32 bit OS and plug in board memory size

[cjm99999]

Does anyone know how this works...  If I get a PC with a 32 bit OS (WinXP 32bit etc.) and it has 4GB of memory installed, how does the OS access a plug in card that has memory or memory mapped I/O on it.  32 bits only allows addressing up to 4GB and that is used up by the installed RAM.  Or, does this not work.  Or, would it require a 64 bit OS to work.

The reason I ask is that I'm looking at the 18 slot PXI express chassis for a new design but it would be populated with many cards that have large amounts of memory on them that the control would need to access.  For example if each card had 256MB, of memory and I put 10 of them in the chassis, and the controller has 4GB of RAM installed, it seems to me that I run out of memory addresses.  The cards would be custom boards using VISA to implement the "device drive" to access them.

I am also interested in running my application under WinXP vs. under LabWindows Real-Time.  I can't seem to find any specifics about LabWindows Real-Time but I am assuming that is a 32 bit OS.

[jr_2005]

In general, if you install an I/O card with on-board memory into a 32 bit PC system, then the BIOS/memory subsystem will assign a range of memory addresses to that card. So, when your application puts out a 32 bit address which matches that given to the card, those memory operations are redirected to the card instead of to main memory. In effect, the main RAM becomes overlaid at that point and that region is inaccessible. So in your example, you would end up with 2.5G memory assigned to your 10 cards, leaving 1.5G RAM available for the rest of the PC. (OS and applications.)

There are other card designs which do not map large amounts of memory into the PC's address space, but use a separate on-board memory controller to manage it independantly (eg high end graphics cards). Such cards would not have such a significant impact on the memory available to the OS.

JR

[StevieZ]

To answer your question about LabWindows/CVI Real-Time. LabWindows/CVI Real-Time is a program that will can compile and run code for in PharLap, a real-time operating system. PharLap is 32-bit only at this point.

Regards,

Steven Zittrower

Applicati Instruments

http://www.ni.com/support

[cjm99999 · Accepted solution]

Thank you very much for both your answers.  Very helpful.