expression size

[romi]

In the expression below, I'm wondering if 'a' is supposed to be
expanded to 32 bits before the ~ is applied to 'a' or after. I
thought that the parenthesis would force the ~ to be done on the 1 bit
value prior to the expansion to 32 bit for the equality.


logic a,b;
assign b = (~a) == 1;


If a=0, should be b be 0 or 1?

Thanks

 

[Jonathan Bromley]

On Fri, 20 Jun 2008 11:52:40 -0700 (PDT), romi <weberrm@gmail.com>
wrote:

In the expression below, I'm wondering if 'a' is supposed to be
expanded to 32 bits before the ~ is applied to 'a' or after. I
thought that the parenthesis would force the ~ to be done on the 1 bit
value prior to the expansion to 32 bit for the equality.

logic a,b;
assign b = (~a) == 1;

If a=0, should be b be 0 or 1?

False (0). The parentheses don't affect the fact that
"~a" is a context-determined expression, so "a" is widened
to the width of integer before the expression is evaluated.

By contrast, if you had hidden the ~a in a _concatenation_
then it would have been self-determined and the negation
would be done in 1-bit width, so your result would be true:

assign c = {~a} == 1; // yields TRUE if a==0

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[Kevin Neilson]

Jonathan Bromley wrote:

On Fri, 20 Jun 2008 11:52:40 -0700 (PDT), romi <weberrm@gmail.com
wrote:

In the expression below, I'm wondering if 'a' is supposed to be
expanded to 32 bits before the ~ is applied to 'a' or after. I
thought that the parenthesis would force the ~ to be done on the 1 bit
value prior to the expansion to 32 bit for the equality.

logic a,b;
assign b = (~a) == 1;

If a=0, should be b be 0 or 1?

False (0). The parentheses don't affect the fact that
"~a" is a context-determined expression, so "a" is widened
to the width of integer before the expression is evaluated.

By contrast, if you had hidden the ~a in a _concatenation_
then it would have been self-determined and the negation
would be done in 1-bit width, so your result would be true:

assign c = {~a} == 1; // yields TRUE if a==0

Using the logical operator probably makes more sense here:

assign c = !a == 1; // yields TRUE if a==0