diff --git a/evm/src/cpu/kernel/asm/memory/core.asm b/evm/src/cpu/kernel/asm/memory/core.asm
index 2b4d2b68..f6bb99b6 100644
--- a/evm/src/cpu/kernel/asm/memory/core.asm
+++ b/evm/src/cpu/kernel/asm/memory/core.asm
@@ -393,3 +393,21 @@
     %mstore_kernel_general_2
     // stack: (empty)
 %endmacro
+
+%macro mload_main
+    // stack: offset
+    DUP1
+    // stack: offset, offset
+    %update_msize
+    // stack: offset
+    %mload_current(@SEGMENT_MAIN_MEMORY)
+%endmacro
+
+%macro mstore_main
+    // stack: offset, value
+    DUP1
+    // stack: offset, offset, value
+    %update_msize
+    // stack: offset, value
+    %mstore_current(@SEGMENT_MAIN_MEMORY)
+%endmacro
diff --git a/evm/src/cpu/kernel/asm/memory/metadata.asm b/evm/src/cpu/kernel/asm/memory/metadata.asm
index 644699e0..1a495682 100644
--- a/evm/src/cpu/kernel/asm/memory/metadata.asm
+++ b/evm/src/cpu/kernel/asm/memory/metadata.asm
@@ -45,3 +45,23 @@
 %macro callvalue
     %mload_context_metadata(@CTX_METADATA_CALL_VALUE)
 %endmacro
+
+%macro msize
+    %mload_context_metadata(@CTX_METADATA_MSIZE)
+%endmacro
+
+%macro update_msize
+    // stack: offset
+    %add_const(32)
+    // stack: 32 + offset
+    %div_const(32)
+    // stack: (offset+32)/32 = ceil_div_usize(offset+1, 32)
+    %mul_const(32)
+    // stack: ceil_div_usize(offset+1, 32) * 32
+    %msize
+    // stack: current_msize, ceil_div_usize(offset+1, 32) * 32
+    %max
+    // stack: new_msize
+    %mstore_context_metadata(@CTX_METADATA_MSIZE)
+%endmacro
+
diff --git a/evm/src/cpu/kernel/constants/context_metadata.rs b/evm/src/cpu/kernel/constants/context_metadata.rs
index a2c460fc..fab74373 100644
--- a/evm/src/cpu/kernel/constants/context_metadata.rs
+++ b/evm/src/cpu/kernel/constants/context_metadata.rs
@@ -23,10 +23,12 @@ pub(crate) enum ContextMetadata {
     /// Pointer to the initial version of the state trie, at the creation of this context. Used when
     /// we need to revert a context.
     StateTrieCheckpointPointer = 9,
+    /// Size of the active main memory.
+    MSize = 10,
 }
 
 impl ContextMetadata {
-    pub(crate) const COUNT: usize = 10;
+    pub(crate) const COUNT: usize = 11;
 
     pub(crate) fn all() -> [Self; Self::COUNT] {
         [
@@ -40,6 +42,7 @@ impl ContextMetadata {
             Self::CallValue,
             Self::Static,
             Self::StateTrieCheckpointPointer,
+            Self::MSize,
         ]
     }
 
@@ -56,6 +59,7 @@ impl ContextMetadata {
             ContextMetadata::CallValue => "CTX_METADATA_CALL_VALUE",
             ContextMetadata::Static => "CTX_METADATA_STATIC",
             ContextMetadata::StateTrieCheckpointPointer => "CTX_METADATA_STATE_TRIE_CHECKPOINT_PTR",
+            ContextMetadata::MSize => "CTX_METADATA_MSIZE",
         }
     }
 }
diff --git a/evm/src/cpu/kernel/interpreter.rs b/evm/src/cpu/kernel/interpreter.rs
index a2c25bf6..f6cae73e 100644
--- a/evm/src/cpu/kernel/interpreter.rs
+++ b/evm/src/cpu/kernel/interpreter.rs
@@ -9,6 +9,7 @@ use plonky2::field::goldilocks_field::GoldilocksField;
 
 use crate::cpu::kernel::aggregator::KERNEL;
 use crate::cpu::kernel::assembler::Kernel;
+use crate::cpu::kernel::constants::context_metadata::ContextMetadata;
 use crate::cpu::kernel::constants::global_metadata::GlobalMetadata;
 use crate::cpu::kernel::constants::txn_fields::NormalizedTxnField;
 use crate::generation::memory::{MemoryContextState, MemorySegmentState};
@@ -45,9 +46,7 @@ impl InterpreterMemory {
 
         mem
     }
-}
 
-impl InterpreterMemory {
     fn mload_general(&self, context: usize, segment: Segment, offset: usize) -> U256 {
         let value = self.context_memory[context].segments[segment as usize].get(offset);
         assert!(
@@ -499,6 +498,8 @@ impl<'a> Interpreter<'a> {
     fn run_keccak_general(&mut self) {
         let context = self.pop().as_usize();
         let segment = Segment::all()[self.pop().as_usize()];
+        // Not strictly needed but here to avoid surprises with MSIZE.
+        assert_ne!(segment, Segment::MainMemory, "Call KECCAK256 instead.");
         let offset = self.pop().as_usize();
         let size = self.pop().as_usize();
         let bytes = (offset..offset + size)
@@ -578,11 +579,10 @@ impl<'a> Interpreter<'a> {
     }
 
     fn run_msize(&mut self) {
-        let num_bytes = self.memory.context_memory[self.context].segments
-            [Segment::MainMemory as usize]
-            .content
-            .len();
-        self.push(U256::from(num_bytes));
+        self.push(
+            self.memory.context_memory[self.context].segments[Segment::ContextMetadata as usize]
+                .get(ContextMetadata::MSize as usize),
+        )
     }
 
     fn run_jumpdest(&mut self) {