 # Unknown condition in if statement is treated in what manner

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Guest

Thu Nov 06, 2008 8:45 am

Hi everyone,
I have run this small code and seeing many variations can you plaese
suggest why is that so:

module top;

reg [1:0] c;
reg [1:0] a = 2'b10;
reg [1:0] b = 2'b01;
reg [1:0] if_out;
wire [1:0] out;

assign out = c ? a : b;

initial
begin
#1 c = 2'b1x;
if(c)
if_out = a;
else
if_out = b;
#1 \$display("out:%b, if_out:%b, c:%b",out,if_out,c);
end
endmodule

output:
out:10, if_out:10, c:1x

Now if i change c to 2'bXX

out:xx, if_out:01, c x

Now if i change c to 2'bx0

out:xx, if_out:01, c 0

Jonathan Bromley
Guest

Thu Nov 06, 2008 10:21 am

On Wed, 5 Nov 2008 23:25:36 -0800 (PST), gtalk.nikhil_at_gmail.com wrote:

Quote:
Hi everyone,
I have run this small code and seeing many variations can you plaese
suggest why is that so:

Interesting, but your example makes things a little too
complicated for easy explanation..... let's try.

You are using the 2-bit register 'c' as a test
condition in two places:

Quote:
assign out = c ? a : b;

if(c)

In both cases the meaning is clear, and in
both cases it is the same:

TRUE if c is definitely non-zero (has at least one 1 bit)
FALSE if c is exactly zero
UNKNOWN if c contains any X/Z bits and no 1 bits

You can get exactly this result by looking at the
reduction-OR operator |c which gives 1'b1, 1'b0, 1'bX
for those three cases.

OK, now we know how a truth-value is handled in Verilog,
let's see what happens to it in those two statements.
We'll do procedural if() first, because it's easier:

if (Condition)
TRUE_branch;
else
FALSE_branch;

It's easy to understand this if Condition is TRUE or
FALSE, but what happens if Condition is UNKNOWN?
There is no third branch for "don't know"! So Verilog
must make a choice; that choice is to do the FALSE branch
if Condition is unknown. (That's guaranteed by the LRM.)
This completely explains your results on "if_out".

Now let's look at the more tricky case:

assign out = c ? a: b;

In this case Verilog can be rather more clever in the way
it handles unknowns. Obviously if c is TRUE you get out=a,
and if c is FALSE you get out=b. But if c is UNKNOWN,
Verilog looks at EACH BIT POSITION of a, b independently.
In a given bit position, if the corresponding bits in a
and b are the same then we assign that value to the
corresponding bit of out!

Here's the reasoning:
* c?a:b models a multiplexer. c is the select input.
* If the select input is unknown, but the two data
inputs are identical, then the output is sure to
be the same as the data inputs.
* If the select input is unknown but the two data
inputs are different, then we can't predict the
outcome; so we drive X on to the output.

Try this example:

reg [3:0] d0, d1, y;
initial begin
d0 = 'b1100;
d1 = 'b0101;
y = 1'bx ? d0: d1;
\$display("d0 = %b", d0);
\$display("d1 = %b", d1);
\$display(" y = %b", y);
end

--
Jonathan Bromley, Consultant

DOULOS - Developing Design Know-how
VHDL * Verilog * SystemC * e * Perl * Tcl/Tk * Project Services

Doulos Ltd., 22 Market Place, Ringwood, BH24 1AW, UK
jonathan.bromley_at_MYCOMPANY.com
http://www.MYCOMPANY.com

The contents of this message may contain personal views which
are not the views of Doulos Ltd., unless specifically stated.

Guest

Thu Nov 06, 2008 12:45 pm

On Nov 6, 2:21 pm, Jonathan Bromley <jonathan.brom...@MYCOMPANY.com>
wrote:
Quote:
On Wed, 5 Nov 2008 23:25:36 -0800 (PST), gtalk.nik...@gmail.com wrote:
Hi everyone,
I have run this small code and seeing many variations can you plaese
suggest why is that so:

Interesting, but your example makes things a little too
complicated for easy explanation..... let's try.

You are using the 2-bit register 'c' as a test
condition in two places:

assign out = c ? a : b;
if(c)

In both cases the meaning is clear, and in
both cases it is the same:

TRUE if c is definitely non-zero (has at least one 1 bit)
FALSE if c is exactly zero
UNKNOWN if c contains any X/Z bits and no 1 bits

You can get exactly this result by looking at the
reduction-OR operator |c which gives 1'b1, 1'b0, 1'bX
for those three cases.

OK, now we know how a truth-value is handled in Verilog,
let's see what happens to it in those two statements.
We'll do procedural if() first, because it's easier:

if (Condition)
TRUE_branch;
else
FALSE_branch;

It's easy to understand this if Condition is TRUE or
FALSE, but what happens if Condition is UNKNOWN?
There is no third branch for "don't know"! So Verilog
must make a choice; that choice is to do the FALSE branch
if Condition is unknown. (That's guaranteed by the LRM.)
This completely explains your results on "if_out".

Now let's look at the more tricky case:

assign out = c ? a: b;

In this case Verilog can be rather more clever in the way
it handles unknowns. Obviously if c is TRUE you get out=a,
and if c is FALSE you get out=b. But if c is UNKNOWN,
Verilog looks at EACH BIT POSITION of a, b independently.
In a given bit position, if the corresponding bits in a
and b are the same then we assign that value to the
corresponding bit of out!

Here's the reasoning:
* c?a:b models a multiplexer. c is the select input.
* If the select input is unknown, but the two data
inputs are identical, then the output is sure to
be the same as the data inputs.
* If the select input is unknown but the two data
inputs are different, then we can't predict the
outcome; so we drive X on to the output.

Try this example:

reg [3:0] d0, d1, y;
initial begin
d0 = 'b1100;
d1 = 'b0101;
y = 1'bx ? d0: d1;
\$display("d0 = %b", d0);
\$display("d1 = %b", d1);
\$display(" y = %b", y);
end

--
Jonathan Bromley, Consultant

DOULOS - Developing Design Know-how
VHDL * Verilog * SystemC * e * Perl * Tcl/Tk * Project Services

Doulos Ltd., 22 Market Place, Ringwood, BH24 1AW, UK
jonathan.brom...@MYCOMPANY.comhttp://www.MYCOMPANY.com

The contents of this message may contain personal views which
are not the views of Doulos Ltd., unless specifically stated.

Hi,
Great explanation thanks a lot for your time

Guest

Thu Apr 02, 2020 4:45 pm

can you please share the page number or context which says .. in unknown case if(exp)
....if exp is unknown .. else will be executed

unfrostedpoptart
Guest

Sun Apr 05, 2020 11:45 pm

On Thursday, April 2, 2020 at 7:49:42 AM UTC-7, vishal...@gmail.com wrote:
Quote:
can you please share the page number or context which says .. in unknown case if(exp)
...if exp is unknown .. else will be executed

Why are you responding to a 12-year-old post?

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