#ifdef UNIVAC #define NULL (0xffff) #endif
Later models used [blah], evidently as a sop to all the extant poorly-written C code which made incorrect assumptions.
Q: Seriously, have any actual machines really used nonzero null
pointers, or different representations for pointers to different
A: The Prime 50 series used segment 07777, offset 0 for the null
pointer, at least for PL/I. Later models used segment 0, offset 0 for
null pointers in C, necessitating new instructions such as TCNP (Test
C Null Pointer), evidently as a sop to [footnote] all the extant
poorly-written C code which made incorrect assumptions. Older,
word-addressed Prime machines were also notorious for requiring larger
byte pointers (char *’s) than word pointers (int *’s).
The Eclipse MV series from Data General has three architecturally
supported pointer formats (word, byte, and bit pointers), two of which
are used by C compilers: byte pointers for char * and void *, and word
pointers for everything else. For historical reasons during the
evolution of the 32-bit MV line from the 16-bit Nova line, word
pointers and byte pointers had the offset, indirection, and ring
protection bits in different places in the word. Passing a mismatched
pointer format to a function resulted in protection faults.
Eventually, the MV C compiler added many compatibility options to try
to deal with code that had pointer type mismatch errors.
Some Honeywell-Bull mainframes use the bit pattern 06000 for
(internal) null pointers.
The CDC Cyber 180 Series has 48-bit pointers consisting of a ring,
segment, and offset. Most users (in ring 11) have null pointers of
0xB00000000000. It was common on old CDC ones-complement machines to
use an all-one-bits word as a special flag for all kinds of data,
including invalid addresses.
The old HP 3000 series uses a different addressing scheme for byte
addresses than for word addresses; like several of the machines above
it therefore uses different representations for char * and void *
pointers than for other pointers.
The Symbolics Lisp Machine, a tagged architecture, does not even have
conventional numeric pointers; it uses the pair (basically a
nonexistent handle) as a C null pointer.
Depending on the “memory model” in use, 8086-family processors (PC
compatibles) may use 16-bit data pointers and 32-bit function
pointers, or vice versa.
Some 64-bit Cray machines represent int * in the lower 48 bits of a
word; char * additionally uses some of the upper 16 bits to indicate a
byte address within a word.