# Icarus Verilog Attributes

## Attribute Naming Conventions

Attributes that are specific to Icarus Verilog, and are intended to be of use to programmers, start with the prefix “ivl_”.

Attributes with the “_ivl_” prefix are set aside for internal use. They may be generated internally by the compiler. They need not be documented here.

## Attributes To Control Synthesis

The following is a summary of Verilog attributes that Icarus Verilog understands within Verilog source files to control synthesis behavior. This section documents generic synthesis attributes. For target specific attributes, see target specific documentation.

These attributes only effect the behavior of the synthesizer. For example, the ivl_combinational will not generate an error message if the Verilog is being compiled for simulation. (It may generate a warning.)

- Attributes for “always” and “initial” statements

(* ivl_combinational *)

> This attribute tells the compiler that the statement models combinational logic. If the compiler finds that it cannot make combinational logic out of a marked always statement, it will report an error.

> This attribute can be used to prevent accidentally inferring latches or flip-flops where the user intended combinational logic.

(* ivl_synthesis_on *)

> This attribute tells the compiler that the marked always statement is synthesizable. The compiler will attempt to synthesize the code in the marked “always” statement. If it cannot in any way synthesize it, then it will report an error.

(* ivl_synthesis_off *)

> If this value is attached to an “always” statement, then the compiler will not synthesize the “always” statement. This can be used, for example, to mark embedded test bench code.

- Attributes for modules

(* ivl_synthesis_cell *)

> If this value is attached to a module during synthesis, that module will be considered a target architecture primitive, and its interior will not be synthesized further. The module can therefore hold a model for simulation purposes.

- Attributes for signals (wire/reg/integer/tri/etc.)

(* PAD = “<pad assignment list>” *)

> If this attribute is attached to a signal that happens to be a root module port, then targets that support it will use the string value as a list of pin assignments for the port/signal. The format is a comma separated list of location tokens, with the format of the token itself defined by the back-end tools in use.

- Other Attributes

[ none defined yet ]

## Misc

(* _ivl_schedule_push *)

> If this attribute is attached to a thread object (always or initial statement) then the vvp code generator will generate code that causes the scheduler to push this thread at compile time. The compiler may internally add this attribute to always statements if it detects that it is combinational. This helps resolve time-0 races.