WIP adding fetch::l1_i_cache

Cargo.lock

@@ -388,7 +388,7 @@ checksum = "e8c02a5121d4ea3eb16a80748c74f5549a5665e4c21333c6098f283870fbdea6"
 [[package]]
 name = "fayalite"
 version = "0.3.0"
-source = "git+https://git.libre-chip.org/libre-chip/fayalite.git?branch=master#1bc835803bf87643c6464eaca0932e30e5febb8c"
+source = "git+https://git.libre-chip.org/libre-chip/fayalite.git?branch=master#c632e5d570d4763e8e18d764e95b7a9e515ebf99"
 dependencies = [
  "base64",
  "bitvec",
@@ -416,7 +416,7 @@ dependencies = [
 [[package]]
 name = "fayalite-proc-macros"
 version = "0.3.0"
-source = "git+https://git.libre-chip.org/libre-chip/fayalite.git?branch=master#1bc835803bf87643c6464eaca0932e30e5febb8c"
+source = "git+https://git.libre-chip.org/libre-chip/fayalite.git?branch=master#c632e5d570d4763e8e18d764e95b7a9e515ebf99"
 dependencies = [
  "fayalite-proc-macros-impl",
 ]
@@ -424,7 +424,7 @@ dependencies = [
 [[package]]
 name = "fayalite-proc-macros-impl"
 version = "0.3.0"
-source = "git+https://git.libre-chip.org/libre-chip/fayalite.git?branch=master#1bc835803bf87643c6464eaca0932e30e5febb8c"
+source = "git+https://git.libre-chip.org/libre-chip/fayalite.git?branch=master#c632e5d570d4763e8e18d764e95b7a9e515ebf99"
 dependencies = [
  "base16ct 0.2.0",
  "num-bigint",
@@ -439,7 +439,7 @@ dependencies = [
 [[package]]
 name = "fayalite-visit-gen"
 version = "0.3.0"
-source = "git+https://git.libre-chip.org/libre-chip/fayalite.git?branch=master#1bc835803bf87643c6464eaca0932e30e5febb8c"
+source = "git+https://git.libre-chip.org/libre-chip/fayalite.git?branch=master#c632e5d570d4763e8e18d764e95b7a9e515ebf99"
 dependencies = [
  "indexmap",
  "prettyplease",

crates/cpu/src/config.rs

@@ -37,6 +37,8 @@ pub struct CpuConfig {
     pub max_branches_per_fetch: NonZeroUsize,
     pub max_fetches_in_flight: NonZeroUsize,
     pub log2_fetch_width_in_bytes: u8,
+    pub log2_cache_line_size_in_bytes: u8,
+    pub l1_i_cache_line_count: NonZeroUsize,
     /// default value for [`UnitConfig::max_in_flight`]
     pub default_unit_max_in_flight: NonZeroUsize,
     pub rob_size: NonZeroUsize,
@@ -63,6 +65,13 @@ impl CpuConfig {
         v
     };
     pub const DEFAULT_LOG2_FETCH_WIDTH_IN_BYTES: u8 = 3;
+    pub const DEFAULT_LOG2_CACHE_LINE_SIZE_IN_BYTES: u8 = 6;
+    pub const DEFAULT_L1_I_CACHE_LINE_COUNT: NonZeroUsize = {
+        let Some(v) = NonZeroUsize::new(256) else {
+            unreachable!();
+        };
+        v
+    };
     pub const DEFAULT_UNIT_MAX_IN_FLIGHT: NonZeroUsize = {
         let Some(v) = NonZeroUsize::new(8) else {
             unreachable!();
@@ -77,6 +86,8 @@ impl CpuConfig {
             max_branches_per_fetch: Self::DEFAULT_MAX_BRANCHES_PER_FETCH,
             max_fetches_in_flight: Self::DEFAULT_MAX_FETCHES_IN_FLIGHT,
             log2_fetch_width_in_bytes: Self::DEFAULT_LOG2_FETCH_WIDTH_IN_BYTES,
+            log2_cache_line_size_in_bytes: Self::DEFAULT_LOG2_CACHE_LINE_SIZE_IN_BYTES,
+            l1_i_cache_line_count: Self::DEFAULT_L1_I_CACHE_LINE_COUNT,
             default_unit_max_in_flight: Self::DEFAULT_UNIT_MAX_IN_FLIGHT,
             rob_size,
         }
@@ -141,6 +152,17 @@ impl CpuConfig {
             .checked_shl(self.log2_fetch_width_in_bytes.into())
             .expect("log2_fetch_width_in_bytes is too big")
     }
+    pub fn cache_line_size_in_bytes(&self) -> usize {
+        1usize
+            .checked_shl(self.log2_cache_line_size_in_bytes.into())
+            .expect("log2_cache_line_size_in_bytes is too big")
+    }
+    pub fn l1_i_cache_size_in_bytes(&self) -> usize {
+        self.l1_i_cache_line_count
+            .get()
+            .checked_mul(self.cache_line_size_in_bytes())
+            .expect("L1 I-Cache is too big")
+    }
 }
 
 #[hdl(get(|c| c.fetch_width.get()))]
@@ -161,6 +183,18 @@ pub type CpuConfigLog2FetchWidthInBytes<C: PhantomConstGet<CpuConfig>> = DynSize
 #[hdl(get(|c| c.fetch_width_in_bytes()))]
 pub type CpuConfigFetchWidthInBytes<C: PhantomConstGet<CpuConfig>> = DynSize;
 
+#[hdl(get(|c| c.log2_cache_line_size_in_bytes.into()))]
+pub type CpuConfigLog2CacheLineSizeInBytes<C: PhantomConstGet<CpuConfig>> = DynSize;
+
+#[hdl(get(|c| c.cache_line_size_in_bytes()))]
+pub type CpuConfigCacheLineSizeInBytes<C: PhantomConstGet<CpuConfig>> = DynSize;
+
+#[hdl(get(|c| c.l1_i_cache_line_count.get()))]
+pub type CpuConfigL1ICacheLineCount<C: PhantomConstGet<CpuConfig>> = DynSize;
+
+#[hdl(get(|c| c.l1_i_cache_size_in_bytes()))]
+pub type CpuConfigL1ICacheSizeInBytes<C: PhantomConstGet<CpuConfig>> = DynSize;
+
 #[hdl(get(|c| c.rob_size.get()))]
 pub type CpuConfigRobSize<C: PhantomConstGet<CpuConfig>> = DynSize;
 

crates/cpu/src/fetch.rs

@@ -0,0 +1,266 @@
+// SPDX-License-Identifier: LGPL-3.0-or-later
+// See Notices.txt for copyright information
+
+use crate::{
+    config::{
+        CpuConfig, CpuConfigCacheLineSizeInBytes, CpuConfigFetchWidthInBytes,
+        CpuConfigL1ICacheLineCount, PhantomConstCpuConfig,
+    },
+    next_pc::{NextPcToFetchInterface, ResetStatus, ResetSteps, SimValueDefault},
+};
+use fayalite::{
+    memory::{ReadWriteStruct, memory_addr_width, splat_mask},
+    prelude::*,
+    util::ready_valid::ReadyValid,
+};
+
+#[hdl]
+pub enum MemoryOperationKind {
+    Read,
+    Write,
+}
+
+#[hdl(no_static)]
+pub struct MemoryOperationStart<C: PhantomConstGet<CpuConfig> + PhantomConstCpuConfig> {
+    pub kind: MemoryOperationKind,
+    pub addr: UInt<64>,
+    pub write_data: ArrayType<UInt<8>, CpuConfigFetchWidthInBytes<C>>,
+    pub config: C,
+}
+
+#[hdl]
+pub enum MemoryOperationErrorKind {
+    Generic,
+}
+
+#[hdl]
+pub enum MemoryOperationFinishKind {
+    Success(MemoryOperationKind),
+    Error(MemoryOperationErrorKind),
+}
+
+#[hdl(no_static)]
+pub struct MemoryOperationFinish<C: PhantomConstGet<CpuConfig> + PhantomConstCpuConfig> {
+    pub kind: MemoryOperationFinishKind,
+    pub read_data: ArrayType<UInt<8>, CpuConfigFetchWidthInBytes<C>>,
+    pub config: C,
+}
+
+#[hdl(no_static)]
+pub struct MemoryInterface<C: PhantomConstGet<CpuConfig> + PhantomConstCpuConfig> {
+    pub start: ReadyValid<MemoryOperationStart<C>>,
+    #[hdl(flip)]
+    pub finish: ReadyValid<MemoryOperationFinish<C>>,
+    pub config: C,
+}
+
+#[hdl(no_static)]
+struct CacheLine<C: PhantomConstGet<CpuConfig> + PhantomConstCpuConfig> {
+    data: ArrayType<UInt<8>, CpuConfigCacheLineSizeInBytes<C>>,
+    addr: HdlOption<UInt<64>>,
+    config: C,
+}
+
+#[hdl(no_static)]
+struct L1ICacheState<C: PhantomConstGet<CpuConfig> + PhantomConstCpuConfig> {
+    config: C,
+}
+
+impl<C: PhantomConstCpuConfig> SimValueDefault for L1ICacheState<C> {
+    #[hdl]
+    fn sim_value_default(self) -> SimValue<Self> {
+        let Self { config } = self;
+        #[hdl(sim)]
+        Self { config }
+    }
+}
+
+impl<C: PhantomConstCpuConfig> ResetSteps for L1ICacheState<C> {
+    #[hdl]
+    fn reset_step(this: &mut SimValue<Self>, step: usize) -> ResetStatus {
+        #[hdl(sim)]
+        let Self { config: _ } = this;
+        ResetStatus::Done
+    }
+}
+
+#[hdl_module(extern)]
+fn l1_i_cache_impl(config: PhantomConst<CpuConfig>) {
+    #[hdl]
+    let cd: ClockDomain = m.input();
+    #[hdl]
+    let memory_interface: MemoryInterface<PhantomConst<CpuConfig>> =
+        m.output(MemoryInterface[config]);
+    #[hdl]
+    let from_next_pc: NextPcToFetchInterface<PhantomConst<CpuConfig>> =
+        m.input(NextPcToFetchInterface[config]);
+    // i_cache_port.clk is externally overridden with cd.clk
+    #[hdl]
+    let i_cache_port: ReadWriteStruct<CacheLine<PhantomConst<CpuConfig>>, DynSize> = m.output(
+        ReadWriteStruct[CacheLine[config]][memory_addr_width(CpuConfigL1ICacheLineCount[config])],
+    );
+    #[hdl]
+    let state_for_debug: L1ICacheState<PhantomConst<CpuConfig>> = m.output(L1ICacheState[config]);
+    m.register_clock_for_past(cd.clk);
+    #[hdl]
+    async fn run(
+        mut sim: ExternModuleSimulationState,
+        cd: Expr<ClockDomain>,
+        memory_interface: Expr<MemoryInterface<PhantomConst<CpuConfig>>>,
+        from_next_pc: Expr<NextPcToFetchInterface<PhantomConst<CpuConfig>>>,
+        i_cache_port: Expr<ReadWriteStruct<CacheLine<PhantomConst<CpuConfig>>, DynSize>>,
+        state_expr: Expr<L1ICacheState<PhantomConst<CpuConfig>>>,
+    ) {
+        let mut state = sim.read(state_expr).await;
+        let config = state.config.ty();
+        let l1_i_cache_line_count = CpuConfigL1ICacheLineCount[config];
+        let cache_line_ty = CacheLine[config];
+        for step in 0usize.. {
+            sim.write(state_expr, state).await;
+            sim.wait_for_clock_edge(cd.clk).await;
+            state = sim.read_past(state_expr, cd.clk).await;
+            sim.write(i_cache_port.en, false).await;
+            let mut reset_status = ResetSteps::reset_step(&mut state, step);
+            if step < l1_i_cache_line_count {
+                reset_status = ResetStatus::Working;
+                #[hdl]
+                let ReadWriteStruct::<_, _> {
+                    addr,
+                    en,
+                    clk: _, // externally overridden with cd.clk
+                    rdata: _,
+                    wmode,
+                    wdata,
+                    wmask,
+                } = i_cache_port;
+                sim.write(addr, step.cast_to(addr.ty())).await;
+                sim.write(en, true).await;
+                sim.write(wmode, true).await;
+                sim.write(
+                    wdata,
+                    #[hdl(sim)]
+                    CacheLine::<_> {
+                        data: repeat(0u8, cache_line_ty.data.len()),
+                        addr: #[hdl(sim)]
+                        HdlNone(),
+                        config,
+                    },
+                )
+                .await;
+                sim.write(wmask, splat_mask(cache_line_ty, true.to_expr()))
+                    .await;
+            }
+            match reset_status {
+                ResetStatus::Done => break,
+                ResetStatus::Working => {}
+            }
+        }
+        todo!();
+    }
+    m.extern_module_simulation_fn(
+        (
+            cd,
+            memory_interface,
+            from_next_pc,
+            i_cache_port,
+            state_for_debug,
+        ),
+        |(cd, memory_interface, from_next_pc, i_cache_port, state_for_debug), mut sim| async move {
+            let config = memory_interface.ty().config;
+            let cache_line_size_in_bytes = CpuConfigCacheLineSizeInBytes[config];
+            let cache_line_ty = CacheLine[config];
+            sim.write(i_cache_port.clk, false).await; // externally overridden with cd.clk, so just write a constant here
+            sim.resettable(
+                cd,
+                |mut sim: ExternModuleSimulationState| async move {
+                    sim.write(memory_interface.start.ready, false).await;
+                    sim.write(memory_interface.finish.ready, false).await;
+                    sim.write(
+                        from_next_pc.next_fetch_block_ids,
+                        from_next_pc.ty().next_fetch_block_ids.HdlNone(),
+                    )
+                    .await;
+                    sim.write(from_next_pc.fetch.ready, false).await;
+                    sim.write(from_next_pc.cancel.ready, false).await;
+                    sim.write(i_cache_port.addr, 0u8.cast_to(i_cache_port.addr.ty()))
+                        .await;
+                    sim.write(i_cache_port.en, false).await;
+                    sim.write(i_cache_port.wmode, false).await;
+                    sim.write(
+                        i_cache_port.wdata,
+                        #[hdl(sim)]
+                        CacheLine::<_> {
+                            data: repeat(0u8, cache_line_size_in_bytes),
+                            addr: HdlNone(),
+                            config,
+                        },
+                    )
+                    .await;
+                    sim.write(
+                        i_cache_port.wmask,
+                        splat_mask(cache_line_ty, false.to_expr()),
+                    )
+                    .await;
+                    sim.write(
+                        state_for_debug,
+                        #[hdl(sim)]
+                        L1ICacheState::<_> { config },
+                    )
+                    .await;
+                },
+                |sim, ()| {
+                    run(
+                        sim,
+                        cd,
+                        memory_interface,
+                        from_next_pc,
+                        i_cache_port,
+                        state_for_debug,
+                    )
+                },
+            )
+            .await;
+        },
+    );
+}
+
+#[hdl_module]
+pub fn l1_i_cache(config: PhantomConst<CpuConfig>) {
+    #[hdl]
+    let cd: ClockDomain = m.input();
+    #[hdl]
+    let memory_interface: MemoryInterface<PhantomConst<CpuConfig>> =
+        m.output(MemoryInterface[config]);
+    #[hdl]
+    let from_next_pc: NextPcToFetchInterface<PhantomConst<CpuConfig>> =
+        m.input(NextPcToFetchInterface[config]);
+    let cache_line_ty = CacheLine[config];
+    let cache_line_count = CpuConfigL1ICacheLineCount[config];
+    #[hdl]
+    let mut i_cache = memory_array(ArrayType[cache_line_ty][cache_line_count]);
+    let i_cache_port = i_cache.new_rw_port();
+    #[hdl]
+    let l1_i_cache_impl = instance(l1_i_cache_impl(config));
+    connect(l1_i_cache_impl.cd, cd);
+    connect(memory_interface, l1_i_cache_impl.memory_interface);
+    connect(l1_i_cache_impl.from_next_pc, from_next_pc);
+    connect(i_cache_port, l1_i_cache_impl.i_cache_port);
+    connect(i_cache_port.clk, cd.clk);
+}
+
+#[hdl_module]
+pub fn fetch(config: PhantomConst<CpuConfig>) {
+    #[hdl]
+    let cd: ClockDomain = m.input();
+    #[hdl]
+    let memory_interface: MemoryInterface<PhantomConst<CpuConfig>> =
+        m.output(MemoryInterface[config]);
+    #[hdl]
+    let from_next_pc: NextPcToFetchInterface<PhantomConst<CpuConfig>> =
+        m.input(NextPcToFetchInterface[config]);
+    #[hdl]
+    let l1_i_cache = instance(l1_i_cache(config));
+    connect(l1_i_cache.cd, cd);
+    connect(memory_interface, l1_i_cache.memory_interface);
+    connect(l1_i_cache.from_next_pc, from_next_pc);
+}

crates/cpu/src/lib.rs

@@ -2,6 +2,7 @@
 // See Notices.txt for copyright information
 pub mod config;
 pub mod decoder;
+pub mod fetch;
 pub mod instruction;
 pub mod next_pc;
 pub mod powerisa_instructions_xml;

crates/cpu/src/next_pc.rs

@@ -2719,13 +2719,13 @@ impl SimValueDefault for BranchPredictionState {
 
 #[derive(Copy, Clone, Debug)]
 #[must_use]
-enum ResetStatus {
+pub(crate) enum ResetStatus {
     Done,
     Working,
 }
 
 impl ResetStatus {
-    fn and(self, other: Self) -> Self {
+    pub(crate) fn and(self, other: Self) -> Self {
         match (self, other) {
             (ResetStatus::Done, ResetStatus::Done) => ResetStatus::Done,
             (ResetStatus::Done | ResetStatus::Working, ResetStatus::Working)
@@ -2734,7 +2734,7 @@ impl ResetStatus {
     }
 }
 
-trait SimValueDefault: Type {
+pub(crate) trait SimValueDefault: Type {
     fn sim_value_default(self) -> SimValue<Self>;
 }
 
@@ -2828,7 +2828,7 @@ impl SimValueDefault for WipDecodedInsn {
     }
 }
 
-trait ResetSteps: Type {
+pub(crate) trait ResetSteps: Type {
     fn reset_step(this: &mut SimValue<Self>, step: usize) -> ResetStatus;
 }
 

crates/cpu/tests/fetch.rs

@@ -0,0 +1,185 @@
+// SPDX-License-Identifier: LGPL-3.0-or-later
+// See Notices.txt for copyright information
+
+use cpu::{
+    config::{CpuConfig, UnitConfig},
+    fetch::{MemoryInterface, fetch},
+    next_pc::NextPcToFetchInterface,
+    unit::UnitKind,
+    util::array_vec::ArrayVec,
+};
+use fayalite::{
+    prelude::*,
+    sim::vcd::VcdWriterDecls,
+    util::{DebugAsDisplay, RcWriter},
+};
+use std::{cell::Cell, collections::VecDeque, num::NonZeroUsize};
+
+const MEMORY_QUEUE_SIZE: usize = 32;
+
+#[hdl]
+struct MemoryQueueEntry {
+    addr: UInt<64>,
+    cycles_left: UInt<8>,
+}
+
+impl MemoryQueueEntry {
+    #[hdl]
+    fn default_sim(self) -> SimValue<Self> {
+        #[hdl(sim)]
+        Self {
+            addr: 0u64,
+            cycles_left: 0u8,
+        }
+    }
+}
+
+#[hdl_module(extern)]
+fn mock_memory(config: PhantomConst<CpuConfig>) {
+    #[hdl]
+    let cd: ClockDomain = m.input();
+    #[hdl]
+    let memory_interface: MemoryInterface<PhantomConst<CpuConfig>> =
+        m.input(MemoryInterface[config]);
+    #[hdl]
+    let queue_debug: ArrayVec<MemoryQueueEntry, ConstUsize<{ MEMORY_QUEUE_SIZE }>> = m.output();
+    m.register_clock_for_past(cd.clk);
+    m.extern_module_simulation_fn(
+        (cd, memory_interface, queue_debug),
+        |(cd, memory_interface, queue_debug), mut sim| async move {
+            // intentionally have a different sequence each time we're reset
+            let delay_sequence_index = Cell::new(0);
+            sim.resettable(
+                cd,
+                async |mut sim| {
+                    sim.write(memory_interface.start.ready, false).await;
+                    sim.write(memory_interface.finish.ready, false).await;
+                    sim.write(
+                        queue_debug,
+                        queue_debug.ty().new_sim(MemoryQueueEntry.default_sim()),
+                    )
+                    .await;
+                },
+                |sim, ()| {
+                    run_fn(
+                        cd,
+                        memory_interface,
+                        queue_debug,
+                        &delay_sequence_index,
+                        sim,
+                    )
+                },
+            )
+            .await;
+        },
+    );
+    #[hdl]
+    async fn run_fn(
+        cd: Expr<ClockDomain>,
+        memory_interface: Expr<MemoryInterface<PhantomConst<CpuConfig>>>,
+        queue_debug: Expr<ArrayVec<MemoryQueueEntry, ConstUsize<{ MEMORY_QUEUE_SIZE }>>>,
+        delay_sequence_index: &Cell<u64>,
+        mut sim: ExternModuleSimulationState,
+    ) {
+        let config = memory_interface.config.ty();
+        let mut queue: VecDeque<SimValue<MemoryQueueEntry>> = VecDeque::new();
+        loop {
+            let mut sim_queue = queue_debug.ty().new_sim(MemoryQueueEntry.default_sim());
+            for entry in &queue {
+                ArrayVec::try_push_sim(&mut sim_queue, entry)
+                    .ok()
+                    .expect("queue is known to be small enough");
+            }
+            sim.write(queue_debug, sim_queue).await;
+            todo!();
+            sim.wait_for_clock_edge(cd.clk).await;
+            println!(
+                "Dump mock memory queue: {:#?}",
+                Vec::from_iter(
+                    queue
+                        .iter()
+                        .map(|v| { DebugAsDisplay(format!("addr={:#x}", v.addr.as_int())) })
+                )
+            );
+        }
+    }
+}
+
+#[hdl_module]
+fn dut(config: PhantomConst<CpuConfig>) {
+    #[hdl]
+    let cd: ClockDomain = m.input();
+    #[hdl]
+    let from_next_pc: NextPcToFetchInterface<PhantomConst<CpuConfig>> =
+        m.input(NextPcToFetchInterface[config]);
+    #[hdl]
+    let fetch = instance(fetch(config));
+    #[hdl]
+    let fetch {
+        cd: fetch_cd,
+        memory_interface: fetch_memory_interface,
+        from_next_pc: fetch_from_next_pc,
+    } = fetch;
+    connect(fetch_cd, cd);
+    connect(fetch_from_next_pc, from_next_pc);
+    #[hdl]
+    let mock_memory = instance(mock_memory(config));
+    #[hdl]
+    let mock_memory {
+        cd: mock_memory_cd,
+        memory_interface: mock_memory_interface,
+        queue_debug: _,
+    } = mock_memory;
+    connect(mock_memory_cd, cd);
+    connect(mock_memory_interface, fetch_memory_interface);
+}
+
+#[hdl]
+#[test]
+fn test_fetch() {
+    let _n = SourceLocation::normalize_files_for_tests();
+    let mut config = CpuConfig::new(
+        vec![
+            UnitConfig::new(UnitKind::AluBranch),
+            UnitConfig::new(UnitKind::AluBranch),
+        ],
+        NonZeroUsize::new(20).unwrap(),
+    );
+    config.fetch_width = NonZeroUsize::new(2).unwrap();
+    config.log2_fetch_width_in_bytes = 4;
+    let m = dut(PhantomConst::new_sized(config));
+    let mut sim = Simulation::new(m);
+    let writer = RcWriter::default();
+    sim.add_trace_writer(VcdWriterDecls::new(writer.clone()));
+    struct DumpVcdOnDrop {
+        writer: Option<RcWriter>,
+    }
+    impl Drop for DumpVcdOnDrop {
+        fn drop(&mut self) {
+            if let Some(mut writer) = self.writer.take() {
+                let vcd = String::from_utf8(writer.take()).unwrap();
+                println!("####### VCD:\n{vcd}\n#######");
+            }
+        }
+    }
+    let mut writer = DumpVcdOnDrop {
+        writer: Some(writer),
+    };
+    sim.write_clock(sim.io().cd.clk, false);
+    sim.write_reset(sim.io().cd.rst, true);
+    for cycle in 0..2000 {
+        todo!("drive m.from_next_pc");
+        sim.advance_time(SimDuration::from_nanos(500));
+        println!("clock tick: {cycle}");
+        sim.write_clock(sim.io().cd.clk, true);
+        sim.advance_time(SimDuration::from_nanos(500));
+        sim.write_clock(sim.io().cd.clk, false);
+        sim.write_reset(sim.io().cd.rst, false);
+    }
+    // FIXME: vcd is just whatever fetch does now, which isn't known to be correct
+    let vcd = String::from_utf8(writer.writer.take().unwrap().take()).unwrap();
+    println!("####### VCD:\n{vcd}\n#######");
+    if vcd != include_str!("expected/fetch.vcd") {
+        panic!();
+    }
+}