Windows Can Only See Up To 4GB of RAM

[essential]

I am aware that Windows XP (32-bit) can only recognize a maximum of 4GB of RAM (usually more like 2.5-3.25GB usable).

What I don?t get is why does it recognize 2.5GB in some machines, and other machines it recognizes 3.25GB, especially if they all have independent video? Why are different random amounts seen?

For example, I recently got a laptop with 3GB of RAM (2GBx1+1GBx1), and Windows recognizes all three GBs (I have ATI Mobility, not shared video). My fathers desktop has 4GB of RAM (not sure if it's 2GBx2 or 1GBx4) and Windows recognizes 3.25GB (with NVIDIA 8600GT), and yesterday I installed XP on the same exact laptop configuration that I have above where Windows recognizes 3GB (2GBx1+1GBx1) except this laptop has 4GB of RAM (2GBx1+2GBx1 with ATI Mobility graphics) and Windows only recognizes 2.67GB. I knew all 4GB wouldn't be seen, but it's not even seeing a full 3GB when my laptop does, why? The only difference is 2GBx1+1GBx1 vs. 2GBx1+2GBx1 in terms of the stick sizes.

Why does this happen? Shouldn?t it see at least 3GB of the RAM if my nearly identical setup can see 3GB, if not 3.25GB?

Is there any post installation way of making Windows see more RAM?

 

[ViRGE]

The amount of usable RAM a 32bit system can see is dependent on the devices in that system. Every device is allocated part of of the physical memory address space for inter-device communication. The difference in the amount of usable RAM then is the difference in the needs of the devices. Your laptops are dissimilar in some fashion, and one of them has devices that require a larger block of address space and/or more devices total.

 

[bsobel]

Originally posted by: ViRGE
The amount of usable RAM a 32bit system can see is dependent on the devices in that system. Every device is allocated part of of the physical memory address space for inter-device communication. The difference in the amount of usable RAM then is the difference in the needs of the devices. Your laptops are dissimilar in some fashion, and one of them has devices that require a larger block of address space and/or more devices total.

Yep its not Windows, its the requirements of the hardware on the box.

 

[hanspeter]

In some cases the limitation is both in hardware and in Windows. The times where the hardware supports 8 or more GB (and memory remapping) then it is limited by Windows.

 

[kylebisme]

Originally posted by: ViRGE
The amount of usable RAM a 32bit system can see is dependent on the devices in that system. Every device is allocated part of of the physical memory address space for inter-device communication. The difference in the amount of usable RAM then is the difference in the needs of the devices. Your laptops are dissimilar in some fashion, and one of them has I've seen this explaination before but I don't understand why it only applies to systems beyond a certain amount of memory. Why does Windows report a full 2GB if that is all you have installed?

 

[ViRGE]

Originally posted by: TheSnowman

Originally posted by: ViRGE
The amount of usable RAM a 32bit system can see is dependent on the devices in that system. Every device is allocated part of of the physical memory address space for inter-device communication. The difference in the amount of usable RAM then is the difference in the needs of the devices. Your laptops are dissimilar in some fashion, and one of them has I've seen this explaination before but

I don't understand why it only applies to systems beyond a certain amount of memory. Why does Windows report a full 2GB if that is all you have installed?

A 32bit processor has 2^32 memory addresses, for 4GB of physical memory addresses. Generally the top XXX-bytes of those addresses are used for inter-device communication as described above. When you only have 2GB of RAM, the other 2GB of physical addresses are still free to use for inter-device communication, hence you do not lose any RAM from those devices.

 

[kylebisme]

Ah, that makes sense. Thanks.