6.4. Memory Hierarchy

6.4.1. The L1 Caches

Each CPU tile has an L1 instruction cache and L1 data cache. The size and associativity of these caches can be configured. The default RocketConfig uses 16 KiB, 4-way set-associative instruction and data caches. However, if you use the NMediumCores or NSmallCores configurations, you can configure 4 KiB direct-mapped caches for L1I and L1D.

import freechips.rocketchip.subsystem.{WithNMediumCores, WithNSmallCores}

class SmallRocketConfig extends Config(
    new WithNSmallCores(1) ++
    new RocketConfig)

class MediumRocketConfig extends Config(
    new WithNMediumCores(1) ++
    new RocketConfig)

If you only want to change the size or associativity, there are configuration mixins for those too.

import freechips.rocketchip.subsystem.{WithL1ICacheSets, WithL1DCacheSets, WithL1ICacheWays, WithL1DCacheWays}

class MyL1RocketConfig extends Config(
    new WithL1ICacheSets(128) ++
    new WithL1ICacheWays(2) ++
    new WithL1DCacheSets(128) ++
    new WithL1DCacheWays(2) ++
    new RocketConfig)

You can also configure the L1 data cache as an data scratchpad instead. However, there are some limitations on this. If you are using a data scratchpad, you can only use a single core and you cannot give the design an external DRAM.

import freechips.rocketchip.subsystem.{WithNoMemPort, WithScratchpadsOnly}

class ScratchpadRocketConfig extends Config(
    new WithNoMemPort ++
    new WithScratchpadsOnly ++
    new SmallRocketConfig)

6.4.2. The SiFive L2 Cache

The default RocketConfig provided in the Chipyard example project uses SiFive’s InclusiveCache generator to produce a shared L2 cache. In the default configuration, the L2 uses a single cache bank with 512 KiB capacity and 8-way set-associativity. However, you can change these parameters to obtain your desired cache configuration. The main restriction is that the number of ways and the number of banks must be powers of 2.

import freechips.rocketchip.subsystem.WithInclusiveCache

# Create an SoC with 1 MB, 4-way, 4-bank cache
class MyCacheRocketConfig extends Config(
    new WithInclusiveCache(
        capacityKB = 1024,
        nWays = 4,
        nBanks = 4) ++
    new RocketConfig)

6.4.3. The Broadcast Hub

If you do not want to use the L2 cache (say, for a resource-limited embedded design), you can create a configuration without it. Instead of using the L2 cache, you will instead use RocketChip’s TileLink broadcast hub. To make such a configuration, you can just copy the definition of RocketConfig but omit the WithInclusiveCache mixin from the list of included mixims.

import freechips.rocketchip.subsystem.{WithNBigCores, BaseConfig}

class CachelessRocketConfig extends Config(
    new WithTop ++
    new WithBootROM ++
    new WithNBigCores(1) ++
    new BaseConfig)

If you want to reduce the resources used even further, you can configure the Broadcast Hub to use a bufferless design.

import freechips.rocketchip.subsystem.WithBufferlessBroadcastHub

class BufferlessRocketConfig extends Config(
    new WithBufferlessBroadcastHub ++
    new CachelessRocketConfig)

6.4.4. The Outer Memory System

The L2 coherence agent (either L2 cache or Broadcast Hub) makes requests to an outer memory system consisting of an AXI4-compatible DRAM controller.

The default configuration uses a single memory channel, but you can configure the system to use multiple channels. As with the number of L2 banks, the number of DRAM channels is restricted to powers of two.

import freechips.rocketchip.subsystem.WithNMemoryChannels

class DualChannelRocketConfig extends Config(
    new WithNMemoryChannels(2) ++
    new RocketConfig)

In VCS and Verilator simulation, the DRAM is simulated using the SimAXIMem module, which simply attaches a single-cycle SRAM to each memory channel.

If you want a more realistic memory simulation, you can use FireSim, which can simulate the timing of DDR3 controllers. More documentation on FireSim memory models is available in the FireSim docs.