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[xls/contrib] Tutorial: XLS[cc] memories.

This tutorial is aimed at walking you through usage of memories. Memories in this context refer to externally implemented SRAMs. The example given will be usage of a dual port "1 read 1 write" memory.

The channels tutorial should be followed as a prerequisite.

C++ Source

Create a source file

template<typename T>
using InputChannel = __xls_channel<T, __xls_channel_dir_In>;
template<typename T>
using OutputChannel = __xls_channel<T, __xls_channel_dir_Out>;
template<typename T, int Size>
using Memory = __xls_memory<T, Size>;

class TestBlock {
    InputChannel<int> in;
    OutputChannel<int> out;
    Memory<short, 32> store;

    int addr = 0;

    #pragma hls_top
    void Run() {
        const int next_addr = (addr + 1) & 0b11111;
        store[addr] =;
        addr = next_addr;

Translate to IR

$ ./bazel-bin/xls/contrib/xlscc/xlscc \
  --block_from_class TestBlock --block_pb block.pb \

Examine the metadata

The memory should appear in the resulting block block.pb:

channels {
  name: "in"
  is_input: true
  type: FIFO
  width_in_bits: 32
channels {
  name: "out"
  is_input: false
  type: FIFO
  width_in_bits: 32
channels {
  name: "store"
  type: MEMORY
  width_in_bits: 16
  depth: 32
name: "TestBlock"

This block description information can be used to generate the RAM rewrites and IO constraints specified in later steps.

Optimize the IR with RAM rewrites

Create rewrites.textproto with the following contents:

rewrites {
  from_config {
    kind: RAM_ABSTRACT
    depth: 32
  to_config {
    kind: RAM_1R1W
    depth: 32
  from_channels_logical_to_physical: {
    key: "abstract_read_req"
    value: "store__read_request"
  from_channels_logical_to_physical: {
    key: "abstract_read_resp"
    value: "store__read_response"
  from_channels_logical_to_physical: {
    key: "abstract_write_req"
    value: "store__write_request"
  from_channels_logical_to_physical: {
    key: "write_completion"
    value: "store__write_response"
  to_name_prefix: "store_"

Then run opt with awareness of the RAM:

$ ./bazel-bin/xls/tools/opt_main --ram_rewrites_pb rewrites.textproto >

Generate Verilog with IO constraints

For this memory, we are assuming a fixed 1-cycle latency for reads and writes. As such, we need to add these constraints to the codegen command, in addition to making it aware of the RAM rewrites, so that the correct ports are generated.

$ ./bazel-bin/xls/tools/codegen_main \
  --generator=pipeline \
  --delay_model="sky130" \
  --output_verilog_path=memory_test.v \
  --module_name=memory_test \
  --top=TestBlock_proc \
  --reset=rst \
  --reset_active_low=false \
  --reset_asynchronous=false \
  --reset_data_path=true \
  --pipeline_stages=2  \
  --flop_inputs=false \
  --flop_outputs=false \
  --io_constraints=store__read_req:send:store__read_resp:recv:1:1,store__write_req:send:store__write_completion:recv:1:1 \

Below is a quick summary of the options.

  1. --io_constraints=store__read_req:send:store__read_resp:recv:1:1... This option ensures that requests to the memory are written exactly 1 cycle before responses are read.
  2. --ram_configurations=ram:1R1W:store__read_req... This option informs codegen of the necessary information about the memory to generate the top level ports in the correct style. "ram" is the name prefix the ports will use.

Additional XLS[cc] examples.

For developers, it is possible to check if a specific feature is supported by checking for unit tests.