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C Pointer Arithmetic for Unusual Architectures

I'm trying to get a better understanding of the C standard. In particular I am interested in how pointer arithmetic might work in an implementation for an unusual machine architecture.

Suppose I have a processor with 64 bit wide registers that is connected to RAM where each address corresponds to a cell 8 bits wide. An implementation for C for this machine defines CHAR_BIT to be equal to 8. Suppose I compile and execute the following lines of code:

char *pointer = 0;
pointer = pointer + 1;

After execution, pointer is equal to 1. This gives one the impression that in general data of type char corresponds to the smallest addressable unit of memory on the machine.

Now suppose I have a processor with 12 bit wide registers that is connected to RAM where each address corresponds to a cell 4 bits wide. An implementation of C for this machine defines CHAR_BIT to be equal to 12. Suppose the same lines of code are compiled and executed for this machine. Would pointer be equal to 3?

More generally, when you increment a pointer to a char, is the address equal to CHAR_BIT divided by the width of a memory cell on the machine?

Solution

  • Would pointer be equal to 3?

    Well, the standard doesn't say how pointers are implemented. The standard tells what is to happen when you use a pointer in a specific way but not what the value of a pointer shall be.

    All we know is that adding 1 to a char pointer, will make the pointer point at the next char object - where ever that is. But nothing about pointers value.

    So when you say that

    pointer = pointer + 1;

    will make the pointer equal 1, it's wrong. The standard doesn't say anything about that.

    On most systems a char is 8 bit and pointers are (virtual) memory addresses referencing a 8 bit addressable memory loacation. On such systems incrementing a char pointer will increase the pointer value (aka memory address) by 1. However, on - unusual architectures - there is no way to tell.

    But if you have a system where each memory address references 4 bits and a char is 12 bits, it seems a good guess that ++pointer will increase the pointer by three.