HP 35s Scientific Calculator - Manual (Page 206)

13-24 Simple Programming
For example, to see the checksum for the current program (the "cylinder" program):
If your checksum does not match this number, then you have not entered this
program correctly.
You will see that all of the application programs provided in chapters 16 and 17
include checksum values with each labeled routine so that you can verify the
accuracy of your program entry.
In addition, each equation in a program has a checksum. See "To enter an
equation in a program line" earlier in this chapter.
Nonprogrammable Functions
The following functions of the HP 35s are not programmable:
Programming with BASE
You can program instructions to change the base mode using   . These
settings work in programs just as they do as functions executed from the keyboard.
Keys:
(In RPN mode)
Display: Description:
 
() 
 

Displays label C, which takes
67 bytes.
  (hold) 
 Checksum and length.
  ()  
  ()   label line number
  
Ш , Ч , Ц , Х  
  
Ш ,  Ч  

:   ()