On: I wrote a linker everyone can understand

[pjmlp]

As alternative, you can try to understand how Project Oberon linker works, given it is quite basic.

Chapter 6

http://people.inf.ethz.ch/wirth/ProjectOberon/PO.System.pdf

Or the re-implementation of Turbo Pascal one for MS-DOS,

http://mail.turbopascal.org/linker