Dan,
Without /UNDEF, this program will allocate successfully until the size of Arr4 exceeds your virtual memory size limit.
As you never access / 'touch' Arr4, it will never be allocated physical memory pages (or virtual memory pages).
If you included 'Arr4 = 1' then all of the array will be allocated memory pages, and when the array size exceeds your physical memory size, your PC will look as if it has stopped (while it is shuffling memory pages to your paging disk/file).
If you included 'Arr4(:,1) = 1' then only 1 memory page will be allocated for the array, while the rest of the untouched array will not have allocated memory pages. This will continue until the array size exceeds the virtual array size limit.
The tricky bit is 'what is the virtual array size limit'
I think this is the sum of physical memory size plus paging space size, rather than the virtual address limit of Win64 OS.
You could put all this in a do loop, where you increase the size of nB and run it while watching Task Manager, which should show no memory required while Arr4 is not touched. (put a pause in your program at the end to see it in task manager)
You can do further tricks to only touch a subset of the array and see only this subset of required array memory being allocated, eg
do j = 1,nB,100000
Arr4(:,j) = 1
end do
It is rare that this type of array use is available.
The practicality is, if you use 'Arr4 = 1', once you exceed physical memory, your computer will look to stop, as it will start madly copying your array to the pagfile.sys. Remember 40 years ago, when all OS ran on a paging disk and we accepted these 1000 x slower performance.
Try this example, which stops when it exceeds memory + paging ( on my pc of 64 GB mem + 32 GB pagefile.sys goes to 128 GB, rather than 96 GB, so confusing ? )
! compilation: ftn95 aaa.f90 /link /64 /debug
!
real*4, dimension(:,:), allocatable :: Arr4
integer, parameter :: million = 1000000
integer :: nA, nB, ierr,j
character :: answer
nA = 1000
do nB = million, 50*million, million
!.. Allocating array
Print*, 'Trying to allocate GB of RAM :', 1.d-9 * 4. * nA * nB
allocate ( Arr4 (nA, nB), stat = ierr)
if (ierr.eq.0) then
write (*,*) 'Allocation success. End Program'
else
Pause 'Failed to allocate. End Program'
exit
endif
do j = 1,nB, (million/10)
arr4(:,j) = 1
end do
write (*,*) ' next size ?'
read (*,*) answer
deallocate ( Arr4 )
end do
End
