The architecture of the x86 processors forces with the use of memory segments to direct the information, the size of these segments is of 64kb.
The cause of being of these segments is that, allowing for that the maximum size of a number that the processor can direct is given by a word of 16 bits or register, it would not be possible to access more than 65536 localities of memory using only one of these registers, other than at the present, if the PC''s memory is separated into groups or segments, each one of 65536 localities, and we use an address on an limited register to discover each segment, and then we make each address of a particular slot with two registers, it is likely for us to access a quantity of 4294967296 bytes of memory, which is, in the present day, additional memory than what we will observe installed in a PC.
In sort for the assembler to be able to manage the data, it is essential that each piece of information or instruction be found in the area that corresponds to its individual segments. The assembler accesses this information taking into account the localization of the segment, specified by the DS, ES, SS and CS registers and inside the register the address of the specified part of information. It is because of this that when we make a program using the Debug on each line that we assemble.