What is the maximum addressable memory?

"There are a number of factors in play here. With a 32 bit architecture the OS is be able to address 4 GB of memory. However, all of this may not be available for applications. For instance a Windows machine with 4 GB of memory will usually not be able to use more than roughly 3,5 GB for OS and applications as some of the address space is used to map hardware.

Also, Windows splits the 32 bit memory address space into two: 2 GB for kernel memory used by the OS and 2 GB for user space applications. I.e. per default each application will only be able to access 2 GB of memory. Windows can be configured to use 1 GB for kernel and 3 GB for user.

On top of that the actual hardware may set certain limitations. When your laptop specification says maximum memory is 2 GB it is most likely because that is the maximum the motherboard will support. It doesn't matter how much the OS is able to address. If the hardware will only recognize 2 GB then that is the limit on physical memory for the machine."