The 386 has six segment registers (CS, DS, SS, ES, FS, and GS) which can select one of 16,383 (8,191 shared and 8,192 private) segment descriptors. These segment descriptors reside in either the Global Descriptor Table (GDT) or the Local Descriptor Table (LDT) and determine underlying characteristics (type attributes, location in linear address space, and segment size). In addition to memory segments, system segments are available to the operating system for special purposes and call gates to facilitate controlled indirection into other possibly hidden segments.

Memory segments can be selected via a dedicated segment register, with different results. The CS register contains program instructions. The DS register selects program data. The SS register selects the program stack. The ES register selects the destination of string instructions. Both the FS and GS registers are undedicated at this time. It is even possible to reassign the segment registers in the machine instructions, so one can view the ES, FS, and GS segment registers as alternative DS segment registers.

Each memory segment has a size, and can be as large as 4 gigabytes. In order for that segment to be active, however, it must consume space (global linear address space) in direct proportion to its size. This means that, although a process may possess a total address space greater than 4 gigabytes, only an aggregate of active segments totaling less than or equal to 4 gigabytes is permitted. While the 386 theoretically can address 2{14} x 2{46} bytes, in practice only 2{32} bytes (4 gigabytes) can be active at any time. If the maximum 4 gigabytes of instruction, data, and stack (for both operating system and each user process) is invoked, managing the global linear address space to allow segments to be active (present) when linear address space is available becomes a significant problem.

Segments can also be overlapped in linear address space. Because through both segments we can access the same memory interchangeably, possibly with different attributes, this overlap is called an alias.

80x86 segments can be either "bottom up" or "top down." A segment that is bottom up means that one begins with segment relative address 0 and "grows up" to the desired address x (that is, [0 ... x]). A segment that is top down means that one begins with segment relative address 0xffffffff and "grows down" to the desired address y ([y ... Oxfffffff]). (Yes, we know this is awkward, but that's how it works). Segments are grown only in accordance to these rules. The stack segment is the only common example of a downward growing segment.