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
- The expression result bit-length should be known before the assignment to prevent unintended data loss.
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