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As for fixnums, there are two different three-bit lowtag codes for other-immediate, allowing 64 other-immediate types:
---------------------------------------------------------------- | Data (24 bits) | Type (8 bits with low-tag) | 1 0 | ----------------------------------------------------------------
The type-code for an other-immediate type is considered to include the two lowtag bits. This supports the concept of a single “type code” namespace for all descriptors, since the normal lowtag codes are disjoint from the other-immediate codes.
For other-pointer objects, the full eight bits of the header type code are used as the type code for that kind of object. This is why we use two lowtag codes for other-immediate types: each other-pointer object needs a distinct other-immediate type to mark its header.
The system uses the other-immediate format for characters, the symbol-value unbound trap value, and header-words for data-blocks on the heap. The type codes are laid out to facilitate range checks for common subtypes; for example, all numbers will have contiguous type codes which are distinct from the contiguous array type codes. See sec-data-blocks-and-o-i for details.