Is Byte Really The Minimum Addressable Unit?

Question

Section 3.6 of C11 standard defines "byte" as "addressable unit of data storage ... to hold ... character".

Section 1.7 of C++11 standard defines "byte" as "the fundamental storage unit in the C++ memory model ... to contain ... character".

Both definitions does not say that "byte" is the minimum addressable unit. Is this because standards intentionally want to abstract from a specific machine ? Can you provide a real example of machine where C/C++ compiler were decided to have "byte" longer/shorter than the minimum addressable unit ?

Answer 1

A byte is the smallest addressable unit in strictly conforming C code. Whether the machine on which the C implementation executes a program supports addressing smaller units is irrelevant to this; the C implementation must present a view in which bytes are the smallest addressable unit in strictly conforming C code.

A C implementation may support addressing smaller units as an extension, such as simply by defining the results of certain pointer operations that are otherwise undefined by the C standard.

Answer 2

One example of a real machine and its compiler where the minimal addressable unit is smaller than a byte is the 8051 family. One compiler I was used to is Keil C51.

The minimal addressable unit is a bit. You can define a variable of this type, you can read and write it. However, the syntax to define the variable is non-standard. Of course, C51 needs several extensions to support all of this. BTW, pointers to bits are not allowed.

For example:

unsigned char bdata bitsAdressable;
sbit bitAddressed = bitsAdressable^5;

void f(void) {
    bitAddressed = 1;
}

bit singleBit;

void g(bit value) {
    singleBit = value;
}