[sv-bc] RE: [sv-ac] Mantis 1974: Definitions of true and false conditions

Brad,
 
I think you misunderstood me. 
 
1364 issue 403 did not refer to how the first and second expressions are
combined when the condition has an ambiguous value.
 
The issue there, and the issue I have brought up now in two other
places, is how to describe when the condition is true, when false, and
when ambiguous. (It is true that in the conditional operator, the
ambiguous result is distinguished from a false result, whereas in
assertions and in if-else, an ambiguous result is treated the same as a
false result.)
 
This is only indirectly related to Mantis 1190.
 
You wrote "The cond_predicate has a scalar type, and cannot usefully be
considered bitwise."
Actually it does not have a scalar type.
If I write "a[1:0] ? 1'b1 : 1'b0" or "if (a[1:0])", the cond_predicate
a[1:0] is still a two-bit vector.
But it is treated as either true, false, or ambiguous.
 
Now, what are true, false, and ambiguous?
 
Draft 3a for the conditional operator does not define true and false and
says that ambiguous is "x or z".
 
12.4 says that true is "a nonzero known value" and false is "0 or x or
z".
 
16.3 says that false is "0 or x or z" and true is "otherwise".
 
So how is 2'b1x to be considered? It is not completely known, nor
completely unknown, nor completely x. However it is definitely non-zero.
This is if any bit is definitely 1.
 
As such, it is considered to be TRUE. The real condition for true is its
non-zero-ness. If the condition is definitely non-zero, it is true. For
if-else and assertions, it is otherwise false.
 
For the conditional operator, if the condition value is definitely zero
(all bits zeroes), then it is false. If it is not definitely zero, nor
definitely non-zero (i.e., no bit is 1, not all are zero), then it is
ambiguous.
 
This is not what 12.4 and 16.3 say. 
 
12.4 say that true requires a 'known' value. 2'b1x could be considered
to not be a known value. I don't known whether the value is 2 or 3.
Nevertheless, it is definitely non-zero and therefore should be
considered true. 
 
11.4.12 avoids this trap by not defining what true and false are, but
still is not precise requiring the ambiguous result.
 
Regards,
Shalom
 



________________________________

	From: owner-sv-ac@server.eda.org
[mailto:owner-sv-ac@server.eda.org] On Behalf Of Brad Pierce
	Sent: Wednesday, August 15, 2007 9:17 AM
	To: sv-bc
	Cc: sv-ac@server.eda-stds.org
	Subject: RE: [sv-ac] Mantis 1974: Definitions of true and false
conditions
	
	
	Shalom,
	 
	The fix for 1364 issue 403 became the following in 11.4.12 of
Draft 3a --
	 
	      "If cond_predicate is true, the operator returns the value
of the first expression; if false, it returns the value of  the second
expression. If cond_predicate evaluates to an ambiguous value (x or z),
then both the first expression and the second expression shall be
evaluated, and their results shall be combined bit by bit using Table
11-22 to calculate the final result".
	 
	The cond_predicate has a scalar type, and cannot usefully be
considered bitwise.  Instead, this paragraph is saying that, if the
cond_predicate is ambiguous, then where the first and second expression
agree about particular bits of the result, a non-x-pessimistic value is
calculated for those bits.
	 
	This is related to enhancement request
	 
	     http://www.eda-stds.org/svdb/view.php?id=1190
<http://www.eda-stds.org/svdb/view.php?id=1190> 
	 
	which was <http://www.boyd.com/1364_btf/report/full_pr/401.html>
http://www.boyd.com/1364_btf/report/full_pr/401.html .
	 
	-- Brad

________________________________

	From: owner-sv-ac@eda.org [mailto:owner-sv-ac@eda.org] On Behalf
Of Bresticker, Shalom
	Sent: Tuesday, August 14, 2007 9:54 PM
	To: sv-bc
	Cc: sv-ac@eda-stds.org
	Subject: [sv-ac] Mantis 1974: Definitions of true and false
conditions
	
	

	Section 12.4 has the following definitions of 'true' and 'false'
conditions: 

	"If the cond_predicate expression evaluates to true (that is,
has a nonzero known value), the first statement shall be executed. If it
evaluates to false (that is, has a zero value or the value is x or z),
the first statement shall not execute."

	These descriptions are incorrect. We corrected them for the
conditional operator (?:) in 1364-2005, but missed this place. See old
1364 issue 403 ( <http://boyd.com/1364_btf/report/full_pr/403.html>
http://boyd.com/1364_btf/report/full_pr/403.html).

	Section 16.3 (Immediate Assertions) has the same mistake: 

	"The expression is nontemporal and is interpreted the same way
as an expression in the condition 
	of a procedural if statement. In other words, if the expression
evaluates to X, Z, or 0, then it is interpreted as being false, and the
assertion is said to fail. Otherwise, the expression is interpreted as
being true, and the assertion is said to pass."

	The difference is where some of the bits are X/Z and some are
0/1. The value is known but also not completely unknown. The value is
not quite X/Z, but not completely 0s and 1s. What is known is that if
any of the bits is 1, then the value is definitely non-zero. In this
case, the condition is true, even though the value is not quite known.
On the other hand, if the known bits are all 0, then the condition is
false, even though its value is not quite 0, X, or Z.

	This is Mantis 1974. 

	Shalom 

	Shalom Bresticker 
	Intel Jerusalem LAD DA 
	+972 2 589-6852 
	+972 54 721-1033 


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