A memory that is addressed starting with 0. Each subsequent byte is referenced by the next sequential number (0, 1, 2, 3, etc.) all the way to the end of memory. Except for PCs, which are based on the Intel CPU architecture, most computers use a flat address space.

A PC running in 16-bit mode (Real Mode) uses a segmented address space. Memory is broken up into 64KB segments, and a segment register always points to the base of the segment that is currently being addressed. The PC's 32-bit mode is considered a flat address space, but it too uses segments. Since one 32-bit segment addresses 4GB, one segment covers all of memory.