What is a midstate?

[Geremia]

This defines "midstate" as the "precomputed hash state after hashing the first half of the data."

I've heard "midstate" used in the context of speeding up hashing. How does using a midstate speed up hashing?

[emrebey]

it's initial hash of the first half of the data - later using as an input to the double-hash of the second part of the data. I don't think "using" it speeding up hashing? where did you heart of it?

you already use it since it's a part of the getwork protocol afaik.