Segmentation

Segmentation

Users prefer to view memory as a collection of variable size segments, with no necessary ordering among segments.

Memory segmentation supports this view by providing addresses with a segment number (mapped to a segment base address) and an offset from the beginning of that segment. Each segment has a name and a length. The addresses specify both the segment name and the offset within the segment. For simplicity of implementation, segments are numbered and are referred to by a segment number, rather than name. Thus, logical address consists of a two tuple: <Segment-number, offset>.

Segmentation Hardware

A segment table maps segment-offset addresses to physical addresses, and simultaneously checks for invalid addresses. Each entry in segment table has a segment base and a segment limit. The segment base contains the starting physical address and the segment limit specifies the length of the segment.

A logical address consists of two parts: a segment number‘s’ and an offset into that segment‘d’. The segment number is used as an index to the segment table. The offset‘d’ must be between 0 and segment limit, if it is not a trap to OS is sent. When an offset is legal, it is added to the segment base to produce the address in physical memory of the desired byte.