- Section 8.12
The null statement performs no action. It is usually used with the case statement to indicate that under certain conditions, no action is required.
case ENCRYPTION is when "00" => CPU_DATA_TMP := (B & A) - OPERAND; when "01" => CPU_DATA_TMP := (B & A) + OPERAND; when "10" => CPU_DATA_TMP := (A & B) - OPERAND; when "11" => CPU_DATA_TMP := (A & B) + OPERAND; when others => null; end case;
The null may have an optional label:
The null statement is supported by synthesis tools.
Note: using a null statement in a “combinational process” can result in latches being inferred, unless all signals driven by the process are given unconditional default assignments.