The Verilog ‘95 Code Generator (-tvlog95)

Icarus Verilog contains a code generator to emit 1995 compliant Verilog from the input Verilog netlist. This allows Icarus Verilog to function as a Verilog > 1995 to Verilog 1995 translator. The main goal of the project was to convert @*, ANSI style arguments and other constructs to something allowed in 1995 Verilog.


To translate a Verilog program to 1995 compliant Verilog, invoke “iverilog” with the -tvlog95 flag:

% iverilog -tvlog95 -o my_design_95.v my_design.v

The generated Verilog will be placed in a single file (a.out by default), even if the input Verilog is spread over multiple files.

Generator Flags

  • -pspacing=N

    Set the indent spacing (the default is 2).

  • -pallowsigned=1

    Allow emitting the various signed constructs as an extension to 1995 Verilog (off by default).

  • -pfileline=1

    Emit the original file and line information as a comment for each generated line (off by default).

Structures that cannot be converted to 1995 compatible Verilog

The following Verilog constructs are not translatable to 1995 compatible Verilog:

  • Automatic tasks or functions.

  • The power operator (**). Expressions of the form (2**N)**<variable> (where N is a constant) can be converter to a shift.

  • Some System Verilog constructs (e.g. final blocks, ++/– operators, etc.). 2-state variables are converted to 4-state variables.

Icarus extensions that cannot be translated:

  • Integer constants greater than 32 bits.

  • Real valued nets.

  • Real modulus.

  • Most Verilog-A constructs.

Known Issues and Limitations

Some things are just not finished and should generate an appropriate warning. Here is a list of the major things that still need to be looked at.

  • There are still a few module instantiation port issues (pr1723367 and partselsynth).

  • inout ports are not converted (tran-VP).

  • Variable selects of a non-zero based vector in a continuous assignment are not converted.

  • There is no support for translating a zero repeat in a continuous assignment. It is currently just dropped.

  • A pull device connected to a signal select is not translated correctly (this may be fixed).

  • L-value indexed part selects with a constant undefined base in a continuous assignment are not translated.

  • Logic gates are not arrayed exactly the same as the input and the instance name is not always the same.

  • The signed support does not generate $signed() or $unsigned() function calls in a continuous assignment expression.

  • The special power operator cases are not converted in a continuous assignment.

  • Currently a signed constant that sets the MSB in an unsigned context will be displayed as a negative value (e.g. bit = 1 translates to bit = -1).

  • Can net arrays, etc. be unrolled?

  • Can generate blocks be converted?