Performance and Evaluation of Lisp Systems - Dreamsongs

§ 2.7 NIL 55
vector whose unit size is a longword such as a Common Lisp simple vector; it need
not be shifted first.
The Vax user address space is loosely segmented into P0 (program) and P1
(stack) spaces. (These two make up one half of the Vax addressability. The other
half is ‘system’ space.) The address field of a NIL pointer permits addressing the
low half of P0 space. By selecting the type bits accordingly, one can, however,
represent objects in other parts of the address space if the mask used to clear
the type bits is different from the type code and if the appropriate high type
bits are present in the type code itself. NIL represents ‘stack vectors,’ which are
simple vectors allocated in the high half of P1 space where the NIL stack lives in
the following way. There are two simple-vector type codes—one for heap-allocated
vectors and one for stack-allocated vectors. The type code assignments are chosen
so that there is a single mask that clears either one of the type codes, producing the
correct address. (Stack vectors are important for a number of things, particularly
for doing efficient &REST arguments without CONSing.)
The fact that 5 bits are used for type code means that there are 32 different
primitive types; therefore a single longword mask can be used to represent the
union of a number of these types. As a result, the computation of
(typep x ’(or single-float double-float))
is identical to
(typep x ’(or single-float double-float short-float long-float))
except for the mask used.
There is also an extended-number type, which implements BIGNUMs, ratios,
and all the complex types.
2.7.2 Function Call
A compiled function is a typed pointer whose address points to a VAX pro-
cedure. It is called using the Vax CALL instructions. Function cells contain only
the address, not the type bits, so getting to the function is done by ‘evaluating’
the function (identical to the way in which special variables are evaluated) and
doing CALLS on the result. The procedure exits with the RET instruction.