Why your PC can't detect 4GB RAM?

blackpearl · 09-03-2007, 03:26 PM

Look at this screenshot. Its a PC with 4GB memory running Vista.

Only 3,454 megabytes. Dude, where's my 4 gigabytes of RAM?

The Explanation
To address 4GB of memory you need 32 bits of address bus. (Assuming individual bytes are addressable.) This gives us a problem - the same problem that IBM faced when designing the original PC. You tend to want to have more than just memory in a computer - you need things like graphics cards and hard disks to be accessible to the computer in order for it to be able to use them. So just as the original PC had to carve up the 8086's 1MB addressing range into memory (640K) and 'other' (384K), the same problem exists today if you want to fit memory and devices into a 32-bit address range: not all of the available 4GB of address space can be given over to memory.

For a long time this wasn't a problem, because there was a whole 4GB of address space, so devices typically lurk up in the top 1GB of physical address space, leaving the bottom 3GB for memory. And 3GB should be enough for anyone, right?

So what actually happens if you go out and buy 4GB of memory for your PC? Well, it's just like the DOS days - there's a hole in your memory map for the IO. (Now it's only 25% of the total address space, but it's still a big hole.) So the bottom 3GB of your memory will be available, but there's an issue with that last 1GB.

Moral of the Story: If you're planning to stick with a 32-bit operating system for the next few years, don't waste your money on 4 GB of RAM. You won't be able to use it all. Buy 3 GB instead.

Read more at the Source

iMav · 09-03-2007, 03:36 PM

nice info ... but does this mean that only 3 gb will be used by the entire pc ... as in i have seen pcs having 256 mb ram but shoing lesser as available to the user so can that be the case in here also ... can the remianing 1 gb be shared for other resources like the grafix card???

rakeshishere · 09-03-2007, 03:37 PM

Nice Piece of Info

uchiha.sasuke · 09-03-2007, 05:52 PM

great info.....

NucleusKore · 09-03-2007, 07:48 PM

Nice info, reps 4 u

10-03-2007, 01:13 AM

i've got an AMD Athlon 64 3200+ which is obviously a 64-bit processor but i use the ordinary 32-bit ver of Windows XP. can i use 4gb ram?

gxsaurav · 10-03-2007, 01:23 AM

Quote:

i've got an AMD Athlon 64 3200+ which is obviously a 64-bit processor but i use the ordinary 32-bit ver of Windows XP. can i use 4gb ram?

Yes, but the OS will not address anything over 4 GB. U need a 64 bit OS for that.

Some CAD applications require a switch to use anything over 1.5GB, such as 3Ds Max & Maya.

shantanu · 10-03-2007, 01:45 AM

as 64 bit OS supports MAX ram in Tera

BYTES

Ch@0s · 10-03-2007, 05:52 AM

Just map the I/O to addresses over the 4GB limit. There should be an option in the bios (memory hole/large memory whatever is called). Any 32bit OS with PAE should easily be able to address 4GB and beyond. However because of limitations of 32bit windows, an app will be limited to max of 3GB of memory. (2GB unless you use the /3GB switch in boot.ini)

teknoPhobia · 10-03-2007, 07:56 AM

Quote:

Originally Posted by shantanu_webmaster

as 64 bit OS supports MAX ram in TETRA BYTES

IS that something like when you take bites out of tetra packs?

FYI Its TERA not TETRA

djmykey · 12-03-2007, 07:53 AM

Ok in my office I gave a pentium d machine installed with 64 bit windows 2003 enterprise edition. But still it showed 3454 MB of RAM and in the BIOS it shows 4096 mb of ram. Any explaination that you can give ??

blackpearl · 12-03-2007, 05:03 PM

^^ Thats strange!! Maybe you should visit the source link I gave and go through the comments the readers posted. Readers have described different types of experiences and there are also some links from microsoft.com that addresses the issue. Maybe that would help you.