Memory (RAM) that is addressed starting with 0, and each subsequent byte is referenced by the next sequential number (1, 2, 3, etc.). A 32-bit computer with a 32-bit address space treats all 4GB of memory (its maximum) as one contiguous segment. However, a 64-bit address register can hold a gigantic number, and 64-bit motherboards limit the maximum memory typically to 128GB (see
binary values).
In contrast, the 16-bit mode (Real Mode) in an x86 PC uses a segmented address space. Memory is broken up into 64KB segments, and a segment register is always being adjusted to point to the base of the segment that is currently being addressed. See
Real Mode and
address register.