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40.1 Tagging

The following is a key of the three bit low-tagging scheme:

000

even fixnum

001

function pointer

010

even other-immediate (header-words, characters, symbol-value trap value, etc.)

011

list pointer

100

odd fixnum

101

structure pointer

110

odd other immediate

111

other-pointer to data-blocks (other than conses, structures, and functions)

This tagging scheme forces a dual-word alignment of data-blocks on the heap, but this can be pretty negligible:

• RATIOS and COMPLEX must have a header-word anyway since they are not a major type. This wastes one word for these infrequent data-blocks since they require two words for the data.
• BIGNUMS must have a header-word and probably contain only one other word anyway, so we probably don’t waste any words here. Most bignums just barely overflow fixnums, that is by a bit or two.
• Single and double FLOATS? no waste, or one word wasted
• SYMBOLS have a pad slot (current called the setf function, but unused.)

Everything else is vector-like including code, so these probably take up so many words that one extra one doesn’t matter.