手册中这段写的很有意思：

Parameter Transfer
The parameters of a block are transferred as a value. With function blocks a copy of the actual
parameter value in the instance data block is used in the called block. With functions a copy of the
actual value lies in the local data stack. Pointers are not copied. Prior to the call the INPUT values are
copied into the instance DB or to the L stack. After the call the OUTPUT values are copied back into
the variables. Within the called block you can only work on a copy. The STL instructions required for
this are in the calling block and remain hidden from the user.