OP_1NEGATE - Pushing Negative One

3. OP_1NEGATE — Pushing Negative One

Overview

OP_1NEGATE is a specialized single-byte opcode that pushes the integer value negative one (-1) onto the Script stack. It occupies byte value 0x4f in the opcode space. Like OP_0 through OP_16, it encodes its pushed value implicitly — no additional bytes in the script stream are needed.

Opcode Value and Encoding

OP_1NEGATE = 0x4f

When executed, it pushes the Script number encoding of -1:

Script number encoding of -1: [0x81]

Breakdown:

- Magnitude: 1 (fits in one byte: 0x01)

- Sign bit: negative, so set the high bit of the last byte: 0x01 | 0x80 = 0x81

- Result: [0x81]

This one-byte representation [0x81] is the canonical Script number for -1. Any arithmetic opcode that consumes this item will interpret it as the integer -1.

Script Number Encoding (Sign-Magnitude)

Bitcoin Script uses a sign-magnitude little-endian encoding for integers, distinct from the two's-complement encoding used in most modern hardware:

Encoding algorithm:

1. Serialize the absolute value in little-endian bytes

2. If result is negative AND the high bit of the last byte is set,
   append an extra 0x80 byte (sign extension byte)

3. If result is negative AND the high bit is NOT set,
   OR it into the last byte with 0x80

Examples:
  0  --> []        (empty = zero)
  1  --> [0x01]
 -1  --> [0x81]    (0x01 | 0x80)
127  --> [0x7f]
-127 --> [0xff]    (0x7f | 0x80)
128  --> [0x80, 0x00]
-128 --> [0x80, 0x80]

Practical Uses of -1

The value -1 appears in specific Script contexts where negative results or flags are needed. Some use cases include:

Arithmetic results: Script arithmetic can produce -1 as a result. For example:

Script: OP_0 OP_1 OP_SUB
Stack evolution:
  Push 0:  [0]
  Push 1:  [0, 1]
  SUB:     [-1]   --> [0x81]

Historical script patterns: Older pre-standard scripts sometimes used -1 as a sentinel or marker value in custom locking logic.

Boolean negation context: In scripts that implement custom boolean logic, -1 can arise from subtraction operations and must be handled correctly by subsequent arithmetic opcodes.

Opcode Position in the Namespace

OP_1NEGATE at 0x4f sits just below OP_RESERVED at 0x50 and just above the direct-byte push range that ends at 0x4b. This positioning is deliberate: the range 0x4f–0x60 forms a compact group of constant-push opcodes:

0x4f = OP_1NEGATE  --> pushes -1
0x50 = OP_RESERVED --> undefined (fails if executed)
0x51 = OP_1        --> pushes 1
0x52 = OP_2        --> pushes 2
...
0x60 = OP_16       --> pushes 16

Bitcoin Core Implementation

// From src/script/interpreter.cpp
case OP_1NEGATE:
{
    stack.push_back(CScriptNum::serialize(-1));
    break;
}

CScriptNum::serialize(-1) produces the byte vector {0x81}, conforming to the sign-magnitude encoding described above.

Minimal Push Compliance

BIP 62 requires that -1 always be pushed using OP_1NEGATE (one byte) rather than an explicit data push. A non-minimal push of -1 would be:

Non-minimal (BIP 62 violation):
0x01 0x81    (push-1-byte opcode followed by 0x81)

Minimal (BIP 62 compliant):
0x4f         (OP_1NEGATE)

Non-standard transactions using non-minimal pushes are rejected from the mempool under modern Bitcoin Core policy.

Summary

OP_1NEGATE is a compact, single-byte opcode (0x4f) that pushes the Script-encoded integer -1 ([0x81]) onto the stack. It occupies a unique position in the opcode space as the only dedicated negative-integer push opcode, complementing the 0 and 1–16 push opcodes in the constant-value group. Its existence enables space-efficient encoding of the -1 constant without requiring a two-byte direct push sequence.