>For an answer to why we want 64-bit MPE on IA-64, read the answer in
HP's own
>words at the following URL.  Oh, yeah, don't forget to substitute "MPE"
>everytime you read "HP-UX".
>
>http://www-dmo.external.hp.com/gsy/software/64bit/64bitwp.html
>

From that very web page:

Caveats

For all these potential benefits of 64-bit computing, there are some
important caveats associated with this technology that have limited its
adoption by end users thus far.

Limited Need
Both the supply of and demand for 64-bit computing are limited and
specialized. Although 64-bit computing has been available for several
years, there are relatively few applications on the market that take
advantage of an underlying 64-bit architecture. This is because
relatively few end users have actual need for the higher capabilities of
64-bit computing. The vast majority of high-end computing needs are met
with a 32-bit operating system and applications. The 32-bit computing
model that has become the mainstream computing standard will be
sufficient for most users for many years to come. Restated, 32-bit
computing still has considerable headroom.

Costs in Time and Money
For those users seeking access to the performance and scalability gains
that 64-bit computing promises, they should be aware that, as with most
things, 64-bit computing does not come without cost. These costs fall
into two categories: those that are direct and measurable, and others
that are less easily quantifiable.

To take fullest advantage of larger memory addressing, it is necessary
to incorporate large amounts of memory into one's computing environment.
With the cost of RAM ranging from $50,000/GB to $70,000/GB, the cost of
memory needed for 64-bit computing can rise significantly for many
customer environments.

Less easily measured are the costs (in a non-HP environment) of possibly
having to port 32-bit applications to the new 64-bit environment. These
are the costs of system admin-istrators' and developers' time spent on
backing up and restoring file systems; application recoding and
recompiling; and certifying (or testing) those newly compiled
applications. HP will not force its user base to go through this
burdensome process, because HP-UX will allow for the coexistence of 32-
and 64-bit applications.

Uncertain Performance Gains
A final caveat is the fact is that not all 32-bit applications
recompiled to 64 bits will run faster in 64-bit mode. This is because of
the "cache-fit" effect. When recompiled, a 32-bit application will
typically result in a larger binary-or executable module-in 64-bit mode.
With a given cache size available on a system, performance may actually
decline because of a greater number of cache misses when running the
64-bit binary.