defines the task interface (similar to a type declaration), the task body specifies the implementation of the task. Depending on the implementation, Ada tasks are either mapped to operating system threads or processes, or are scheduled internally by the Ada runtime. Tasks can have entries for synchronisation (a form of synchronous message passing). Task entries are declared in the task specification. Each task entry can have one or more accept statements within the task body. If the control flow of the task reaches an accept statement, the task is blocked until the corresponding entry is called by another task (similarly, a calling task is blocked until the called task reaches the corresponding accept statement). Task entries can have parameters similar to procedures, allowing tasks to synchronously exchange data. In conjunction with select statements it is possible to define guards on accept statements (similar to Dijkstra's guarded commands). Ada also offers protected objects for mutual exclusion. Protected objects are a monitor-like construct, but use guards instead of conditional variables for signaling (similar to conditional critical regions). Protected objects combine the data encapsulation and safe mutual exclusion from monitors, and entry guards from conditional critical regions. The main advantage over classical monitors is that conditional variables are not required for signaling, avoiding potential deadlocks due to incorrect locking semantics. Like tasks, the protected object is a built-in limited type, and it also has a declaration part and a body. A protected object consists of encapsulated private data (which can only be accessed from within the protected object), and procedures, functions and entries which are guaranteed to be mutually exclusive (with the only exception of functions, which are required to be side effect free and can therefore run concurrently with other functions). A task calling a protected object is blocked if another task is currently executing inside the same protected object, and released when this other task leaves the protected object. Blocked tasks are queued on the protected object ordered by time of arrival. Protected object entries are similar to procedures, but additionally have guards. If a guard evaluates to false, a calling task is blocked and added to the queue of that entry; now another task can be admitted to the protected object, as no task is currently executing inside the protected object. Guards are re-evaluated whenever a task leaves the protected object, as this is the only time when the evaluation