Cells as Data Storage
Everything in TOS is stored in cells. A cell is a data structure containing:
- up to 1023 bits of data (not bytes!)
- up to 4 references to other cells
Bits and references are not intermixed (they are stored separately). Circular references are forbidden: for any cell, none of its descendant cells can have this original cell as reference.
Thus, all cells constitute a directed acyclic graph (DAG). Here is a good picture to illustrate:
Cell types
Currently, there are 5 types of cell: ordinary and 4 exotic. The exotic types are the following:
- Pruned branch cell
- Library reference cell
- Merkle proof cell
- Merkle update cell
For more on exotic cells see: TVM Whitepaper, Section 3.
Cell flavors
A cell is an opaque object optimized for compact storage.
In particular, it deduplicates data: if there are several eqivalent sub-cells referenced in different branches, their content is only stored once. However, opaqueness means that a cell cannot be modified or read directly. Thus, there are 2 additional flavors of the cells:
- Builder for partially constructed cells, for which fast operations for appending bitstrings, integers, other cells and references to other cells can be defined.
- Slice for 'dissected' cells representing either the remainder of a partially parsed cell or a value (subcell) residing inside such a cell and extracted from it via a parsing instruction.
Another special cell flavor is used in TVM:
- Continuation for cells containing op-codes (instructions) for TOS Virtual Machine, see TVM bird's-eye overview.
Serialization of data to cells
Any object in TOS (message, message queue, block, whole blockchain state, contract code and data) serializes to a cell.
The process of serialization is described by a TL-B scheme: a formal description of how this object can be serialized into Builder or how to parse an object of a given type from the Slice. TL-B for cells is the same as TL or ProtoBuf for byte-streams.
If you want to know more details about cell (de)serialization, you could read Cell & Bag of Cells article.
What's next?
- Navigate to the TL-B section for more information.