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A structure descriptor has the structure lowtag type code, making structurep a fast operation. A structure data-block has the following format:
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| length (24 bits) | Structure header type (8 bits) |
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| structure type name (a symbol) |
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| structure slot 0 |
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| ... structure slot length - 2 |
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The header word contains the structure length, which is the number of words (other than the header word.) The length is always at least one, since the first word of the structure data is the structure type name.