The conditional operator selects an expression for evaluation depending on the value of condition.
condition ? expression1 : expression2;
If the condition is evaluated as false (or zero value) then expression2 is evaluated and used as a result of an entire expression. If condition is evaluated as true (or nonzero value) then expression1 is evaluated. In case condition is evaluated as x or z value, then both expresion1 and expression2 are evaluated, and the result is calculated bit by bit on the basis of the following table:

0 
1 
x 
z 
0 
0 
x 
x 
x 
1 
x 
1 
x 
x 
x 
x 
x 
x 
x 
z 
x 
x 
x 
x 
Table 5 Results of bit by bit calculation.
If one of the expressions is of real type then the result of the whole expression should be 0 (zero). If expressions have different lengths, then length of an entire expression will be extended to the length of the longer expression. Trailing 0s will be added to the shorter expression.
The conditional operator can be nested (Example 3) and its behavior is identical with the case statement behavior.
Example 1
(a) ? 4'b110x : 4'b1000;
If 'a' has a nonzero value then the result of this expression is 4'b110x. If 'a' is 0, then the result of this expression is 4'b1000. If 'a' is x value then the result is 4'b1x0x (this is due to the fact that the result must be calculated bit by bit on the basis of the Table 1).
Example 2
assign data_out = (enable) ? data_reg : 8'bz;
The above example shows modeling tristate buffers.
Example 3
reg [4:0] mux;
reg [1:0] addr;
mux = (addr == 2'b00) ? i0 :
((addr == 2'b01) ? i1 :
((addr == 2'b10) ? i2 :
((addr == 2'b11) ? i3 :
4'bz)));
case (addr)
2'b00: mux = i0;
2'b01: mux = i1;
2'b10: mux = i2;
2'b11: mux = i3;
default: mux = 4'bz;
endcase
Two different methods of modeling a multiplexer.
Conditional operator can be used for tristate buffer modeling.
Conditional operator can be nested (its behavior is identical with the case statement behavior).
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