RE: [sv-bc] packed struct with 2 and 4 val fields

Hi,

The LRM says in 7.2.1,

"If any data type within a packed structure is 4-state, the structure as a whole is treated as a 4-state vector. If there are also 2-state members in the structure, there is an implicit conversion from 4-state to 2-state when reading those members and from 2-state to 4-state when writing them."

I agree this is not clear enough.

I believe the meaning is this:

The entire structure, i.e., all its members, become 4-state. The LRM says, "A packed structure can be used as a whole with arithmetic and logical operators."

When you read a 2-state member, like s.b, you expect to get a 2-state entity. However, internally it is 4-state. So, when the read operation is performed, a conversion is performed from 4-state to 2-state. However, internally it remains 4-state.

Less clear is the write operation. When you write a 2-state member, the value will get converted to 4-state internally, because it is being stored internally as 4-state.

I think the question is what happens when you write

s.b = 'x;

Does the 'x get written internally without change, or is 'x converted to a 2-state '0 since you are writing a 2-state variable, and then the 2-state '0 get converted to a 4-state '0?

I ran this test on 2 different simulators and indeed got different results.
One gave xxxxxxxx and another gave xxxx0000.

However, even the tool that returned xxxx0000 gave xxxxxxxx if the assignment was s = 'x.

Regards,
Shalom

> -----Original Message-----
> From: owner-sv-bc@eda.org [mailto:owner-sv-bc@eda.org] On Behalf Of
> Daniel Mlynek
> Sent: Tuesday, April 26, 2011 1:57 PM
> To: sv-bc@eda.org
> Subject: [sv-bc] packed struct with 2 and 4 val fields
>
> struct packed {reg[1:0][1:0] r; bit [1:0][1:0]b;} s;
> module top;
> initial begin
> s.b='x;
> $display("%b", s); //should print all x or xxxx0000???
> end
> endmodule
>
> In above example s is 4 value as stated in LRM
> My question is what should be written into s.b when it is assigned with
> 'x.
> LRM says that when writing to a 2-val field then implicit 2val to 4
> val
> cast occurs.
> But in this case I'm writing 4 value value into 2 val field.
> In my opinion this LRM statement about 2val to 4val implicit cast is
> not
> clear.
>
> DANiel

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