Introduction

Tangram is a VLSI-programming language based on CSP, and has much in common with the programming language OCCAM [INM89] (see Section 2.7 for some of the differences). The main construct of Tangram is the command. Commands are either primitive commands, such as a?x and x := x + 1, or composite commands, such as R; S and R ∥ S, where R and S are commands themselves.

Execution of a command may result in a number of communications with the environment through external ports. Another form of interaction with the environment is the reading from and writing into external variables. A Tangram program is a command without external variables, prefixed by an explicit definition of its external ports.

Not all compositions of commands are valid in Tangram. For instance, in a sequential composition the two constituent commands must agree on the input/output direction of their common ports. Also, two commands composed in parallel may not write concurrently into a common variable. Similarly, concurrent reading from and writing into a common variable is not allowed. Section 6.1 defines the syntax of Tangram, including these composition rules. The meaning of each command is described informally.

For a subset of the Tangram commands the handshake-process denotations are given in Section 6.3. This subset is referred to as Core Tangram.

Tangram

The main syntactic constructs of Tangram are program, command, guarded-command set, and expression. With each construct we associate a so-called alphabet structure: a set of typed ports and variables.