diff --git a/doc/internal/opcode.md b/doc/internal/opcode.md
index d93d018d4..0d8c9851f 100644
--- a/doc/internal/opcode.md
+++ b/doc/internal/opcode.md
@@ -7,19 +7,23 @@ reducing the memory consumption of mruby VM.
 # Instructions
 
 Instructions are bytes. There can be 256 instructions. Currently, we
-have 94 instructions. Instructions can take 0 to 3 operands.
+have 106 instructions. Instructions can take 0 to 3 operands.
 
 ## operands
 
 The size of operands can be either 8bits, 16bits or 24bits.
-In the table.1 below, the second field describes the size (and
-sign) of operands.
+In the table.1 below, the second field describes the size
+of operands.
 
 - B: 8bit
 - S: 16bit
-- sS: signed 16bit
 - W: 24bit
 
+If the first and second operands are of type `B` (8bits), they may be
+extended to 16bits by the operand extension instruction immediately
+preceding them.
+See also `OP_EXT1`, `OP_EXT2` and `OP_EXT3`.
+
 ## table.1 Instruction Table
 
 | Instruction Name   | Operand type   | Semantics                                                  |