PoC.cache.par2

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Cache with parallel tag-unit and data memory. For the data memory, PoC.mem.ocram.sp is used.

Configuration

Parameter Description
REPLACEMENT_POLICY Replacement policy. For supported policies see PoC.cache_replacement_policy.
CACHE_LINES Number of cache lines.
ASSOCIATIVITY Associativity of the cache.
ADDR_BITS Number of address bits. Each address identifies exactly one cache line in memory.
DATA_BITS Size of a cache line in bits. DATA_BITS must be divisible by 8.

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 0 1 Read cache line before replace.
0 1 0 1 Replace cache line.

Operation

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

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 (pipelined) if HIT_MISS_REG is true, otherwise the result is outputted immediately (combinational).

Upon writing a cache line, the new content is given by CacheLineIn. Only the bytes which are not masked, i.e. the corresponding bit in WriteMask is ‘0’, are actually written.

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

Replacing a cache line requires two steps, both with Replace = '1':

  1. Read old contents of cache line by setting ReadWrite to ‘0’. The old content is outputed on CacheLineOut and the old tag on OldAddress, both with a latency of one clock cycle.
  2. Write new cache line by setting ReadWrite to ‘1’. The new content is given by CacheLineIn. All bytes shall be written, i.e. WriteMask = 0. The new cache line content will be outputed again on CacheLineOut in the next clock cycle (latency = 1).

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_par2 is
  generic (
    REPLACEMENT_POLICY : string   := "LRU";
    CACHE_LINES        : positive := 32;
    ASSOCIATIVITY      : positive := 32;
    ADDR_BITS          : positive := 8;
    DATA_BITS          : positive := 8;
    HIT_MISS_REG       : boolean  := true  -- must be true for Cocotb.
  );
  port (
    Clock : in std_logic;
    Reset : in std_logic;

    Request    : in std_logic;
    ReadWrite  : in std_logic;
    WriteMask  : in std_logic_vector(DATA_BITS/8 - 1 downto 0) := (others => '0');
    Invalidate : in std_logic;
    Replace    : in std_logic;
    Address    : in std_logic_vector(ADDR_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(ADDR_BITS-1 downto 0)
  );
end entity;