I found this article and I think will be a good resource for users of FASM:

The PUSHA and POPA instructions provide a mechanism to save the state of all the registers to the stack. This provides a potentially convenient macro-like instruction for assembly language programmers. It is complicated by the fact that it also stores and restores the stack pointer itself, which eliminates its potential use of allowing a lazy coder to blindly store registers at the start of a function and restore them at the end of the function.