Channel types

Channels are similar to data types. Instead of relating a user-defined channel to built-in data, we relate them to a built-in channel types instead. The methods required for supporting the full functionality of a channel are operations necessary to send and receive data on the channel, rather than a read and write operation on a data value.

defchan e1of2 <: chan(bool) (bool d0,d1,e)
{
   spec {
    exclhi(d0,d1)
   }
}

Port lists for channel types can be data types (built-in or user-defined) or channels. Processes are not valid types for ports of a channel type.

There are eight possible methods that can be defined for a channel type:

• Methods for sending and receiving values on the channel
  • set, send_up, send_rest: together these three operations implement a send operation on the channel. The send operation consists of two parts: (i) setting the data value to be sent (set); (ii) completing the synchronization (send_up); and (iii) completing the rest of the protocol (send_rest).
  • get, recv_up, recv_rest: together these three operations implement a receive operation on the channel. The receive operation consists of three parts: (i) getting the value that has been transmitted along the channel (get); (ii) completing the synchronization operation (recv_up); and (iii) completing the rest of the protocol (recv_rest).
• Methods for initializing the channel on reset, if needed.
  • send_init : this is used to initialize the sender end of the channel on reset.
  • recv_init : this is used to initialize the receiver end of the channel on reset.

For channels, there are two special methods that are used for probe operations with different syntax. Both of these have to be specified via an expression, rather than the normal method syntax.

• send_probe: this is the probe operation for the sending end of the channel. It corresponds to the receiver being ready to communicate.
• recv_probe: this is the probe operation for the receiving end of the channel. It corresponds to the sending being ready to communicate.

The send operation X!e in the CHP language corresponds to two parts: setting the data value, followed by the synchronization operation, and possibly the reset phase of the handshake. Setting the data value also indicates that the sender is ready to communicate. It is illegal to set the data value multiple times without an intervening synchronization operation. Finally, attempting to set the data value might block if the previous channel operation has not completed as yet. Whether or not this could occur depends on the channel protocol.

The receive operation X?v in the CHP language corresponds to three parts: receiving the data value, followed by the synchronization operation, and finally the rest of the handshake. Attempting to get the data value from the channel will block if the sender has not provided any value. Once a value has been extracted from the channel, the synchronization operation can be executed. Prior to the synchronization, multiple get operations can be executed; the channel must be designed so that subsequent get operations will return the same value as the first one, and will be guaranteed not to block. The get operation is used to implement a CHP value probe, where the receiver can peek at the value pending in the channel without attempting a synchronization operation.

An example definition of a Boolean channel where the channel has an lazy-active send and passive receive is below. Note that for a channel, self corresponds to the type of the data being sent or received on the channel.

defchan e1of2 <: chan(bool) (bool d0,d1,e)
{
   spec {
    exclhi(d0,d1)
   }
   methods {
    set {
      [e];[self->d1+[]~self->d0+]
    }
    send_up {
      [~e]
    }
    send_rest {
      d0-,d1-
    }
    get {
     [d0->self-[]d1->self+]
    }
    recv_up {
     e-
    }
    recv_rest {
     [~d0&~d1];e+
    }
    recv_probe = (d0|d1);
   }
}

In the example above, the set, send_up, and send_rest methods specify the sequence of operations on the channel variables that are invoked for a send action. The get, recv_up, and recv_rest methods specify the sequence of operations used to perform a receive. The special variable self is used to specify the bool value that is being either sent or received on the channel.

This channel has an active send and passive receive, and hence probes are only supported at the receiver. The recv_probe method expression specifies the Boolean expression corresponding to the probe at the receiver end of the channel. A send_probe can be specified in a similar way when the sender is passive and receiver is active.

The e1of2 channel has been specified to perform a four-phase handshake protocol. If the channel were to correspond to a two-phase protocol, a different sequence of actions can be specified instead.

When defining an exchange channel, the special variable selfack is used to specify the value being received by the sender, and being sent by the receiver.