diff --git a/ceno_emul/src/syscalls.rs b/ceno_emul/src/syscalls.rs
index 3c37b117a..e5b3413cd 100644
--- a/ceno_emul/src/syscalls.rs
+++ b/ceno_emul/src/syscalls.rs
@@ -1,9 +1,18 @@
-use crate::{
-    Change, EmuContext, Platform, RegIdx, Tracer, VMState, WORD_SIZE, Word, WordAddr, WriteOp,
-};
+use crate::{RegIdx, Tracer, VMState, Word, WordAddr, WriteOp};
 use anyhow::Result;
-use itertools::{Itertools, chain, izip};
-use tiny_keccak::keccakf;
+use itertools::chain;
+
+mod keccak_permute;
+
+pub const KECCAK_PERMUTE: u32 = 0x00_01_01_09;
+
+/// Trace the inputs and effects of a syscall.
+pub fn handle_syscall(vm: &VMState, function_code: u32) -> Result<SyscallEffects> {
+    match function_code {
+        KECCAK_PERMUTE => Ok(keccak_permute::keccak_permute(vm)),
+        _ => Err(anyhow::anyhow!("Unknown syscall: {}", function_code)),
+    }
+}
 
 /// A syscall event, available to the circuit witness generators.
 #[derive(Clone, Debug, Default, PartialEq, Eq)]
@@ -47,75 +56,3 @@ impl SyscallEffects {
         self.witness
     }
 }
-
-pub const KECCAK_PERMUTE: u32 = 0x00_01_01_09;
-
-/// Trace the inputs and effects of a syscall.
-pub fn handle_syscall(vm: &VMState, function_code: u32) -> Result<SyscallEffects> {
-    match function_code {
-        KECCAK_PERMUTE => Ok(keccak_permute(vm)),
-        _ => Err(anyhow::anyhow!("Unknown syscall: {}", function_code)),
-    }
-}
-
-const KECCAK_CELLS: usize = 25; // u64 cells
-const KECCAK_WORDS: usize = KECCAK_CELLS * 2; // u32 words
-
-/// Trace the execution of a Keccak permutation.
-///
-/// Compatible with:
-/// https://github.com/succinctlabs/sp1/blob/013c24ea2fa15a0e7ed94f7d11a7ada4baa39ab9/crates/core/executor/src/syscalls/precompiles/keccak256/permute.rs
-///
-/// TODO: test compatibility.
-fn keccak_permute(vm: &VMState) -> SyscallEffects {
-    let state_ptr = vm.peek_register(Platform::reg_arg0());
-
-    // Read the argument `state_ptr`.
-    let reg_accesses = vec![WriteOp::new_register_op(
-        Platform::reg_arg0(),
-        Change::new(state_ptr, state_ptr),
-        0, // Set later by Tracer.
-    )];
-
-    let addrs = (state_ptr..)
-        .step_by(WORD_SIZE)
-        .take(KECCAK_WORDS)
-        .map(WordAddr::from)
-        .collect_vec();
-
-    // Read Keccak state.
-    let input = addrs
-        .iter()
-        .map(|&addr| vm.peek_memory(addr))
-        .collect::<Vec<_>>();
-
-    // Compute Keccak permutation.
-    let output = {
-        let mut state = [0_u64; KECCAK_CELLS];
-        izip!(state.iter_mut(), input.chunks_exact(2)).for_each(|(cell, chunk)| {
-            let lo = chunk[0] as u64;
-            let hi = chunk[1] as u64;
-            *cell = lo | hi << 32;
-        });
-        keccakf(&mut state);
-        state.into_iter().flat_map(|c| [c as u32, (c >> 32) as u32])
-    };
-
-    // Write permuted state.
-    let mem_writes = izip!(addrs, input, output)
-        .map(|(addr, before, after)| WriteOp {
-            addr,
-            value: Change { before, after },
-            previous_cycle: 0, // Set later by Tracer.
-        })
-        .collect_vec();
-
-    assert_eq!(mem_writes.len(), KECCAK_WORDS);
-    SyscallEffects {
-        witness: SyscallWitness {
-            mem_writes,
-            reg_accesses,
-        },
-        next_pc: None,
-    }
-}
diff --git a/ceno_emul/src/syscalls/keccak_permute.rs b/ceno_emul/src/syscalls/keccak_permute.rs
new file mode 100644
index 000000000..05c04c872
--- /dev/null
+++ b/ceno_emul/src/syscalls/keccak_permute.rs
@@ -0,0 +1,68 @@
+use itertools::{Itertools, izip};
+use tiny_keccak::keccakf;
+
+use crate::{Change, EmuContext, Platform, VMState, WORD_SIZE, WordAddr, WriteOp};
+
+use super::{SyscallEffects, SyscallWitness};
+
+const KECCAK_CELLS: usize = 25; // u64 cells
+const KECCAK_WORDS: usize = KECCAK_CELLS * 2; // u32 words
+
+/// Trace the execution of a Keccak permutation.
+///
+/// Compatible with:
+/// https://github.com/succinctlabs/sp1/blob/013c24ea2fa15a0e7ed94f7d11a7ada4baa39ab9/crates/core/executor/src/syscalls/precompiles/keccak256/permute.rs
+///
+/// TODO: test compatibility.
+pub fn keccak_permute(vm: &VMState) -> SyscallEffects {
+    let state_ptr = vm.peek_register(Platform::reg_arg0());
+
+    // Read the argument `state_ptr`.
+    let reg_accesses = vec![WriteOp::new_register_op(
+        Platform::reg_arg0(),
+        Change::new(state_ptr, state_ptr),
+        0, // Cycle set later in finalize().
+    )];
+
+    let addrs = (state_ptr..)
+        .step_by(WORD_SIZE)
+        .take(KECCAK_WORDS)
+        .map(WordAddr::from)
+        .collect_vec();
+
+    // Read Keccak state.
+    let input = addrs
+        .iter()
+        .map(|&addr| vm.peek_memory(addr))
+        .collect::<Vec<_>>();
+
+    // Compute Keccak permutation.
+    let output = {
+        let mut state = [0_u64; KECCAK_CELLS];
+        izip!(state.iter_mut(), input.chunks_exact(2)).for_each(|(cell, chunk)| {
+            let lo = chunk[0] as u64;
+            let hi = chunk[1] as u64;
+            *cell = lo | hi << 32;
+        });
+        keccakf(&mut state);
+        state.into_iter().flat_map(|c| [c as u32, (c >> 32) as u32])
+    };
+
+    // Write permuted state.
+    let mem_writes = izip!(addrs, input, output)
+        .map(|(addr, before, after)| WriteOp {
+            addr,
+            value: Change { before, after },
+            previous_cycle: 0, // Cycle set later in finalize().
+        })
+        .collect_vec();
+
+    assert_eq!(mem_writes.len(), KECCAK_WORDS);
+    SyscallEffects {
+        witness: SyscallWitness {
+            mem_writes,
+            reg_accesses,
+        },
+        next_pc: None,
+    }
+}
diff --git a/ceno_emul/src/tracer.rs b/ceno_emul/src/tracer.rs
index fd9a12cf0..a7e5b6fd6 100644
--- a/ceno_emul/src/tracer.rs
+++ b/ceno_emul/src/tracer.rs
@@ -48,9 +48,8 @@ pub struct MemOp<T> {
 
 impl<T> MemOp<T> {
     pub fn new_register_op(idx: RegIdx, value: T, previous_cycle: Cycle) -> MemOp<T> {
-        let addr = Platform::register_vma(idx).into();
         MemOp {
-            addr,
+            addr: Platform::register_vma(idx).into(),
             value,
             previous_cycle,
         }