6.11. Memory Hierarchy

6.11.1. 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 WithNMedCores or WithNSmallCores configurations, you can configure 4 KiB direct-mapped caches for L1I and L1D.

If you only want to change the size or associativity, there are config fragments for those too. See Config Fragments for how to add these to a custom Config.

new freechips.rocketchip.subsystem.WithL1ICacheSets(128) ++  // change rocket I$
new freechips.rocketchip.subsystem.WithL1ICacheWays(2) ++    // change rocket I$
new freechips.rocketchip.subsystem.WithL1DCacheSets(128) ++  // change rocket D$
new freechips.rocketchip.subsystem.WithL1DCacheWays(2) ++    // change rocket D$

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. Note that these configurations fully remove the L2 cache and mbus.

class ScratchpadOnlyRocketConfig extends Config(
  new testchipip.WithSerialPBusMem ++
  new freechips.rocketchip.subsystem.WithNMemoryChannels(0) ++ // remove offchip mem port
  new freechips.rocketchip.subsystem.WithNBanks(0) ++
  new freechips.rocketchip.subsystem.WithNoMemPort ++
  new freechips.rocketchip.subsystem.WithScratchpadsOnly ++    // use rocket l1 DCache scratchpad as base phys mem
  new freechips.rocketchip.subsystem.WithNBigCores(1) ++
  new chipyard.config.AbstractConfig)

This configuration fully removes the L2 cache and memory bus by setting the number of channels and number of banks to 0.

6.11.2. The System Bus

The system bus is the TileLink network that sits between the tiles and the L2 agents and MMIO peripherals. Ordinarily, it is a fully-connected crossbar, but TestChipIP provides a version that uses a ring network instead. This can be useful when taping out larger systems. To use the ring network system bus, simply add the WithRingSystemBus config fragment to your configuration.

class RingSystemBusRocketConfig extends Config(
  new testchipip.WithRingSystemBus ++                       // Ring-topology system bus
  new freechips.rocketchip.subsystem.WithNBigCores(1) ++
  new chipyard.config.AbstractConfig)

6.11.3. 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.

Refer to the CacheParameters object defined in sifive-cache for customization options.

6.11.4. 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 config fragment from the list of included mixims.

If you want to reduce the resources used even further, you can configure the Broadcast Hub to use a bufferless design. This config fragment is freechips.rocketchip.subsystem.WithBufferlessBroadcastHub.

6.11.5. 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.

new freechips.rocketchip.subsystem.WithNMemoryChannels(2)

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

Instead of connecting to off-chip DRAM, you can instead connect a scratchpad and remove the off-chip link. This is done by adding a fragment like testchipip.WithBackingScratchpad to your configuration and removing the memory port with freechips.rocketchip.subsystem.WithNoMemPort.

class MbusScratchpadRocketConfig extends Config(
  new testchipip.WithBackingScratchpad ++                   // add mbus backing scratchpad
  new freechips.rocketchip.subsystem.WithNoMemPort ++       // remove offchip mem port
  new freechips.rocketchip.subsystem.WithNBigCores(1) ++
  new chipyard.config.AbstractConfig)

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.