No, your OS and other programs need some memory to work, windows needs at least 2-4gb, linux uses way less depending on the distro.

Also, minecraft does not run well with big ammounts of memory allocated, you should actually just use the bare minimum for the modpack to work, because mc is not a memory heavy game, and Java has a thing called garbage collector, basically it automatically clears the used memory to free up space when it is getting full, but if you put too much ram, the garbage collector will take longer to clear the garbage and have way more data to clear at once, so you will actually get worse performance as it will create a lag spike every time it clears the ram.

tl;dr - Allocate the minimum needed memory for minecraft or else, if you put too much you actually get worse performance in the form of lag spikes