How do patches work?

Because the patches contain revised versions of the same files and overwrite them.

IIRC whenever there's a new patch it just dumps all the new information into patch.mpq (Blizzard archive) overwriting as it goes, and the game itself just loads whatever is in the patch.mpq with a prority over the release mpq's. If they've changed all the monster and item files every patch then you don't store (and have no need to store) every revision. I imagine most of the updates to Diablo II are along the lines of stat tweaks, opposed to new graphics.

Actually overwriting the files that are updates is inefficient- Diablo 2 and LOD together is just under 2 gigs total of data. Intelligent patch installers can find the file it needs to update, and change it to match the new version instead of fully deleting it and copying over a full copy of the new one.

Quote from Jetsetlemming

Actually overwriting the files that are updates is inefficient- Diablo 2 and LOD together is just under 2 gigs total of data. Intelligent patch installers can find the file it needs to update, and change it to match the new version instead of fully deleting it and copying over a full copy of the new one.

uhh... yeah, that's called overwriting. patching is like using svn/subversion; it makes sure the files are in sync.

I think he meant it only changes the part of the file that's changed, which is what some games' patchers do.

Quote from Thrik

I think he meant it only changes the part of the file that's changed, which is what some games' patchers do.

yeah but that's still overwriting.

It probably just does a binary diff on a load of files. It knows which bytes in a file it has to change to get to the desired state, so it doesn't have to store all of the information.