The formation of ordered phases from block copolymers is driven by a delicate balance between the monomer-monomer interaction and chain configurational entropy. The configurational entropy can be regulated by designed chain architecture, resulting in a new entropy-driven mechanism to control the self-assembly of ordered phases from block copolymers. An effective routine to regulate the configurational entropy is to utilize multiarm architecture, in which the entropic contribution to the free energy could be qualitatively controlled by the fraction of bridging configurations. As an illustration of this mechanism, the phase behavior of two $AB$-type multiarm block copolymers, ${B_0-(B_i-A_i)}_m$ and ${(B_1-A_i-B_2)}_m$ where the minority $A$ blocks form cylindrical or spherical domains, are examined using the self-consistent field theory (SCFT). The SCFT results demonstrate that the packing symmetry of the cylinders or spheres can be controlled by the length of the bridging $B$ blocks. Several nonclassical ordered phases, including a novel square array cylinder with $p4mm$ symmetry, are predicted to form from the $AB$-type multiarm block copolymers.