Re: Calling conventions (particularly 32-bit ARM)

According to Anton Ertl <anton@mips.complang.tuwien.ac.at>:

Such things happended many times in the past.  AFAIK standard
setup on a VAX was that accessing data at address 0 gave you 0.
A lot of VAX programs needed fixes to run on different machines.

>
That case is interesting.  It's certainly a benefit to programmers if
most uses of NULL produce a SIGSEGV, but for existing programs mapping
allowing to have accessible memory in page 0 is an advantage.  So how
did we get from there to where we are now?
>
First, my guess is that the VAX is only called out because it was so
popular, and it was one of the first Unix machines where doing it
differently was possible.  I am sure that earlier Unix tragets without
virtual memory used memory starting with address 1 because they would
otherwise have wasted precious memory.

It was a bug. As I recall, the first thing in the address space in Berkeley Unix
was a register save mask where the low byte happened to be zero, and a lot of
sloppy programs written by students accidentally depended on it, e.g.

  if(*p == 0) /* no string */

For a while ports to 68K and other architectures ensured there was a zero byte at
location zero so the Berkeley programs wouldn't crash, but eventually people fixed
the code.

Location 0 on the PDP-11 had nothing memorable and we did our string tests correctly.
You could deferenence a null pointer but you got a string of junk.




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