Protected Shared Variables in VHDL: IEEE Std 1076a

The VHDL Standard current allows concurrent access to variables shared between processes, but does not define any semantics for concurrency control. The IEEE 1076a Shared Variables Working Group has developed a form of monitors, called protected types, to provide mutually exclusive access to shared variables. This article identifies the problems that can arise from unprotected concurrent access to shared variables, and reviews the idea of monitors, which forms the basis of the proposed language change. It then describes protected types, gives some guidelines on using them for hardware modeling, and includes an example to illustrate their use.