Interesting information on reasons for choosing a 1GB hash size, thanks.
> A0 should have let Stockfish use its opening book though.
Has it been confirmed that it didn't? As far as I can tell, DeepMind never said that Stockfish didn't use an opening book or endgame tablebase, only that AlphaZero didn't: https://twitter.com/demishassabis/status/938347604462542849
The only time opening books are mentioned in the arxiv paper is when they are described as standard components of chess engines; while it doesn't explicitly state whether Stockfish was allowed to use them in the tests, I would think it likely: https://arxiv.org/abs/1712.01815
The rumor that Stockfish didn't have an opening book seems widespread but unsourced.