Verilog Always block using (*) symbol

verilog
all_by_grace picture all_by_grace · May 15, 2011 · Viewed 46.8k times · Source

I have a simple question regarding how to write an always block in a Verilog module.
If I have the following inputs in my Verilog module: 

input        [31:0] PCplus4 ;       // Value of PC + 4
input        [31:0] A;          // Value A, i.e. RSbus (Use Forwarded Value)
input        [31:0] B;          // Value B, i.e. RTbus (Use Forwarded Value)
input        [31:0] IMM;            // Extended Immediate Value
input        [25:0] TARGET;         // Target Address for Jumps
input         [3:0] BR;         // Branch Selector Input

Is there any difference if I use 

always @ (*)

instead of 

always @ (PCplus4  or A or B or IMM or TARGET or BR)

Is this always @ (*) syntax valid for all versions of Verilog?

Answer

toolic picture toolic · May 15, 2011

The always @(*) syntax was added to the IEEE Verilog Std in 2001. All modern Verilog tools (simulators, synthesis, etc.) support this syntax.

Here is a quote from the LRM (1800-2009):

An incomplete event_expression list of an event control is a common source of bugs in register transfer level (RTL) simulations. The implicit event_expression, @*, is a convenient shorthand that eliminates these problems by adding all nets and variables that are read by the statement (which can be a statement group) of a procedural_timing_ control_statement to the event_expression.

So, your two lines of code may be equivalent (it depends on the code in the body of your always block). However, the @* syntax is easier to maintain.

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