Why does Salsa20 algorithm require incrementing word[8] and word[9] after producing cipher text message I'm trying to get understanding of Salsa20 algorithm. There is an official specification from Daniel J. Bernstein at link. Everything in spec is clear and I could write an implementation on Swift. 

But I met Salsa20 cipher algorithm implemented by other developers Crypto and TLS for C++11 , Salsa20 written on Python
The developers perform some operations with words at positions 8 and 9 after generating ciphertext message:


I didn't saw these operations in the original Salsa20 spec. What do these increments mean?
This code only increments word 9 when word 8 is zero. Just like when counting you increment the next digit whenever the previous digit is zero, going from 9 to 10.

This is equivalent to having a 64-bit counter and putting the lower half into word 8 and the upper half into word 9.