WASM Binary Instruction Utility

WebAssembly Binary Instruction Decoder

Decode raw WebAssembly binary instruction bytes into readable WASM mnemonics, immediate operands, LEB128 values, local and global indexes, branch depths, constants, memory alignments and offsets.

✓ Opcodes ✓ LEB128 ✓ i32 / i64 ✓ Memory ✓ Branches ✓ Locals / Globals

WASM

Instruction Stream Decoder

● Ready

WebAssembly Instruction Hex Bytes Enter raw instruction bytes only. Spaces, commas, colons, hyphens and continuous hexadecimal are accepted. The default sample is loaded automatically by JavaScript.

Decode Mode Sequence mode continues until all supplied bytes are decoded.

Unknown Opcodes

Decoder scope: this tool decodes a raw WebAssembly instruction stream. It does not parse the WASM module header, sections, function bodies or validation context. Immediate integer operands use WebAssembly’s LEB128 representation where applicable.

WebAssembly Decode Result Decoded

Decoded Instruction Stream —

Instructions —

Input Bytes —

First Opcode —

First Instruction —

Control Instructions —

Memory Instructions —

Numeric Instructions —

Bytes Consumed —

Instruction Listing —

Byte-by-Byte Breakdown —

What Is a WebAssembly Binary Instruction?

WebAssembly instructions are encoded as compact binary opcodes followed by zero or more immediate operands. A WebAssembly function body is ultimately represented as a sequence of these instruction bytes.

Many integer immediates and indexes use LEB128 encoding, which allows small values to occupy fewer bytes than a fixed-width integer representation.

WebAssembly Opcode Examples

Opcode	Instruction	Immediate
0x0B	end	None
0x0C	br	Label index
0x0D	br_if	Label index
0x10	call	Function index
0x20	local.get	Local index
0x21	local.set	Local index
0x23	global.get	Global index
0x28	i32.load	memarg
0x36	i32.store	memarg
0x41	i32.const	Signed LEB128
0x42	i64.const	Signed LEB128
0x6A	i32.add	None

LEB128 in WebAssembly

LEB128 is a variable-length integer representation. Seven data bits are stored per byte, while bit 7 indicates whether another byte follows.

Example:

2A

Binary:
00101010

High continuation bit:
0

Unsigned value:
42

i32.const

The i32.const instruction uses opcode 0x41 followed by a signed LEB128 32-bit integer.

41 2A

41:
i32.const

2A:
42

Decoded:
i32.const 42

i64.const

i64.const uses opcode 0x42 and a signed LEB128 64-bit immediate. This allows both small and large signed 64-bit constants to be encoded compactly.

Local and Global Instructions

Instructions such as local.get, local.set, local.tee, global.get and global.set are followed by unsigned LEB128 indexes.

20 03

0x20:
local.get

0x03:
local index 3

Decoded:
local.get 3

WebAssembly Memory Arguments

Load and store instructions include a memory argument containing alignment and offset values. Both values are encoded as unsigned LEB128 integers.

28 02 10

0x28:
i32.load

alignment exponent:
2

offset:
16

Decoded:
i32.load align=4 offset=16

The alignment immediate stores an exponent, so an encoded value of 2 represents 2² = 4-byte alignment.

Control Flow Instructions

WebAssembly uses structured control flow with block, loop, if, else and end instructions. Branch instructions such as br and br_if reference a relative label depth rather than a raw machine-code address.

WebAssembly Numeric Instructions

Numeric opcodes cover integer and floating-point comparisons, arithmetic, bitwise operations, shifts, conversions and reinterpretations.

41 2A
41 08
6A

Decoded:

i32.const 42
i32.const 8
i32.add

WebAssembly Binary Instruction Decoder FAQs

What does opcode 0x41 mean in WebAssembly?

Opcode 0x41 is i32.const and is followed by a signed LEB128 i32 immediate.

What does opcode 0x42 mean?

Opcode 0x42 is i64.const followed by a signed LEB128 64-bit integer.

What does opcode 0x6A mean?

0x6A is the WebAssembly i32.add instruction.

What is LEB128?

LEB128 is a variable-length integer encoding used extensively by the WebAssembly binary format.

Does this decoder parse complete .wasm modules?

No. It focuses on raw instruction bytes. Module sections and function-body framing require a complete WebAssembly module parser.

How are memory offsets encoded?

Load and store memory arguments contain unsigned LEB128 alignment and offset values.

What does WebAssembly end opcode 0x0B do?

The end instruction terminates a structured expression such as a block, function expression or conditional construct.

Decode Raw WebAssembly Instructions

Inspect WASM instruction bytes from disassemblers, compiler output, browser tools, runtime traces and binary analysis with opcode, LEB128 and immediate operand decoding.