Introduction

Compiling a VHDL description into a simulation program is done in four phases:

1.

Scanner/parser phase

2.

Code generation phase

3.

Intermediate language compilation phase

4.

Linking phase.

During the first phase the VHDL source is scanned and parsed. The scanner/parser checks the source syntactically and semantically and builds an abstract syntax tree which includes all information necessary to enter the code generation phase. During code generation the nodes of the tree are transformed into corresponding C++ (intermediate language) constructs which are compiled during the next phase (intermediate language compilation stage) into object code. Finally, all object modules of the VHDL model are linked together with the simulation kernel and a interface module to form a simulation program. The interface module is connected with the SCS (simulation control centre) via network. The SCS reads in user commands (i.e. start/stop simulation, show signal values, ...) and send them to the user interface module which translates them into corresponding kernel actions. In turn, all (stdout) output generated by the simulator is send to the SCS.

The code generator creates

• the structure elaboration code,
• the code to emulate the behaviour of the VHDL processes, functions, procedures and finally
• the type handling code

from the syntax tree. The job of the structure elaboration code is to instantiate all signal and process instances of the model and bind them to the simulation kernel. After structure elaboration has finished the VHDL model consists of a set of processes and signals. Controlled by the simulation kernel processes are executed and values are assigned on signals.

The type handling code implements functionality used to perform basic operations (assignment, compare, ...) on objects (signals, variables ...) of user defined types. Additionally, it provides some special handling functions to the simulation kernel. This functions are required because user defined types are unknown to the kernel at kernel compile time while code generated by the code generator can handle user defined types directly.