I have a computer which has an ASUS P5B motherboard and an Intel Pentium D
Processor and runs Windows XP.
It has 4GB of DDR2 RAM (4 x 1GB). All RAM chips are of the same type and
manufucturer. In BIOS setup, when i go to "system information" it shows that
the pc has 3GB of RAM instead of 4 and In Windows XP, in "System properties"
window (right click on my computer -> properties) it shows 2,93 GB of RAM.
But  in "System Information" (Start menu -> All programs -> Accessories ->
System tools -> System Information) It says
Total Physical Memory: 4,096.00 MB
Available Physical Memory: 2,44 GB
Why the computer only uses 3GB of RAM ?
Is there a maximum for the amount of RAM memory which can be
used in Windows XP ?
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A 32-bit operating system can address 4GB at most.  Some of this 4GB is unusable by you because the hardware needs the addresses for devices (BIOSes, controllers); these are usually located in high address space.  What is left over is available to the OS, but applications are limited to 2GB unless a /PAE or /3GB switch is in the boot.ini.

See http://www.microsoft.com/whdc/system/platform/server/PAE/PAEmem.mspx and

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I am no 'Expert' on memory but here's a way to think of it.

Think of memory addresses as phone numbers in a phone book.
For our puropse 1Gb = 1000 numbers.

Numbers 1 to 3000 (first 3 GB)
These are listed in the book if used.

Numbers 3001 to 4000 (4th Gb block)
Are numbers reserved for special uses and not 'listed' in the phone book even if used.
These 'phone numbers' are 'reserved' for this regardless of how many numbers are in the book.

Numbers 4001 and up (over 4 Gb)
Are available for use but they cannot be -listed- in the phone book.
(So if you wanna call one of these phones you have to already know the number.)


"system information"  
-- Can only show you -listed- phone -numbers-. (Memory addresses allocated for the operating system)
-- It does not show all the -phones- being used. (Memory actually being used.)
Gary CaseRetiredCommented:
(a)  XP systems with 4GB of memory "see" anywhere from the low 2+ GB (you're seeing with 2.93) up to about 3.5GB.  The upper end of the address space is used for a variety of reserved items -- BIOS shadowing, AGP aperture, I/O addresses, video shadowing, etc. -- and for these reasons systems with 4GB of installed memory never report a full 4GB of available memory.   Typically you'll "see" somewhere between 2.75gb and 3.5gb in systems that actually have 4GB installed (although it actually go below 2GB if you install 2 1GB video cards).

(b)  XP restricts the memory allocated to individual processes to 2GB, but uses any memory above for system processes that would have occupied lower memory if the system had 2GB or less of memory;  the /3GB switch will alter that balance -- but has NOTHING to do with "seeing" more memory in systems with 4GB of memory;

(c)  the /PAE switch, on systems that have chipset, CPU, and OS support for it, will allow you to use all of 4GB (or even more if the motherboard supports it) by remapping the additional memory above the 4GB barrier imposed by 32-bit addressing (it does this by adding 4-bits to the address registers and dynamically managing the mappings) => but it has NOTHING to do with allowing applications to use more than 2GB of virtual space (you have to use the /3GB switch for that).   Note that while the /PAE switch MAY (if your system supports it) allow you to use more of your installed memory; but does so at a cost in performance, since the CPU must then manage the 4 additional bits it uses for memory addressing.   I generally do not recommend using this switch.

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