Q. What do you mean by Linker?
For modularity of your programs it is better to break your program in numerous sub routines. It's even better to put common routine such as reading a hexadecimal number or writing hexadecimal number etc. which could be used by numerous other programs into a separate file. These files are assembled distinctly. After every file has been successfully assembled they can be linked together to form a large file that constitutes your complete program. The file comprising the common routines can be linked to your other program also. The program that links your program is known as the linker.
The linker produces a link file that comprises binary code for all compound modules. The linker also generates link maps that comprises the address information about linked files. The linker though doesn't assign absolute addresses to your program. It just assigns continuous relative addresses to all modules linked starting from zero. This form a program is said to be relocatable since it can be put anywhere in memory to be run.