The Scripting Context (CScript)

Bitcoin Script is a stack-based language that handles numbers in a very specific way. To save space, it uses a Variable-Length Little Endian format for integers placed on the stack.

1. Minimally Encoded Integers

In most programming languages, an integer is always 4 or 8 bytes. In Bitcoin Script, the number 1 is stored as a single byte: 0x01.

• The number 128 is stored as 0x80 0x00.

• The number 256 is stored as 0x00 0x01.

2. Little Endian Signed Magnitude

Bitcoin Script uses Little Endian for its math, but it also uses a "Sign Bit."

• If the highest bit of the last byte is 1, the number is negative.

• Example: The number -1 is represented as 0x81.

• Example: The number 1 is represented as 0x01.

3. The "Arithmetic" Rule

When you use an opcode like OP_ADD:

1. The node pops two items off the stack.

2. It interprets them as Little Endian integers.

3. It performs the addition.

4. It pushes the result back to the stack as a Minimally Encoded Little Endian integer.

4. Opcodes and Data

It's important to distinguish between the OpCode and the Data.

• OpCodes: Like OP_DUP (0x76), are single bytes. Endianness does not exist for single bytes.

• PushData: If you want to push a public key, you use a length byte (e.g., 0x21 for 33 bytes) followed by the data. The data inside a push is typically Big Endian (if it's a key or signature).

5. Why this matters for Developers

If you are writing a script manually, you must ensure your numbers are encoded correctly.

• If you want to check a sequence number, that number in the transaction is 4-byte Little Endian.

• If you want to use that same number in a script for comparison, it must be converted to the script's minimal-length format.

Value	Standard Int (4-byte LE)	Script Int (Minimal LE)
1	01 00 00 00	01
127	7F 00 00 00	7F
128	80 00 00 00	80 00

In the final section, we will build a Python Context Auditor.