FIFO and LIFO Buffers

First-In First-Out Buffer (FIFO)

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• It is digital device/block to regulate flow of data received in irregular time intervals

• Data can be read while the data fill up the buffer

M x N-bit (or simply M x N) FIFO indicates a capacity of M data word, N-bit each.

可以存M个Nbit的字符

External Interface

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entity FIFO_MN is
generic (N: positive := 4; M: positive := 4);
port ( D_IN: in std_logic_vector(N-1 downto 0);
       Read_WriteN: in std_logic;
       Enable: in std_logic;
       Reset: in std_logic;
       CLK: in std_logic;
       D_OUT: out std_logic_vector(N-1 downto 0);
       Empty, Full: out std_logic);
end FIFO_MN;

　GENERIC（常数名 数据类型 := 设定值）;

  1. 定义实体的端口大小；

  2. 设计实体的物理特征；

    在例化时应该加generic map来修改不同需求下的元件例化参数

利用移位寄存器来设计FIFO

Suppose we want to implement a M x N FIFO, it is clearly that we will need at least M x N D flip flops to store the bits.

we make use of the N M-bit shift register to build the FIFO

我们使用N个M比特的移位寄存器来构建FIFIO

  1. when a new word is added, each of the bits in this word will be shifted into the N shift registers separately.

  2. the older words are also shifted, therefore it is important that we remember where the oldest word is by a counter of a suitable size.

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If it is full, we should read the output (and free the space) before we can receive more word(s); otherwise the buffer overflows

Overflow a buffer should be avoided at all costs. i.e. don’t write when Full = 1

Dout一直在读，R/W’直影响计数器加减以及移位寄存器保持还是移动

之所以把计数器初始化为1111，是为了统一计数器和shifer的索引，这样可以保证索引始终指在最后一个有效数字上。且由于M=4所以

Last-In First-Out Buffer (LIFO)

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push-down stack.

• Data words are pushed down into the buffer.

• The last word that goes into the stack (push) will be the first one be taken out (pop).

PUSH place a word onto the top of stack

POP remove a word from the top of stack

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entity LIFO_MN is
generic (N: positive := 4; M: positive := 4);
port ( D_IN: in std_logic_vector(N-1 downto 0);
       Push_PopN, Enable, Reset: in std_logic;
       CLK: in std_logic;
       D_OUT: out std_logic_vector(N-1 downto 0);
       Empty, Full: out std_logic);
end LIFO_MN;

shift register has to shift to both left and right, so that after a word is shifted in to the right (push), it can be shifted out to the left (pop, last in first out).

use N M-bit bi-directional shift registers to build the LIFO

  1. when a new word is added (push), each of the bits will be shifted right into the N shift registers in parallel. At the same time, the older words are all shifted right.

  2. the top of the stack is always at the leftmost.

image-20221026122316485

因为这里读数据只读第一个位置，索引与计数器无关（不随计数器变化），所以计数器不用初始化为1111；FIFO和LIFO区别就在于索引动不动，其他没有本质区别

When it is full, we should pop data out before we push more words in ; otherwise the buffer overflows, which means that bottom of the stack - AAword0 is lost permanently

image-20221026122429721