Types and Variables

Variables are the basic data objects in ACT. Instantiations specify which variables are created, and state what type they have. The type of an object completely specifies what the object is and how it can be used.

Types come in two flavors: parameters and circuit elements. Parameters are variables whose types are pint, preal, pbool, or arrays thereof. All other types refer to circuit elements. The basic circuit element is a Boolean value bool. Circuit element types are broken down into three categories: processes (created with defproc), channels (created with defchan), and data (created with deftype).

There are some restrictions on variable names. Ordinarily a variable identifier can be constructed as an arbitrary sequence of underscores, letters, and digits. Identifier names are case sensitive, so case and Case are different identifiers.

Basic types

The following basic types are supposed by ACT:

  • Parameter types
    • pint, for integer parameters.
    • preal, for real-valued parameters.
    • pbool, for Boolean-valued parameters.
    • ptype, for type parameters.
  • Data types
    • bool, for Boolean circuit signals.
    • int, for unsigned integer-valued data.
    • enum, for enumerations.
    • chan, for channels.

The first group of types (and arrays of them) are referred to as parameter types or meta-language types, and they begin with the character p. This is because they do not represent physical entities in the circuit itself, but rather values that are used to construct the circuit or specify circuit parameters.

The bool type corresponds to an electrical node in the circuit. Eventually all types get implemented using circuit elements and bools.

The int, enum, and chan types are used for higher-level representations of the circuit. These types support parameters, and are described in more detail later.

Variables of these basic types can be created by specifying the type name followed by a comma-separated list of identifier names.

bool a,b,c,n1,n1x2;
pint x,y,z;
preal w2,w_3;

The statements above are referred to as instantiations, since they create variables that are instances of the basic types. It is an error to have more than one instantiation of a variable in the same scope.

bool a;
pint a;
-[ERROR]-> Duplicate instance for name `a'

A parameter instantiation can be accompanied by a single initializer which initializes the value of a variable.

pint a=5, c=8;
preal b=8.9;

The order of initialization of variables is left to right. Using constructs such as

pint a=c, c=5;
-[ERROR]-> The identifier `c' does not exist in the current scope

should be avoided, as this leads to the error shown above indicating that ACT does not know about variable c in the initialization of a. Constructs where the two instances and initializers are listed in an order that does not lead to an error are deprecated even they are well-defined.

Array types

An array of a basic type or user-defined type can be created using ACT's array syntax. The syntax is based on C-style arrays, and examples of creating arrays are shown below:

int ar1[4]
preal ar2[7]
bool ar3[1..6]

The number in square brackets specifies the range of the array. In the first two examples, valid array indices range from zero to three and zero to six respectively. The third example specifies the array indices to range from one to six. In general, if the array index range is specified by a single integer, the lower bound of the range is zero, and the upper bound is the specified integer minus one. Instead of simple integers, arbitrary integer expressions can also be used as array range specifications, as shown below.

int ar4[5*3]
preal ar5[7*x+(y%2)-p] // here x, y, and p must be integer parameter types

Expressions used to specify array ranges must be of integer type. Variables used must always be parameter types (typically pint).

preal a = 4.3;
bool ar6[7*a+5];
-[ERROR]-> Expression must be of type int

Multidimensional arrays are specified by additional square brackets. Two and three-dimensional arrays of bools are specified as shown in the example below.

bool x[5,3];
bool y[1..6][9][2..10];

ACT provides a mechanism for constructing sparse arrays, i.e., those whose range need not be a single contiguous block. It is possible to create an array of nodes whose elements exist only at, say, positions 4 and 6 of the array. The syntax for creating the aforementioned array is shown below.

bool n[4..4], n[6..6];

These sparse array instantiations can be mixed with ordinary instantiations, permitting the definition of arrays which can be dynamically extended in ACT.

bool n[5];
bool n[10..12]; // n is now defined at positions 0 to 4, 10 to 12

The definition below specifies an instantiation of elements of array m at positions [6][5], [6][6], …, [6][10].

bool m[6..6][5..10]

Note that this is quite different from the statement

bool m[6][5..10];

which indicates that array m is to be instantiated at positions [0][5], …, [5][10].

Unlike ordinary instances, array instantiations cannot be followed by initializers.

bool x[10];
bool y[10] = x;
-[ERROR]-> Connection can only be specified for non-array instances

For type-checking purposes, an array is defined by its base type (bool in the example above), number of dimensions, and the shape of the array in each dimension.

Parameterized types

Parameterized types give ACT considerable flexibility in type definitions. Parameterized types come in two flavors: built-in types, and user-defined types. For user-defined types, ACT guarantees that the order in which parameters are created and initialized is from left to right. Therefore, one can use the value of one parameter in the definition of another one.

Although we have been describing the types int and chan as simple types, they are in fact parameterized. Omitting the parameters makes ACT use implicit default parameters for both of them.

The int type is parameterized by the number of bits used to specify the integer. This bit-width can be specified using angle brackets, as shown below:

int<1> x; // x is a one bit integer
int<37> y; // y is a thirty-seven bit integer

When interpreting these bits as integers, ACT assumes an unsigned binary representation. The default bit-width is thirty-two.

The channel type chan can be parameterized by the type that is being sent and received on the channel.

chan(bool) x; // x is a Boolean channel
chan(int<16>) y; // y is a 16-bit integer channel

The default data type for a channel is assumed to be the default int, namely int<32>.

Channels are almost always unidirectional, with data being transferred from sender to receiver. In a few cases, it is useful to be able to transfer data from the sender to the receiver, and from the receiver to the sender in one channel action. To declare a channel where data are transferred in both directions, use:

// a bool is transferred from sender to receiver, and
// an int is transferred from the receiver to the sender
chan(bool,int) x;

These are sometimes called exchange channels, since data is exchanged between the sender and receiver.

Another built-in data type is the enumeration type. An enumeration type corresponds to integer-valued variables with a restricted range.

enum<5> x; // x can take on values 0, 1, 2, 3, 4

For convenience, these values are treated as integers for the purposes of expressions. Also, enumerations that have power-of-two ranges are type-equivalent to the approprate int type. For instance, enum<2> is equivalent to int<1>. Enumerations are useful when specifying a data value that is a one-hot code.

Directional types

Data and channel types also support access permissions in terms of valid operations on the types. To illustrate this, consider the simplest data type, namely a bool. There are three different ways a bool type can be defined, and they are shown below:

bool x;  // Boolean that may be read or written
bool! y; // Boolean that must be written, and may be read
bool? z; // Boolean that must be read, and cannot be written

The ! and ? suffixes constrain the way in which the type can be accessed. The primary use of this is in port lists, where one can specify what variables are read and written by a process. The same syntax can be used (with the same meaning) for user-defined data types.

The following example shows a possible definition for a two-input nand gate that takes two inputs a and b, and produces its output on c.

defcell nand2 (bool? a, b; bool! c) { ... }

Channels support a similar syntax, but the meaning is slightly different.

chan(int) x;  // Sends or receives are permitted
chan!(int) y; // Only sends permitted
chan?(int) z; // Only receives permitted

Again, the same syntax is valid for user-defined channels. These constructs are useful in libraries for additional error checking, and conveying more information to the user of the library.