PoC.cache.par

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Implements a cache with parallel tag-unit and data memory.

Note

This component infers a single-port memory with read-first behavior, that is, upon writes the old-data is returned on the read output. Such memory (e.g. LUT-RAM) is not available on all devices. Thus, synthesis may infer a lot of flip-flops plus multiplexers instead, which is very inefficient. It is recommended to use PoC.cache.par2 instead which has a slightly different interface.

All inputs are synchronous to the rising-edge of the clock clock.

Command truth table:

Request ReadWrite Invalidate Replace Command
0 0 0 0 None
1 0 0 0 Read cache line
1 1 0 0 Update cache line
1 0 1 0 Read cache line and discard it
1 1 1 0 Write cache line and discard it
0   0 1 Replace cache line.

All commands use Address to lookup (request) or replace a cache line. Address and OldAddress do not include the word/byte select part. Each command is completed within one clock cycle, but outputs are delayed as described below.

Upon requests, the outputs CacheMiss and CacheHit indicate (high-active) whether the Address is stored within the cache, or not. Both outputs have a latency of one clock cycle.

Upon writing a cache line, the new content is given by CacheLineIn. Upon reading a cache line, the current content is outputed on CacheLineOut with a latency of one clock cycle.

Upon replacing a cache line, the new content is given by CacheLineIn. The old content is outputed on CacheLineOut and the old tag on OldAddress, both with a latency of one clock cycle.

Warning

If the design is synthesized with Xilinx ISE / XST, then the synthesis option “Keep Hierarchy” must be set to SOFT or TRUE.

Entity Declaration:

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entity cache_par is
  generic (
    REPLACEMENT_POLICY : string   := "LRU";
    CACHE_LINES        : positive := 32;--1024;
    ASSOCIATIVITY      : positive := 32;--4;
    ADDRESS_BITS       : positive := 8; --32-6;
    DATA_BITS          : positive := 8  --64*8
  );
  port (
    Clock : in std_logic;
    Reset : in std_logic;

    Request    : in std_logic;
    ReadWrite  : in std_logic;
    Invalidate : in std_logic;
    Replace    : in std_logic;
    Address    : in std_logic_vector(ADDRESS_BITS - 1 downto 0);

    CacheLineIn  : in  std_logic_vector(DATA_BITS - 1 downto 0);
    CacheLineOut : out std_logic_vector(DATA_BITS - 1 downto 0);
    CacheHit     : out std_logic := '0';
    CacheMiss    : out std_logic := '0';
    OldAddress   : out std_logic_vector(ADDRESS_BITS - 1 downto 0)
  );
end entity;