Why does 4GB memory XP only recognize 3GB?

My motherboard is ASUS A8N-SLI SE, the operating system is XP sp2, 4G memory is installed, 4G memory can be recognized when booting, but only 3G memory is recognized after entering the system. Is there any way to solve it, please advise, thank you!

Answer: 4G memory can only recognize 3G after entering the system is normal, due to the addressing capability of 32-bit system platform The upper limit is 4GB, and the last 1GB address space of 4GB is basically occupied by devices such as video cards, so the system cannot actually access the physical memory of the 3G~4G address segment. Specific to Windows XP 32Bit, it can only recognize up to 3.5GB of memory, and a single process can only use up to 2GB of space. Even the 32-bit version of Windows Vista can't take advantage of this addressing limitation, and only recognizes 3.5GB at most. What if I have bought 4GB of memory? You can play the virtual hard disk RamDisk, open a space from the memory as a hard disk to use very fast, and the swap space for BT download can protect the hard disk - the memory is not afraid of repeated reading and writing; the second is to install the 64-bit version Windows XP or Vista, although the software is very scarce, but can use 4GB of memory. It is recommended that the memory of non-professional users should not exceed 2GB, otherwise it will be wasted