brk's zero-fill behavior on VAXen (useful undefined checks)

[Henry Spencer]

> ... When a new() is done, a unique key is tacked on top of the object
> allocated, that must match the key in the pointer referencing it, otherwise
> a "pointer-invalid" run-time error occurs.
> 
> On the cyber, this is easy, since there are so many bits available in a word.

Yet another similar trick:  in the Algol 68 implementation for the Cyber,
from CDC Netherlands I think it was, the garbage collector uses the extra
bits in the pointers as mark bits, thereby using zero extra storage!
-- 
				Henry Spencer @ U of Toronto Zoology
				{allegra,ihnp4,decvax,pyramid}!utzoo!henry