Windows 9x was designed to use up to 2Gb of RAM:
http://support.microsoft.com/default.aspx?scid=kb;[LN];181594
Windows 95 Can Access Up to Two GB of RAM

Windows Me and Windows 98 are not designed to handle more than 1 GB of RAM.
http://support.microsoft.com/default.aspx?scid=kb;[LN];304943
Computer May Reboot Continuously with More Than 1.5 GB of RAM

The 4Gb limit is the Intel processor memory model, not DOS. DOS was invented
when the x86 could address 1Mb of RAM, so if you want to talk about a DOS
memory model you would be referring to the 640k/1Mb arrangement. Add-ons
such as extended or expanded memory drivers pushed this limit up, but the
4Gb address limit is imposed by the x86 architecture of the Intel
processors. It is not physically possible to address more RAM than this in
the x86 instruction set. It makes no sense to talk about a DOS memory model
with virtual memory - DOS doesn't know anything about it. Perhaps the
references you have are using a different meaning for DOS (IBM?). NT
operates within the Intel x86 architecture of 4Gb address space but it makes
better use of memory than w9x. Intel makes many other types of
microprocessor, but they are not relevant to Windows.

Windows 95 can get into problems with a lot of RAM due to design
deficiencies in the memory management routines. So the problem with
performance is due to the operating system, not the processor type.  It
doesn't happen at a specific amount of RAM, but it is dependant on what
programs are running and how the machine has been used. W98 can have similar
problem in some circumstances with caching. There are specific issues at
about 512Mb and 1Gb, but nothing that is specific to 256Mb and the problems
are not associated with latency in allocating or using any particular
address.

Dos is based on the 16-bit real mode architecture of the Intel
CPU's. 2^16 = 1 Mb (in actual fact you can address a little more
before wrap around occurs, the so-called HI MEMORY). It's
possible (I've done it - 80386 or higher CPU's) using HIMEM.SYS
and suitable code to switch the CPU to 32-bit protected mode (2^32
= 4Gb) so that you can access all of that space, you paid for :-).

In 'protected mode' all memory addressing is now 32-bit, including
that first 1 Mb of ram. There are several other architectural changes
too. Segmented addressing is replaced by 'descriptors', paging is
available, etc., etc.

Virtual memory: when program demands exceed the available RAM,
maintenance programs save the content of part of that RAM to disk,
so the main program can reallocate that RAM memory. Theoretically,
there is no limit to 'available' virtual memory, but every time the program
'swaps RAM to/from disk' the whole process is slowed, since read/writes
to disk are much slower than read/writes to RAM.

If you write Windows programs in ASM or C or C++, there is a wealth
of functions for allocating memory, including virtual memory, e.g.

VirtualAlloc
The VirtualAlloc function reserves or commits a region of pages in the
virtual address space of the calling process. Memory allocated by this
function is automatically initialized to zero, unless the MEM_RESET
flag is set.

LPVOID VirtualAlloc(
 LPVOID lpAddress, // address of region to reserve or commit
 DWORD dwSize,     // size of region
 DWORD AllocationType,  // type of allocation
 DWORD Protect   // type of access protection
);

Alan
email: replace illemann with alananne