Back when I started computing (the nineties), the coolest thing to to know was assembly. If you knew assembly you could make the computer do anything because the computer was essentially executing assembly.
Back in those days the way to inject code into the system was something called a Terminate And Stay Resident (TSR). This mini application could be invoked via a hotkey and run arbitrary code in your memory space - this was a great way to hack applications, or provide additional capabilities to programs.
If you'd like to play with bookmarklets the code is below and also available @ bitbucket.
 Technically the computer didn't execute assembly, but there was a 1:1 mapping between assembly and what the computer executes.