现在的FPGA基本上都提供了Block RAM供用户使用，所以在自己的工程怎么用VHDL来使用这些Block RAM就非常重要。当然，还是推荐使用开发商提供的软件来生成一个RAM模块，但是这样的RAM块不论移植性还是灵活性都降低了。所以这里采用VHDL来描述一个RAM,对于不同的综合工具和约束条件，综合出的结果可能有所不同，尽管这样，其移植性和灵活性还是要比使用工具生成的RAM块好。使用不同的综合工具，只要稍加修改就可以满足自己设计的要求。

       下面的VHDL代码使用ISE的XST综合，综合结果使用了Block RAM，这是我们期望的。当然有时对于用到的容量很小的RAM，我们并不需要其使用Block RAM，那么只要稍微修改一下就可以综合成Distribute RAM。具体如何修改后面再做介绍。

--------------------------------------------------------------------------------
-- Engineer:       skycanny
-- Module Name:    ram - Behavioral
-- Tool versions:  ISE 7.1 
-- Description:    This module is designed to generate a RAM
--------------------------------------------------------------------------------
library IEEE;
use IEEE.STD_LOGIC_1164.ALL;
use IEEE.STD_LOGIC_ARITH.ALL;
use IEEE.STD_LOGIC_UNSIGNED.ALL;

entity ram is
 generic(width: integer := 16;   -- used to change the memory data's width
    depth: integer := 8);   -- used to change the memery address' width during
             -- instantiation.
 port(
  clk : in std_logic;                           --clock
  addr : in std_logic_vector(depth - 1 downto 0); --address bus
  cs : in std_logic;       --chip select
  oe : in std_logic;       --output enable
             --high for write
             --low for read
  data_i: in std_logic_vector(width - 1 downto 0); --write data bus
  data_o: out std_logic_vector(width - 1 downto 0)  --read data bus
  );
end ram;

architecture Behavioral of ram is

type ram is array(2 ** depth - 1 downto 0) of std_logic_vector(width - 1 downto 0);
signal ram1  : ram;

begin
 process(clk)
 begin
  if(clk'event and clk = '1') then
   if(cs = '0') then 
    if(oe = '0') then
     data_o <= ram1(conv_integer(addr));
    else
     ram1(conv_integer(addr)) <= data_i;
    end if;
   end if;
  end if;
 end process;
end Behavioral;
=========================================================================
* HDL Synthesis *
=========================================================================

Synthesizing Unit .
Related source file is "G:/vhdl/ram/ram.vhd".
Found 256x16-bit single-port block RAM for signal .
-----------------------------------------------------------------------
| mode | no-change | |
| aspect ratio | 256-word x 16-bit | |
| clock | connected to signal | rise |
| enable | connected to signal | low |
| write enable | connected to signal | high |
| address | connected to signal | |
| data in | connected to signal | |
| data out | connected to signal | |
| ram_style | Auto | |
-----------------------------------------------------------------------
Summary:
inferred 1 RAM(s).
Unit synthesized.

可见确实综合成了Block RAM。并且只要改变Width和Depth，就可以改变生成RAM的宽度和容量 。

下面是把上面的代码稍做修改后生成Distribute RAM的代码和其综合报告，可见改动是非常小的。

entity ram is
generic(width: integer := 16; -- used to change the memory data's width
depth: integer := 8); -- used to change the memery address' width during
-- instantiation.
port(
clk : in std_logic; --clock
addr : in std_logic_vector(depth - 1 downto 0); --address bus
cs : in std_logic; --chip select
oe : in std_logic; --output enable
--low for read
wr : in std_logic; --low for write --add for distribute ram
data_i: in std_logic_vector(width - 1 downto 0); --write data bus
data_o: out std_logic_vector(width - 1 downto 0) --read data bus
);
end ram;

architecture Behavioral of ram is

type ram is array(2 ** depth - 1 downto 0) of std_logic_vector(width - 1 downto 0);
signal ram1 : ram;

begin
process(clk)
begin
if(clk'event and clk = '1') then
if(cs = '0') then
if(oe = '0') then
data_o <= ram1(conv_integer(addr));
elsif(wr = '0') then --change for distribute ram
ram1(conv_integer(addr)) <= data_i;
end if;
end if;
end if;
end process;
end Behavioral;

=========================================================================
* HDL Synthesis *
=========================================================================

Synthesizing Unit .
Related source file is "G:/vhdl/ram/ram.vhd".
Found 256x16-bit single-port distributed RAM for signal .
-----------------------------------------------------------------------
| aspect ratio | 256-word x 16-bit | |
| clock | connected to signal | rise |
| write enable | connected to internal node | high |
| address | connected to signal | |
| data in | connected to signal | |
| data out | connected to internal node | |
| ram_style | Auto | |
-----------------------------------------------------------------------
Found 16-bit register for signal .
Summary:
inferred 1 RAM(s).
inferred 16 D-type flip-flop(s).
Unit synthesized.

由以上的讨论我们可以看出，用VHDL来描述RAM其实是非常方便，更加重要的这种方法在移植是时候有着更加优越的特点。