Re: Forth for Mac OS X Leopard (Intel) - what are the options?

According to Josh Grams  <josh@[EMAIL PROTECTED]
>:
> I believe (someone please correct me if I'm wrong) that x86 processors
> don't distinguish between readable and executable, so if you can read
> it, you can execute it.

Modern CPU from AMD and Intel can distinguish between readable and
executable. AMD calls it "NX" while Intel uses the brand name "XD".
For details, see :

  http://en.wikipedia.org/wiki/NX_bit

Some OS use that functionality, for supposedly enhanced security.

Some programmers from Hungary had found a convoluted way to achieve
the same functionality on some previous types of x86 CPU, using the
fact that on these CPU, the TLB caches were distinct for code and
for data, hence you could put some data in the cache and yet arrange
for the CPU to trap upon access for execution. There is a penalty,
which is that cache misses basically become exceptions handled at
the OS level. But the penalty is not as high as one could expect
with typical application code. For details, see :

  http://pax.grsecurity.net/

I find it quite improbable that MacOS implements that trick. However,
it is quite possible that MacOS uses the NX/XD bit when available.
Hence you should refrain from executing data which has not been
explicitely marked as executable.


	--Thomas Pornin