1.component（元件）：元件可放在library內，供所有使用者多次調用。
元件聲明：
component label is
port( port_name: signal_mode signal_type;...);
end component;
元件實例化：
label：component_name port map(port_list);
2.幾種常見庫：
use ieee.std_logic_unsigned.all 使得std_logic_vector類型的數據像unsigned類型一樣操作。
use ieee.std_logic_arith.all 包括有符號數和無符號數的算術運算和比較運算。
3.信號與變量：
常量（默認值）：constant const_name: type := value;
可在包、實體或結構體中聲明。
信號（實際連線）：signal name：type [range] [:= initial_value];
表示端口或內部連接。不能簡單理解為某端口的電壓，0、1、Z，一個信號對應一部分的觸發和相應的動作。比如temp<=temp+1對應某個寄存器中數值加一。
變量（局部）：variable name：type [range] [:= initial_value];
特點：立即賦值。綜合時會生成額外的寄存器。
4. others => '0' 用于對數組中的各個元素賦值‘0’；others => NULL 用于某些語句（例如case語句）中的子句（例如when子句）不做任何賦值。
5.Example: Legal and illegal operations with std_logic_vector.
LIBRARY ieee;
USE ieee.std_logic_1164.all;? ? -- no extra package required
...
SIGNAL a: IN STD_LOGIC_VECTOR (7 DOWNTO 0);
SIGNAL b: IN STD_LOGIC_VECTOR (7 DOWNTO 0);
SIGNAL x: OUT STD_LOGIC_VECTOR (7 DOWNTO 0);
...
v <= a + b;? ? ? -- illegal (arithmetic operation not OK)
w <= a AND b;? ? -- legal (logical operation OK)
Despite the constraint mentioned above, there is a simple way of allowing data of
type STD_LOGIC_VECTOR to participate directly in arithmetic operations. For
that, theieeelibrary provides two packages,std_logic_signedandstd_logic_unsigned,
which allow operations with STD_LOGIC_VECTOR data to be performed as if the
data were of type SIGNED or UNSIGNED, respectively.
6.Description
The following error occurs in NGDBuild:
"Error: 432 - Logical block 'xxxx' with type 'xxx' is unexpanded."
Solution
NGDBuild issues this error when it cannot resolve all of the components/modules for the entire design.
For example, suppose you have an HDL design that instantiates black-box modules, and the module description for these black-boxes is contained in an EDIF file. If the EDIF file is not in the macro search path or the project directory, NGDBuild issues this error.
To avoid this problem, verify that the module description file has the correct name and is located in the macro search path or project directory.
