The method of constant-stress molecular dynamics using the Parrinello-Rahman Lagrangian is described, particularly as applied to Coulomb systems. The method is used to simulate the phase behavior of silver iodide. The \ensuremath{\beta}, \ensuremath{\alpha}, rocksalt, and liquid phases are reproduced and the boundaries are similar to those observed for real AgI. Evidence is found for the order-disorder transition in the \ensuremath{\alpha} phase proposed by Perrott and Fletcher. This is characterized by a heat-capacity anomaly and a rise in the cation diffusion coefficient. The supercooled \ensuremath{\alpha} phase and the equilibrium rocksalt phase both have diffuse fast-ion transitions and the latter cuts the rocksalt--\ensuremath{\alpha}-phase boundary, causing it to cusp in to an apparent triple point with the \ensuremath{\alpha}-phase order-disorder transition.