Re: Re: pointer calculations

"Terrence Enger" <tenger@xxxxxxxxxxxxxxxx> wrote in
message news:3.0.3.32.20030904125417.00f3ddf4@xxxxxxxxxxxxxx
> Hmm.  The bit "- (char*)_LSPCO(p)", if I understand it,
> caters for the possibility that the value of the space
> origin is not a multiple of page size.  Am I reading the
> code correctly?  Is such a thing possible?  The code makes
> no comparable allowance for page size being other than a
> power of two.

_LSPCO returns a pointer to the space origin, which is the address of the
first addressable byte in the space located by the given pointer.  It's
possible that the space origin is not a multiple of a page size.

The reason for subtracting the space origin from the original pointer value
is to obtain the space offset associated with the pointer.  A second way of
obtaining the space offset (associated with a space pointer) is to convert
the pointer to an integer.  So, you could rewrite NEXT_PAGE as follows:

#define NEXT_PAGE(p) ( (char *)_LSPCO(p) \
         + ( (int)p + (PAGESIZE-1) ) & (~(PAGESIZE-1))) )

As you observed, this assumes that PAGESIZE is a power of two.  If p is a
null pointer value, the expression

  p - _LSPCO(p)

might behave differently than

  (int)p

, but I haven't tested that.

Also, if p is a pointer to Teraspace, the offset may require more than 32
bits to represent it.  Therefore, it is safer to convert the pointer to a
long long instead of an int.

> Along the way to this followup question, I worked my way to
> <http://publib.boulder.ibm.com/iseries/v5r2/ic2924/info/mi/index.htm>,
> expecting that the definition of LSPCO would tell me
> something about space origins, but I do not see LSPCO there.
> Can you suggest any other readings?

Sorry, but I couldn't find a description either.

- Bob