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Answer» From SystemVerilog LRM 3.1a:-
- always_comb get executed once at TIME 0, always @* waits till a change occurs on a signal in the INFERRED sensitivity list
- Statement within always_comb can't have blocking timing, event control, or fork-join statement. No such restriction of always @*
- Optionally EDA tool might PERFORM additional checks to warn if the behavior within always_comb procedure doesn't represent combinatorial logic
- Variables on the left-hand side of assignments within an always_comb procedure, including variables from the contents of a called function, shall not be WRITTEN to by any other processes, whereas always @* permits multiple processes to write to the same variable.
- always_comb is sensitive to changes within content of a function, whereas always @* is only sensitive to changes to the arguments to the function.
A small SystemVerilog code snippet to illustrate #5
01.module dummy;
02.logic a, b, c, x, y;
03.
04.// Example VOID function
05.function void my_xor;
06.input a; // b and c are hidden input here
07.x = a ^ b ^ c;
08.endfunction : my_xor
09.
10.function void my_or;
11.input a; // b and c are hidden input here
12.y = a | b | c;
13.endfunction : my_xor
14.
15.always_comb // equivalent to always(a,b,c)
16.my_xor(a); // Hidden inputs are also added to sensitivity list
17.
18.always @* // equivalent to always(a)
19.my_or(a); // b and c are not added to sensitivity list
20.endmodule From SystemVerilog LRM 3.1a:- A small SystemVerilog code snippet to illustrate #5 01.module dummy;
02.logic a, b, c, x, y;
03.
04.// Example void function
05.function void my_xor;
06.input a; // b and c are hidden input here
07.x = a ^ b ^ c;
08.endfunction : my_xor
09.
10.function void my_or;
11.input a; // b and c are hidden input here
12.y = a | b | c;
13.endfunction : my_xor
14.
15.always_comb // equivalent to always(a,b,c)
16.my_xor(a); // Hidden inputs are also added to sensitivity list
17.
18.always @* // equivalent to always(a)
19.my_or(a); // b and c are not added to sensitivity list
20.endmodule
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