VHDL Simulator?

Question

What is a VHDL Simulator, and what are the best options?

Answer 1

A VHDL simulator is a very important tool used in the standard work-flow of VHDL design ( second only to a good text editor ). It's used for executing the code in a sequential manner as if it was any other programming language code.

The results can be presented to the user via a waveform, file i/o or console. The designer then examines the results, fixes bugs in the code, and verifies it for correct functionality under different types of stimuli.

Examples:

Modelsim
Questasim
Active HDL

Other less good options for VHDL :
Vivado simulator ( free )
VCS

Answer 2

In general this is a software to simulate the behavior of devices' HDL models. Usually, it gets a HDL model and a testbench which defines:
- an external environment model and cases in wich the device need to work
- test control, log and checks

A simulator gets a set of stimulus from testbenches and applies it to a simulated HDL model. Results of simulation are:
- waves (graphical representation of signals changes during model time)
- logs, HDL-dependant assertions, responses, and other reports and messages.

Basicaly each simultor consists of an HDL compiler/translator, simulation kernel (in which a time loop is implemented - this loop performs object change events control and merging, object value solving), text editor, and a scope/wave to image an object's change history.

I mostly work with Modelsim and do it from command line using make files. This is a great way to use simulators (perhaps except for the simulation in Vivado, because it is convenient to run it in Vivado GUI while designing for an FPGA). The best option for both VHDL and Verilog/SystemVerilog - is to switch off any optimizations, or to strictly control it for modules/entities when you know what want to do and what you need to do.

Answer 3

VHDL simulators are software packages that can be used to compile and run VHDL code. Xilinx ISE is one of the most commonly used simulators, but for older parts. However, there are other good options such as Modelsim which is suitable for students.