Slots and sidechains
256 numbered places, each able to hold one sidechain.
3 min read256 numbered places
BIP300 keeps a list of sidechains on L1. Each one lives in a numbered slotSlotOne of 256 numbered places (0–255) a sidechain can occupy. A slot can host different sidechains over time. from 0 to 255; the slot number is stored as a single byte. Everything that concerns a sidechain, from deposits to withdrawal votes, refers to it by this number.
Slots are like numbered parking spaces. A space can be empty or occupied, and over time a different car may park there.
What a sidechain declares
When a sidechain is proposed, it comes with a declarationDeclarationWhat a proposal says about the sidechain: title, description and two hashes intended to identify its software.: a title, a description and two hashes meant to identify its software, the hash of a release archive and a git commit.
All 256 slots
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Slot is not identity
A slot can be taken over by a new sidechain if miners vote for it (you will see how in the next concept). So the slot number alone does not identify a sidechain. The Observatory names each instanceSidechain instanceOne specific sidechain in a slot, identified by its slot, proposal, activation height and declaration hash. by its slot, the block where it was proposed, the block where it activated, and the hash of its declaration.
Go deeper: how the declaration hash is computed
The declaration hash is SHA256d of the declaration's encoded bytes (without the length prefix), shown in display order. Miners vote for a proposal by quoting this hash, which is why it also works as the sidechain's identity.
Browse every instance observed in a slot from its sidechain page.
Read the source
The rules in this lesson come from these documents and code:
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