I'm going from memory here but I believe there is a wizard that helps write the code for integrating analog I/O modules.
The wizard will install code in the program that moves the data from the analog input to the data register you specify.
They are pretty low level but actually quite powerful if you can wade through the hoops. The instruction that amazed me most when I found it was one that takes latitude, longitude and time and spits out coordinates to point at the sun for optimal solar energy.
Other things are frustrating until you understand them as well. Like you don't specify the time base of a timer, certain timers have certain time bases and that's that (unless you flip a certain bit then some of them change...ya?)
Same with whether or not memory addresses are retentive or not. Certain addresses are and the rest aren't.
Different processors have different register/memory layouts so you have to be looking at the right place in the manual or you could get in trouble.