Block and distributed RAM's on Xilinx FPGA's

Distributed RAM's:

  The configuaration logic blocks(CLB) in most of the Xilinx FPGA's contain small single port or double port RAM. This RAM is normally distributed throughout the FPGA than as a single block(It is spread out over many LUT's) and so it is called "distributed RAM". A look up table on a Xilinx FPGA can be configured as a 16*1bit RAM , ROM, LUT or 16bit shift register. This multiple functionality is not possible with Altera FPGA's.

  For Spartan-3 series, each CLB contains upto 64 bits of single port RAM or 32 bits of dual port RAM. As indicated from the size, a single CLB may not be enough to implement a large memory. Also the most of this small RAM's have their input and output as 1 bit wide. For implementing larger and wider memory functions you can connect several distributed RAM's in parallel. Fortunately you need not know how these things are done, because the Xilinx synthesiser tool will infer what you want from your VHDL/ Verilog code and automatically does all this for you.

Block RAM's:

   A block RAM is a dedicated (cannot be used to implement other functions like digital logic) two port memory containing several kilobits of RAM. Depending on how advance your FPGA is there may be several of them. For example Spartan 3 has total RAM, ranging from 72 kbits to 1872 kbits in size.While Spartan 6 devices have block RAM's of upto 4824 Kbits in size.

Difference between Distributed and Block RAM's:

1. As you can see from the definition distributed RAM, a large sized RAM is implemented using a parallel array of large number of elements. This makes distributed RAM, ideal for small sized memories. But when comes to large memories, this may cause a extra wiring delays.
   But Block RAM's are fixed RAM modules which comes in 9 kbits or 18 kbits in size. If you implement a small RAM with a block RAM then its wastage of the rest of the space in RAM.
   So use block RAM for large sized memories and distributed RAM for small sized memories or FIFO's.
2. Another notable difference is how they are operated. In both, the WRITE operation is synchronous(data is written to ram only happens at rising edge of clock). But for the READ operation, distributed RAM is asynchronous (data is read from memory as soon as the address is given, doesn't wait for the clock edge) and block RAM is synchronous. 

How to tell XST which type of RAM you want to use?

   When you declare a RAM in your code, XST(Xilinx synthesizer tool) may implement it as either block RAM or distributed RAM. But if you want, you can force the implementation style to use block RAM or distributed RAM resources. This is done using the ram_style constraint. See the following code to understand how it is done:

library IEEE;
use IEEE.STD_LOGIC_1164.ALL;

entity ram_example is
port (Clk : in std_logic;
        address : in integer;
        we : in std_logic;
        data_i : in std_logic_vector(7 downto 0);
        data_o : out std_logic_vector(7 downto 0)
     );
end ram_example;

architecture Behavioral of ram_example is

--Declaration of type and signal of a 256 element RAM
--with each element being 8 bit wide.
type ram_t is array (0 to 255) of std_logic_vector(7 downto 0);
signal ram : ram_t := (others => (others => '0'));

begin

--process for read and write operation.
PROCESS(Clk)
BEGIN
    if(rising_edge(Clk)) then
        if(we='1') then
            ram(address) <= data_i;
        end if;
        data_o <= ram(address);
    end if;
END PROCESS;

end Behavioral;

The above code declares and defines a single port RAM. Code is also written to specify how the read and write process is implemented. when we synthesis this design, XST uses the block RAM resources by default for implementing the memory. In certain cases you may want to change it. For instance, if I want the memory to be implemented using distributed RAM then add the following two lines before the begin statement in the architecture section:

attribute ram_style: string;

attribute ram_style of ram : signal is "distributed";

Here ram is the signal name. By changing the word distributed to block we can force XST to use block RAM resources. The default value of the attribute ram_style is Auto. 

Notes:- The code was synthesized successfully using Xilinx Webpack version 12.1. The results may vary if you are using an older version of the XST.