation is called a trap representation. For example, on that decimal machine, a C implementation may choose to use only values 0..7 of each digit, to store three bits of information. Any value with an 8 or 9 in it could then be a trap representation. (A trap representation does not have to actually trap; undefined behaviour can include doing something sensible to someone who knows what's really going on under the hood.) I *think* this would conform; I'll ask someone I know who knows this stuff better than I. /~\ The ASCII Mouse \ / Ribbon Campaign X Against HTML mouse at rodents-montreal.org / \ Email! 7D C8 61 52 5D E7 2D 39 4E F1 31 3E E8 B3 27 4B