BA-01 · LESSON 03

Bits, bytes, hex, and endianness

Computers store Bitcoin data as bits grouped into bytes. Hexadecimal is a compact way to display those bytes, while byte order determines how a number is read.

55 min Difficulty 1/5 Not started

VISUAL MODEL

See the whole mechanism before going step by step

Use the diagrams to orient yourself. Then the guided explanation below teaches every part in depth.

4 explanatory views

CONCEPT-SPECIFIC MAP · byte orderOne number can be displayed and serialized in opposite byte orderSelect any state to inspect its exact role.

RELATIONSHIP MAPOrdered mechanism flow

01
INTEGER258 decimal
The numeric value is independent of how bytes are displayed.
02
HEX0x0102
Hexadecimal writes each byte as two digits.
03
BIG ENDIAN01 02
Most-significant byte first matches ordinary written-number order.
04
LITTLE ENDIAN02 01
Bitcoin serializes many fixed-width integers least-significant byte first.
05
PARSE258 either way when rule matches
The reader must apply the same field rule as the writer.

Rule shown: Bitcoin protocols define field-by-field byte order; reversing bytes changes representation, not the underlying integer.

STAGE 1 OF 5INTEGER: 258 decimal

The numeric value is independent of how bytes are displayed.

Governing relationship: Bitcoin protocols define field-by-field byte order; reversing bytes changes representation, not the underlying integer. Complete path: 1. INTEGER, 258 decimal. The numeric value is independent of how bytes are displayed. 2. HEX, 0x0102. Hexadecimal writes each byte as two digits. 3. BIG ENDIAN, 01 02. Most-significant byte first matches ordinary written-number order. 4. LITTLE ENDIAN, 02 01. Bitcoin serializes many fixed-width integers least-significant byte first. 5. PARSE, 258 either way when rule matches. The reader must apply the same field rule as the writer.

1. State transitionWhat exists, what changes, and what results

BEFORE

Convert decimal 500 into a two-byte little-endian field.

MECHANISM

The binary value 10101100 is one byte. Its high four bits are 1010 and its low four bits are 1100.

AFTER

Hex describes byte values; endianness describes byte order. They are related but different.

Text equivalent: Starting from Convert decimal 500 into a two-byte little-endian field. The mechanism begins when The binary value 10101100 is one byte. Its high four bits are 1010 and its low four bits are 1100. The resulting lesson is Hex describes byte values; endianness describes byte order. They are related but different.

2. Mechanism pathSelect a stage to inspect its job

STAGE 1 OF 5

Group bits into bytes

The binary value 10101100 is one byte. Its high four bits are 1010 and its low four bits are 1100.

Complete path: 1. Group bits into bytes: The binary value 10101100 is one byte. Its high four bits are 1010 and its low four bits are 1100. 2. Translate each nibble to hex: A four-bit group is a nibble. Binary 1010 is hex a and binary 1100 is hex c, so the byte is written ac. 3. Serialize numbers as bytes: A protocol specifies a fixed width and byte order. The number 4660 is hex 0x1234 and needs two bytes: 12 and 34. 4. Apply endianness: Big-endian stores the most significant byte first as 12 34. Little-endian stores the least significant byte first as 34 12. 5. Keep display conventions separate: Bitcoin fields can be serialized little-endian while user-facing identifiers are displayed with their bytes reversed. The rule is field-specific, not a command to reverse every hex string.

3. Rule and failure boundaryWhat accepts the result and where it can stop

ACCEPTED PATH

The chosen field width and byte order must round-trip to the original integer without truncation.

CHANGED CONDITION

Reversing hexadecimal characters instead of byte pairs changes the value, while 65,536 must be rejected by a two-byte serializer as overflow.

Important distinction: Little-endian means reversing every hexadecimal character. Endianness reverses byte order. Characters within each byte stay together, so 12 34 becomes 34 12, not 43 21.

Text equivalent: The accepted path is enforced by The chosen field width and byte order must round-trip to the original integer without truncation. If the required condition changes, Reversing hexadecimal characters instead of byte pairs changes the value, while 65,536 must be rejected by a two-byte serializer as overflow.

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Learn the idea

A bit is one binary digit, either 0 or 1. Eight bits form a byte. Hexadecimal, or hex, writes each group of four bits as one character from 0 to 9 or a to f. Endianness is the order in which a multi-byte number is stored or displayed.

GUIDED EXPLANATION1/5 ideas inspected

Group bits into bytes

The binary value 10101100 is one byte. Its high four bits are 1010 and its low four bits are 1100.

The worked example follows the explanation

Inspect every idea above to open it.

Important distinction“Little-endian means reversing every hexadecimal character.”

Endianness reverses byte order. Characters within each byte stay together, so 12 34 becomes 34 12, not 43 21.

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