diff --git a/evm/src/cpu/kernel/asm/bignum/add.asm b/evm/src/cpu/kernel/asm/bignum/add.asm
index d38c9701..038c7675 100644
--- a/evm/src/cpu/kernel/asm/bignum/add.asm
+++ b/evm/src/cpu/kernel/asm/bignum/add.asm
@@ -1,7 +1,8 @@
 // Arithmetic on little-endian integers represented with 128-bit limbs.
 // All integers must be under a given length bound, and are padded with leading zeroes.
 
-// Replaces a with a + b, leaving b unchanged.
+// Adds two bignums of the same given length. Assumes that len > 0.
+// Replaces a with a + b, leaving b unchanged, and returns the final carry.
 global add_bignum:
     // stack: len, a_start_loc, b_start_loc, retdest
     PUSH 0