diff --git a/evm/src/cpu/kernel/asm/core/call.asm b/evm/src/cpu/kernel/asm/core/call.asm
index 28f0f6e2..2c8eec75 100644
--- a/evm/src/cpu/kernel/asm/core/call.asm
+++ b/evm/src/cpu/kernel/asm/core/call.asm
@@ -169,6 +169,14 @@ global after_call_instruction:
     // stack: new_ctx
 %endmacro
 
+%macro set_new_ctx_calldata_size
+    // stack: calldata_size, new_ctx
+    %stack (calldata_size, new_ctx)
+        -> (new_ctx, @SEGMENT_CONTEXT_METADATA, @CTX_METADATA_CALLDATA_SIZE, calldata_size, new_ctx)
+    MSTORE_GENERAL
+    // stack: new_ctx
+%endmacro
+
 %macro set_new_ctx_gas_limit
     // stack: gas_limit, new_ctx
     %stack (gas_limit, new_ctx)
diff --git a/evm/src/cpu/kernel/asm/core/process_txn.asm b/evm/src/cpu/kernel/asm/core/process_txn.asm
index b97a2d93..a626cd39 100644
--- a/evm/src/cpu/kernel/asm/core/process_txn.asm
+++ b/evm/src/cpu/kernel/asm/core/process_txn.asm
@@ -220,8 +220,14 @@ global process_message_txn_code_loaded:
     %non_intrinisic_gas %set_new_ctx_gas_limit
     // stack: new_ctx, retdest
 
-    // TODO: Copy TXN_DATA to CALLDATA
+    // Set calldatasize and copy txn data to calldata.
+    %mload_txn_field(@TXN_FIELD_DATA_LEN)
+    %stack (calldata_size, new_ctx, retdest) -> (calldata_size, new_ctx, calldata_size, retdest)
+    %set_new_ctx_calldata_size
+    %stack (new_ctx, calldata_size, retdest) -> (new_ctx, @SEGMENT_CALLDATA, 0, 0, @SEGMENT_TXN_DATA, 0, calldata_size, process_message_txn_code_loaded_finish, new_ctx, retdest)
+    %jump(memcpy)
 
+process_message_txn_code_loaded_finish:
     %enter_new_ctx
     // (Old context) stack: new_ctx, retdest