How access arbitrary memory addresses (r/w)?

9 Aug 2012

      Dear Marc,

Here is something that is really key for some lowlevel things: I wish to do
the equivalent of the C expression * ((byte*) x) in the Scheme world, i.e.
the same as something like MOV AL,[EAX] .

So, something like the C variant:

for (int i = 12345; i < 12445; i++)
  print("%i ",* ((byte*) i));

would, if we call the read operator ##sysmem-byteref , be expressed
in Scheme as something like:

(let loop ((i 12345))
  (if (< i 12445)
    (begin
      (print (##sysmem-byteref i) " ")
      (loop (##fxnum+ i 1)))))

We can call the write operator  ##sysmem-byte-set! .  Also it would be of
great benefit to have one for 16bit, one for 32bit, one for 48bit and one
for 64bit words.

I think it's justified that there is some way to reach this with highest
efficiency, i.e. a way that does this inlined in the code thus producing
something like * ((byte*) x) / MOV AL,[EAX] .

Indeed this would provide of good efficiency on 64bit machines only, where
fixnums can go up to ~10^18 (unlike 32bit machines where it's ~0.5 * 10^9,
so pointers would generally be bignums).

Is this in place already, if not how is it best implemented? (with
##c-codeand some magick around it perhaps; I'd suppose with
##c-lambda there's always some handling around that would make it clearly
more than a single-cpu-instruction operation, even if compiled with (declare
(not safe)) ).

I may not be correct about this one but I got the impression that
##u8vector-ref/set! would *not* work for this, though perhaps a u8vector
object could be produced that is immovable and has zero as its offset
memory address?

Thanks,
Mikael

Mikael

Alex Queiroz

Mikael More

Mikael More

Bradley Lucier

Mikael

tags

participants (4)