diff --git a/compiler/optimizing/code_generator_x86.cc b/compiler/optimizing/code_generator_x86.cc
index a835aed..63bd841 100644
--- a/compiler/optimizing/code_generator_x86.cc
+++ b/compiler/optimizing/code_generator_x86.cc
@@ -5104,9 +5104,25 @@
     __ testl(value, value);
     __ j(kEqual, &is_null);
   }
+  // Load the address of the card table into `card`.
   __ fs()->movl(card, Address::Absolute(Thread::CardTableOffset<kX86PointerSize>().Int32Value()));
+  // Calculate the offset (in the card table) of the card corresponding to
+  // `object`.
   __ movl(temp, object);
   __ shrl(temp, Immediate(gc::accounting::CardTable::kCardShift));
+  // Write the `art::gc::accounting::CardTable::kCardDirty` value into the
+  // `object`'s card.
+  //
+  // Register `card` contains the address of the card table. Note that the card
+  // table's base is biased during its creation so that it always starts at an
+  // address whose least-significant byte is equal to `kCardDirty` (see
+  // art::gc::accounting::CardTable::Create). Therefore the MOVB instruction
+  // below writes the `kCardDirty` (byte) value into the `object`'s card
+  // (located at `card + object >> kCardShift`).
+  //
+  // This dual use of the value in register `card` (1. to calculate the location
+  // of the card to mark; and 2. to load the `kCardDirty` value) saves a load
+  // (no need to explicitly load `kCardDirty` as an immediate value).
   __ movb(Address(temp, card, TIMES_1, 0),
           X86ManagedRegister::FromCpuRegister(card).AsByteRegister());
   if (value_can_be_null) {