PoC.cache.cpu¶

This unit provides a cache (PoC.cache.par2) together with a cache controller which reads / writes cache lines from / to memory. The memory is accessed using a PoC.Mem Interface interfaces, the related ports and parameters are prefixed with mem_.

The CPU side (prefix cpu_) has a modified PoC.Mem interface, so that this unit can be easily integrated into processor pipelines. For example, let’s have a pipeline where a load/store instruction is executed in 3 stages (after fetching, decoding, ...):

Execute (EX) for address calculation,
Load/Store 1 (LS1) for the cache access,
Load/Store 2 (LS2) where the cache returns the read data.

The read data is always returned one cycle after the cache access completes, so there is conceptually a pipeline register within this unit. The stage LS2 can be merged with a write-back stage if the clock period allows so.

The stage LS1 and thus EX and LS2 must stall, until the cache access is completed, i.e., the EX/LS1 pipeline register must hold the cache request until it is acknowledged by the cache. This is signaled by cpu_got as described in Section Operation below. The pipeline moves forward (is enabled) when:

pipeline_enable <= (not cpu_req) or cpu_got;

If the pipeline can stall due to other reasons, care must be taken to not unintentionally executing the cache access twice or missing the read data.

Of course, the EX/LS1 pipeline register can be omitted and the CPU side directly fed by the address caculator. But be aware of the high setup time of this unit and high propate time for cpu_got.

This unit supports only one outstanding CPU request. More outstanding requests are provided by PoC.cache.mem.

Configuration¶

Parameter	Description
REPLACEMENT_POLICY	Replacement policy of embedded cache. For supported values see PoC.cache_replacement_policy.
CACHE_LINES	Number of cache lines.
ASSOCIATIVITY	Associativity of embedded cache.
CPU_ADDR_BITS	Number of address bits on the CPU side. Each address identifies one memory word as seen from the CPU. Calculated from other parameters as described below.
CPU_DATA_BITS	Width of the data bus (in bits) on the CPU side. CPU_DATA_BITS must be divisible by 8.
MEM_ADDR_BITS	Number of address bits on the memory side. Each address identifies one word in the memory.
MEM_DATA_BITS	Width of a memory word and of a cache line in bits. MEM_DATA_BITS must be divisible by CPU_DATA_BITS.

If the CPU data-bus width is smaller than the memory data-bus width, then the CPU needs additional address bits to identify one CPU data word inside a memory word. Thus, the CPU address-bus width is calculated from:

CPU_ADDR_BITS=log2ceil(MEM_DATA_BITS/CPU_DATA_BITS)+MEM_ADDR_BITS

The write policy is: write-through, no-write-allocate.

Operation¶

Alignment of Cache / Memory Accesses¶

Memory accesses are always aligned to a word boundary. Each memory word (and each cache line) consists of MEM_DATA_BITS bits. For example if MEM_DATA_BITS=128:

memory address 0 selects the bits 0..127 in memory,
memory address 1 selects the bits 128..256 in memory, and so on.

Cache accesses are always aligned to a CPU word boundary. Each CPU word consists of CPU_DATA_BITS bits. For example if CPU_DATA_BITS=32:

CPU address 0 selects the bits 0.. 31 in memory word 0,
CPU address 1 selects the bits 32.. 63 in memory word 0,
CPU address 2 selects the bits 64.. 95 in memory word 0,
CPU address 3 selects the bits 96..127 in memory word 0,
CPU address 4 selects the bits 0.. 31 in memory word 1,
CPU address 5 selects the bits 32.. 63 in memory word 1, and so on.

Shared and Memory Side Interface¶

A synchronous reset must be applied even on a FPGA.

The memory side interface is documented in detail here.

CPU Side Interface¶

The CPU (pipeline stage LS1, see above) issues a request by setting cpu_req, cpu_write, cpu_addr, cpu_wdata and cpu_wmask as in the PoC.Mem Interface interface. The cache acknowledges the request by setting cpu_got to ‘1’. If the request is not acknowledged (cpu_got = '0') in the current clock cycle, then the request must be repeated in the following clock cycle(s) until it is acknowledged, i.e., the pipeline must stall.

A cache access is completed when it is acknowledged. A new request can be issued in the following clock cycle.

Of course, cpu_got may be asserted in the same clock cycle where the request was issued if a read hit occurs. This allows a throughput of one (read) request per clock cycle, but the drawback is, that cpu_got has a high propagation delay. Thus, this output should only control a simple pipeline enable logic.

When cpu_got is asserted for a read access, then the read data will be available in the following clock cycle.

Due to the write-through policy, a write will always take several clock cycles and acknowledged when the data has been issued to the memory.

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:

entity cache_cpu is
  generic (
    REPLACEMENT_POLICY : string   := "LRU";
    CACHE_LINES        : positive;
    ASSOCIATIVITY      : positive;
    CPU_DATA_BITS      : positive;
    MEM_ADDR_BITS      : positive;
    MEM_DATA_BITS      : positive
  );
  port (
    clk : in std_logic; -- clock
    rst : in std_logic; -- reset
    
    -- "CPU" side
    cpu_req   : in  std_logic;
    cpu_write : in  std_logic;
    cpu_addr  : in  unsigned(log2ceil(MEM_DATA_BITS/CPU_DATA_BITS)+MEM_ADDR_BITS-1 downto 0);
    cpu_wdata : in  std_logic_vector(CPU_DATA_BITS-1 downto 0);
    cpu_wmask : in  std_logic_vector(CPU_DATA_BITS/8-1 downto 0);
    cpu_got   : out std_logic;
    cpu_rdata : out std_logic_vector(CPU_DATA_BITS-1 downto 0);
    
    -- Memory side
    mem_req   : out std_logic;
    mem_write : out std_logic;
    mem_addr  : out unsigned(MEM_ADDR_BITS-1 downto 0);
    mem_wdata : out std_logic_vector(MEM_DATA_BITS-1 downto 0);
    mem_wmask : out std_logic_vector(MEM_DATA_BITS/8-1 downto 0);
    mem_rdy   : in  std_logic;
    mem_rstb  : in  std_logic;
    mem_rdata : in  std_logic_vector(MEM_DATA_BITS-1 downto 0)
    );
end entity;