diff --git a/src/matrix.rs b/src/matrix.rs
index 200ae04..9ef9946 100644
--- a/src/matrix.rs
+++ b/src/matrix.rs
@@ -114,6 +114,8 @@ impl<T> Msg<T> {
 // endregion: --- impls
 
 // region:    --- functions
+// AB -> a.col == b.row (左乘)
+// 最后的矩阵是一个 a.row * b.col 的矩阵
 pub fn multiply<T>(a: &Matrix<T>, b: &Matrix<T>) -> Result<Matrix<T>>
 where
     T: Copy + Default + Add<Output = T> + AddAssign + Mul<Output = T> + Send + 'static,
@@ -127,6 +129,7 @@ where
     // for i in 0..a.row {
     //     for j in 0..b.col {
     //         for k in 0..a.col {
+    //             // data[i][j] += a[i][k] * b[k][j]
     //             data[i * b.col + j] += a.data[i * a.col + k] * b.data[k * b.col + j];
     //         }
     //     }
@@ -156,14 +159,14 @@ where
     // map/reduce: map phase
     for i in 0..a.row {
         for j in 0..b.col {
-            let row = Vector::new(&a.data[i * a.col..(i + 1) * a.col]);
+            let row = Vector::new(&a.data[i * a.col..(i + 1) * a.col]); // a[i][k]
             let col_data = b.data[j..]
                 .iter()
                 .step_by(b.col)
                 .copied()
                 .collect::<Vec<_>>();
-            let col = Vector::new(col_data);
-            let idx = i * b.col + j;
+            let col = Vector::new(col_data); // b[k][j]
+            let idx = i * b.col + j; // i,j -> idx
             let input = MsgInput::new(idx, row, col);
             let (tx, rx) = oneshot::channel();
             let msg = Msg::new(input, tx);