Memory Segmentation
Four registers are used to refer to four segments on the 16-bit x86 segmented memory architecture. DS (data segment), CS (code segment), SS (stack segment), and ES (extra segment). Another 16-bit register can act as an offset into a given segment, and so a logical address on this platform is written segment:offset, typically in hexadecimal notation. In real mode, in order to calculate the physical address of a byte of memory, one left-shifts the contents of the appropriate register 4 bits, and then adds the offset.
For example the logical address 7522:F139 yields the 20-bit physical address:
Note that this process leads to aliasing of memory, such that any given physical address may have multiple logical representations. This means that comparison of pointers in different segments is a complicated process.
In protected mode, the GDT and LDT are used for this purpose.
Since segments refer to successive 16-byte areas of memory, 16 bytes is known as a "paragraph".
Read more about this topic: Intel Memory Model
Famous quotes containing the word memory:
“There must be a solemn and terrible aloneness that comes over the child as he takes those first independent steps. All this is lost to memory and we can only reconstruct it through analogies in later life....To the child who takes his first steps and finds himself walking alone, this moment must bring the first sharp sense of the uniqueness and separateness of his body and his person, the discovery of the solitary self.”
—Selma H. Fraiberg (20th century)