Ok, let me explain it better.
VAS is Virtual Address Space.
We all know that FSX is a 32 bit application and it is limited to 4 GB of Virtaul Address Space.
And 4 GB is the limit...no more.
If you are running a program called Process Explorer it displays it.
Everything FSX loading is put in that 4 GB VAS.
Than includes sceneries, planes etc.
Now with the new PMDG 773 and sceneries like Dubai rebooted, out of memory error crashes becomes all too frequent nowadays.
You can take off from airport at A, overflying airport B and land at airport C (all payware sceneries) and you are almost guaranteed to have a near or total OOM.
If you enter this value's in FSPIUC, the VAS available is always shown at the top bar.
Add-ons
FSUIPC
Click on the logging tab:
Log details
Enable IPC reads and IPC writes
In the specific value check boxes, enter this,
Base - Offset - Type - Hex
IPC - 024C - U32 - Leave unticked
Monitor IPC:024 (U32) = 2930004 is displayed on the top center bar if you are running FSX in windowed mode. (Remember this value is not the same you'll have and is constantly changing.)
That means I have 2.93 GB of memory left in the VAS
If you reach say 400000 or 400 MB the memory load becomes critical and an OOM is imminent.
I hope this explains the VAS issue I'm talking about.
You'll never get this OOM crash in X-Plane 10 64 bits, only because it does not have that VAS limitation FSX have.
Running FSX in DX 10 preview mode with Steve's DX 10 Fixer somehow help a bit, because some of the texture load is now put into the graphics card memory, which FSX does not do in DX9 mode.