Performance and Evaluation of Lisp Systems - Dreamsongs

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2.8 Spice Lisp
Spice Lisp is an implementation of Common Lisp written mostly in Common
Lisp and partly in microcode. The initial implementation of Spice Lisp was done
by the Spice Lisp Group at Carnegie-Mellon University (CMU) on the Perq, a
user-microcodable machine built by Perq Systems Corporation.
2.8.1 Data Types
The Perq is a 16-bit machine, but the Lisp instruction set deals with 32-bit
immediate and pointer objects. The typing scheme described here will probably
be retained in other implementations of Spice Lisp. The most significant 5 bits
of a 32 bit-object determine its type. Immediate objects (such as characters and
fixnums) use the low-order bits to hold information.
Type (5) Immediate Data (27)
Pointer objects (such as CONSes and arrays) have an additional 2-bit space
field below the type field. All 32 bits are used as the virtual address of a piece of
storage.
Type (5) Space (2) Other Address Bits (25)
Fixnums and short floats use two consecutive type codes each to encode
positive and negative numbers. This yields 28 bits of precision. Fixnums look like
this:
And short floats like this:
Type (4) Two’s Complement Integer (28)
Type (4) Sign (1) Exponent (8) Mantissa (19)
CONSes are pointer types. Two consecutive words of storage at the virtual
address specified by the pointer hold the CAR and CDR of the CONS.