# 2.1 Binary Format

_Peios / Advanced Peios / PCDS / GUID_

> The 128-bit GUID — its four-field binary layout, byte order, and the nil GUID sentinel.

A GUID (Globally Unique Identifier) is a 128-bit identifier with
global uniqueness guarantees, used to identify registry hives,
layers, object types, and other entities that require stable
identity across systems and reboots. The GUID with all 128 bits set
to zero is the **nil GUID**, a sentinel value meaning "no GUID" or
"unset."

A GUID is a 128-bit (16-byte) value with the following binary layout:

| Offset | Size | Field | Type |
|--------|------|-------|------|
| 0 | 4 | Data1 | uint32, little-endian |
| 4 | 2 | Data2 | uint16, little-endian |
| 6 | 2 | Data3 | uint16, little-endian |
| 8 | 8 | Data4 | uint8[8] |

The total size of a GUID MUST be exactly 16 bytes with no
padding.

Data1, Data2, and Data3 MUST be stored in little-endian byte
order.

Data4 is a raw byte array with no endianness interpretation.

> [!NOTE]
> This is the mixed-endian layout inherited from DCE RPC and
> MS-DTYP. The first three fields follow the platform's native byte
> order (little-endian on x86), while Data4 is a byte sequence with
> no integer interpretation.

## 2.1.1 Nil GUID

The nil GUID is the GUID with all 16 bytes set to zero.

The nil GUID is a valid GUID value. Specifications that require a
non-nil GUID MUST state this requirement explicitly.

The nil GUID MUST NOT be produced by the generation algorithm
defined in §2.4.