commented thoroughly the most obvious sections of the code, while those obscure 3 lines that make the key of the program are left uncommented

That is true, but a more often seen problem is worthless comments. As an example of worthless commenting:

hello:
         push acc;             save accumulator
         mov a, step;          move step to a
         cjne step,#4,hello1;  branch if step is not 4
         mov a,#18;            move 18 to acc
         mov step,a 18;        step to acc value
hello1:  pop acc;              restore accumulator
         ret;                  return from subroutine

The above would be so much better served with one line of comment above the routine
; if step = 4, set it to 18, otherwise leave untouched.

or - even better
: if step = NEW_MAIL, set step = CHECK_MAIL

Erik