Under Windows Server 2003, Enterprise Edition, SP2 (/3GB switch not enabled)
As I understand it, and I may be wrong, the maximum addressable memory for a process is 4GB.
Is that 2GB of private bytes and 2GB of virtual bytes?
Do you get "out of memory" errors when the private byte limit or virtual byte limit is reached?
Mark Russinovich started a series of posts on this.. Pushing the Limits of Windows: Physical Memory
While 4GB is the licensed limit for 32-bit client SKUs, the effective limit is actually lower and dependent on the system's chipset and connected devices. The reason is that the physical address map includes not only RAM, but device memory as well, and x86 and x64 systems map all device memory below the 4GB address boundary to remain compatible with 32-bit operating systems that don't know how to handle addresses larger than 4GB. If a system has 4GB RAM and devices, like video, audio and network adapters, that implement windows into their device memory that sum to 500MB, 500MB of the 4GB of RAM will reside above the 4GB address boundary.
You can only access 2Gb of memory in total (without the 3Gb switch) on 32bit Windows platforms.
You could run multiple 32bit VMs on a 64bit OS so that each app has access to as much memory as possible if your machine has more than 4Gb.
A lot of people are just starting to hit these barriers, I guess it's easier if your app is in .net or Java as the VMs happily go up to 32Gb of memory on 64bit os.
On 32 bits, if there is enough physical memory and disk space for virtual memory, memory runs out around 3GB since the kernel reserves the address space above 0xC0000000 for itself. On a 64 bits kernel running a 64 bits application, the limit is at 8TB.
For more details, check out MSDN - Memory Limits for Windows Releases
Maximum addressable memory for a 32bit machine is 4GB, for a 64bit machine you can address loads more. (Although some 32bit machines have extension systems for accessing more, but I don't think this is worth bothering with or considering for use).
You get out of memory errors when the virtual limit is reached. On Windows Server 2003, task manager tells you the limit on the performance tab labelled 'Commit Charge Limit'.
It is correct that the maximum address space of a process is 4GB, in a sense. Half of the address space is, for each process, taken up by the operating system. This can be changed with the 3GB switch but it might cause system instability. So, we are left with 2GB of addressable memory for the process to use on its own. Well, not entirely. It turns out that a part of this space is taken up by other stuff such as DLLs an other common code. The actual memory available to you as a programmer is around 1.5GB - 1.7GB.
I'm not sure about how you can handle accidentally going above this limit but I know of games which crash in large multiplayer maps for this reason. Another thing to note is that a 32bit program cannot use more than the 2GB address space on a 64bit system unless they enable the /LARGEADDRESSAWARE:YES linker flag.