Expression Bit Length
Formal Definition
This chapter explains values for evaluation of size of expressions.
Description
Every expression has a result with a specified bit length. Therefore
operand lengths should be checked before the expression is calculated
to prevent the loss of important bits. This problem occurs when the
result of an expression is assigned to a variable that cannot store
all the result bits.
Expression 
Bit length 
Unsized constant number 
Same as integer 
Sized constant number 
As given 
i op j,
op: +  / % &  ^ ^~ ~^ 
Max(length of i, length of j) 
i * j 
Length of i + length of j 
op i,
op: +  ~ 
Length of i 
i op j,
op: == != === !== &&  > >= < <= 
1 bit 
op i,
op: & ~&  ~ ^ ~^ ^~ 
1 bit 
i op j,
op: >> << 
Length of i 
i ? j : k 
Max(length of j, length of k) 
{i,j} 
Length of i + length of j 
{i{j,k}} 
i * (length of j + length of k) 
Table 2 Expression bit length rules
Examples
Example 1
reg [3:0] a;
reg [7:0] b;
reg [15:0] c;
a + b > 8 bits
a * b > 12 bits
a  b > 1 bit
a >> 2 > 4 bits
(c) ? a : b > 8 bits
{i,j} > 12 bits
{2(a,b}} > 24 bits
Important Notes
