# Scope
The crate is allocation-governance infrastructure. It does not implement stable
collections, prove application schema compatibility, authorize controllers, or
perform data migrations.
Its narrower responsibility is to validate declarations against historical
allocation facts before application memory is opened.
## Native Substrate
The native substrate is the `MemoryManager` from `ic-stable-structures`. Its
usable ID domain is:
$$
S = \{0,1,\ldots,254\}
$$
ID `255` is reserved by `ic-stable-structures` as the unallocated sentinel. In
the default runtime, `ic-memory` reserves IDs `0..=9` for governance and uses ID
`0` for the allocation ledger. Lower-level bootstrap owners that bypass the
default runtime must enforce equivalent policy themselves.
## Substrate Correspondence
The allocation slot model is intentionally aligned with the current
`ic-stable-structures` `MemoryManager` implementation used by this crate. The
upstream manager defines `MemoryId` as a one-byte value, uses `255` as the
unallocated-bucket marker, and stores each bucket owner as one byte in the
manager metadata.
Therefore `ic-memory`'s current slot descriptor is intentionally small: it is a
`MemoryManagerId(u8)` in the same durable ID domain that the underlying manager
can distinguish.
The same implementation reserves the first stable-memory page for manager
metadata. Its V1 header stores the magic value `MGR`, a layout version, the
allocated-bucket count, the bucket size, 32 reserved bytes, and one page-count
entry for each managed memory `0..=254`. The bucket-owner table follows the
header. With the default bucket size of 128 Wasm pages and 32,768 managed
buckets, the manager can address 256 GiB of bucket-backed stable memory through
this layout.
`ic-memory`'s default ledger anchor also relies on
`ic-stable-structures::Cell`. The relevant Cell V1 envelope begins with magic
`SCL`, a one-byte layout version, and a four-byte value length before the
encoded value. `ic-memory` preflights that envelope before opening the ledger
Cell so corrupt ledger storage is classified as a bootstrap error instead of
escaping as a decode panic.