User-defined types

User-defined type can be used to create complex circuit structures. A new user-defined type name is introduced by using defproc, defcell, defchan, or deftype statements. All user-defined types have the same basic structure:

1. a type signature, that provides information about the interface to the type and the ports that are externally visible; and
2. a body, contained in braces, that specifies the detailed definition of the user-defined type.

The type chosen for each port must be the most specific type used by that port in the body (see the implementation relation section).

User-defined types can also be parameterized, and this is covered in detail later.

A process is a user-defined type that corresponds to a circuit entity. Other hardware description languages sometimes call it a module or a subcircuit. The basic syntax of a process definition is shown below.

defproc test (bool n, m; bool p, q)
{
 ...
}

The definition above creates a new process, called test, that has a port list consisting of four bools. This port list cannot contain any parameter types (pint, etc).

If the body of the user-defined type is replaced by a single semi-colon or is empty, the statement corresponds to a type declaration. Declarations are typically used when defining mutually recursive types. The declaration corresponding to type test is:

defproc test (bool n, m; bool p, q);

If the process is never defined, ACT assumes that it has an empty body. If a process declaration is followed by a definition, the type signature must match exactly.

defproc test (bool n, m; bool p, q);
defproc test (bool n, m; bool p) { }

-[ERROR]-> Name `test' previously defined as a different process

A type can only have one definition in a given scope.

defproc test (bool n) { ... }
defproc test (bool n) { ... }

-[ERROR]-> Process `test': duplicate definition with the same type signature

The body of a process specifies its implementation. This can use a combination of instances of other processes, connections, and other languages like production rules. Loops and conditional statements can also be used to construct a process.

Port lists have a syntax similar to instantiations. A type specifier can be followed by a list of identifiers rather than just a single identifier, similar to an instantiation. Semicolons are used to separate parameters of differing types, as shown in the example below.

defproc test2(bool n,m; d1of2 p,q) { ... }

In this example we assumed that there was a user-defined type (or channel) called d1of2 that was used in the port list. Any user-defined type in the port list must be either a data or channel type. Processes are supposed to correspond to circuit blocks, and so cannot be port parameters to other circuit blocks.

Square brackets can also be used following the identifier names to specify array ports. The meaning of these square brackets is identical to the ordinary array instantiation. However, the arrays in port lists are restricted to be dense arrays indexed at zero. This restriction is enforced by syntax, and will be reported as a parse error.

defproc test1 (bool a,b,c, d[10]) { }  // success!
defproc test2 (bool a,b,c, d[0..9]) { }

-[ERROR]-> Expecting token `]', got `.'

The ports themselves cannot be converted to sparse arrays within the body of a definition. This means that the following is illegal:

defproc test1 (bool a, b, c, d[10])
{
  bool d[11..12];
  ...
}

-[ERROR]-> Array instance for `d': cannot extend a port array

Type names and variable names do not share the same name space. Creating a type definition with the same name as an instance variable or vice versa is allowed, but deprecated.

Cells follow the same rules for definition as processes, except the keyword defcell is used in place of defproc. The reason for separating cells from processes is that processes are supposed to correspond to logical entities that are meaningful semantic objects. For example, a process ordinarily has its origins in a CHP language description. Cells, on the other hand, can be fragments of logical processes. Examples of cells are standard gates like C-elements, NAND, or NOR gates, or commonly used circuit structures like completion detection logic. Cells are distinguished from processes to make it easier to write automation tools.

A data type is defined using deftype. A data type corresponds to an integer or Boolean value, although it could also be a composite construct like a record or structure (from software programming languages). The syntax is similar to a process, and the constraints about declarations/etc. apply here as well.

Often data types have some additional structure that is not required for a process. In particular, the body of the data type and its type signature provide information that relates a user-defined data type to a previously defined or built-in data type. When a user-defined data type is specified, a method for setting the value of the data type and reading its value must also be specified. If omitted, certain features of data types will not be enabled for the defined type.

The following is a simple example of a datatype that creates a dual-rail representation for a Boolean variable. The first line specifies that d1of2 is a new data type, and it implements the built-in type int<1>—a one-bit integer.

deftype d1of2 <: int<1> (bool d0,d1)
{
  spec {
    exclhi(d0,d1)
  }
}

The body of the type is similar to a process, except it can only contain connections, spec bodies, and special methods. The following would result in an error:

deftype d1of2 <: int<1> (bool d0,d1)
{
  bool p;
  spec {
    exclhi(d0,d1)
  }
}
-[ERROR]-> Expecting bnf-item `methods_body', got `bool'

Port lists for data types can be either built-in data types or user-defined data types. Channels (built-in or user-defined) and processes are not valid types for ports of a data type, since a data type is supposed to represent a circuit structure that is used to represent a data value.

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.

The deftype syntax can also be used to define structures/record types. These types can be used to group data fields or channel fields together. A structure is defined using deftype, except it is not related to a built-in type as in the examples above. So, for instance:

deftype mystruct (int<4> a; int<5> b) { }

would declare a structure with two fields: a and b of the specified type.

If all the fields within a structure are data values, this is a special case of a structure consisting purely of data.

The distinction between a structure and another data type is that other data types are implementations of one of the built-in types like int or bool.

Process, channel, and data types can include methods that provide mechanisms to manipulate the type or access parts of the type. There are a number of built-in method names that can be specified for data types and channel types.

There are two methods that can be specified for a data type:

1. a set method, used to write a value to the type;
2. a get method, used to read the value of the type.

One can think of these as type conversion methods invoked automatically to read or write the data type. When a normal data type is used, the special variable self is implicitly defined to be the built-in type that is implemented by the user-defined data type.

deftype d1of2 <: int<1> (bool d0,d1)
{
   spec {
    exclhi(d0,d1)
   }
   methods {
     set {
       [self=1->d1-;d0+ [] self=0->d0-;d1+]
     }
     get {
       [d0->self:=1 [] d1->self:=0]
     }
   }
}

In the example above, the set method says that the way to set a d1of2 data type to the value 0 is to set d0 to false and d1 to true. The special variable self is used to specify the int<1> value of the type, and the methods specify conversion operations.

The selection statement in the get method uses the deterministic selection operator [] (see the hse sublanguage). This is an implicit check that when the get method is invoked, signals d0 and d1 cannot both be true. We have also made this explicit in the specification body. Also, if both d0 and d1 are false (i.e. an illegal state in which to execute a get operation), the variable self is not assigned; the operation waits for at least one of d0 or d1 to be true. This is viewed as an error for a data type. (This is different in the case of a channel, where the semantics of the channel permit waiting.)

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.

User-defined type variables can be instantiated in much the same manner as ordinary type variables.

defproc test(bool N, n) { ... }
test x;
// x.N and x.n refer to the ports of ''x''

The ACT description above creates an instance of type test named x. Creating an instance of a type creates instances of all the ports listed as well as creating whatever is specified by the body of the type definition. The list of ports of a user-defined type can be accessed from the scope outside the type definition by using dot-notation. These externally visible ports are analogous to the fields of structures or record types in standard programming languages.

This analogy to records can be used to build complex data types, albeit with slightly different syntax compared to traditional programming languages. The following is a simple example that illustrates this.

deftype mystruct <: int<16> (int<4> f1, f2; int<8> f3)
{
  methods {
   set {
     f1:=self >> 12;
     f2:=(self >> 8) & 0xf;
     f3:=self & 0xff
   }
   get {
     self:=(f1 << 12) | (f2 << 8) | f3
   }
  }
}

Processes, channels, and datatypes created using defproc, defchan, and deftype all support parameterization. Parameters are specified using the template keyword.

Since the syntax for all three is the same, we use a process definition to illustrate this. To create a parameterized type, the definition of the type is preceeded by a template specifier as shown below.

// A generic adder block
template<pint N> 
defproc adder (e1of2 a[N], b[N]; e1of2 s[N])
{
  ...
}

This example defines an adder that takes N as a parameter. Note that the value of N determines the size of the arrays in the port list for the process. Instances of this adder can be created in the following way:

adder<4> a1;  // a1 is a 4-bit adder
adder<16> a2; // a2 is a 16-bit adder

The value of a1.N is 4, while the value of a2.N is 16. To illustrate how one might define this adder block, assume we have processes fulladder, zerosource, and bitbucket already defined that implement a full-adder, a constant source of zeros, and a constant sink respectively. One possible definition of the adder would be:

template<pint N>
defproc adder (e1of2 a[N], b[N]; e1of2 s[N])
{
   fulladder fa[N];
   ( i : N-1 : fa[i].a = a[i]; fa[i].b = b[i]; fa[i].s = s[i];
                fa[i].co = fa[i+1].ci; )
   zerosource z;
   bitbucket w;
   fa[0].ci=z.x;
   fa[N-1].co = w.x;
}

This creates a parameterized ripple-carry adder. Notice the use of loops and arrays to connect the carry chain for the adder, and the inputs and outputs of the process to the fulladder ports.

As shown in the example above, the arguments in the template parameter list are specified by listing them next to the type name. Trailing arguments can be omitted from the parameter list attached to the type as shown in the example below.

template<pint N; preal w[N]>
defproc test (bool n[N]) { ... }
 
test<5> x;

Channels and data types can also be parameterized in the same way. For example, the following might be an N-bit dual rail definition.

template<pint N>
deftype d1of2 <: int<N> (bool d0[N], d1[N]) { ... }

Since the body of the type can use loops and selection statements in arbitrary ways, changing the parameters for the type can completely change the structure of the circuit. It can also change the ports for the type. Hence, when checking for type compatibility, the values of parameters are also taken into account. Hence, the full type for instance a2 above is in fact adder<5>, not just adder. Types such as fulladder that do not have parameters are more completely specified as fulladder<>, although the angle brackets can be omitted. Arrays can only correspond to instances of the same type—so an array cannot contain a three-bit adder and five-bit adder.

Direction specifications can be used for built-in data and channel types, as well as user-defined types. Consider the e1of2 user-defined channel type that we saw earlier:

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

When we use e1of2? or e1of2!, we need some mechanism to specify the access permissions for the port parameters of the user-defined type. The convention used is that there are five possible ways to specify any constraints on access to port parameters. For our example, we can use one of the following variations in the port parameter list:

bool d0; // No constraints; this port could be read or written
bool! d0; // Both e1of2? and e1of2! have bool! permissions
bool? d0; // Both e1of2? and e1of2! have bool? permissions
bool?! d0; // e1of2? has bool? permissions, and e1of2! has bool! permissions
bool!? d0; // e1of2? has bool! permissions, and e1of2! has bool? permissions

Hence, a better definition of an e1of2 channel would more completely specify the access permissions for the port parameters in the following way.

defchan e1of2 <: chan(bool) (bool?! d0,d1; bool!? e) { ... }

A careful examination of the type signature reveals that the sender and receiver have the appropriate permissions. There is a subtle interaction between connections and directional types in ACT, and this is detailed in the section on connections.