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Add documentation of NEWCDRCODING cdr codes

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Nick Briggs
2020-09-16 15:12:58 -07:00
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The cdr-code is a 4-bit field of a 32-bit cons cell.
It is treated as an on-page indicator bit and a 3-bit scaled offset.
Certain combinations are treated specially.
on-page| offset | interpretation
-------|--------|----------------------------------------------------------------------------
1 | 0 | CDR is NIL
1 | 1 - 7 | CDR is at 2*offset (counted in 32-bit cells) on same page
0 | 0 | CDR is indirect, CDR(car)
0 | 1 - 7 | CDR is not a cons cell but is in the car of cell at 2*offset on same page
-------|--------|----------------------------------------------------------------------------