The sieve bit vector deals with numbers in chunks of 30. Therefore,
the number 90 (say) is the 91st 'element' of the vector. Each byte
deals with some range {0,1,...,29}+30n so to have the number 90, you
need four bytes.
Rather pleasingly, I bumped into this bug and it reduced to the
incantation:
2010 2010 sieve marked-prime?
In any given 30 successive integers greater than 5, there are at most
8 prime numbers. Use this to tightly pack the result of the Eratostene
sieve. This lets us store more prime numbers than before in less space.