Currently the 32 bit x86 architecture is the most popular type of computer. In this architecture, traditionally the Linux kernel has split the 4GB of virtual memory into 3GB for user programs and 1GB for the kernel.  However, many people insist on using more than 1GB of physical memory on such a 32 bit system.  This makes it necessary for the Linux kernel to jump through some interesting hoops...


Note: 
32-bit architectures can reference 4 GB of physical memory
(2^32). Processors that have an MMU (Memory Management Unit) support
the concept of virtual memory: page tables are set up by the kernel
which map "virtual addresses" to "physical addresses"; this basically
means that each process can access 4 GB of memory, thinking it's the
only process running on the machine (much like multi-tasking, in which
each process is made to think that it's the only process executing on
a CPU).
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CategoryReadAhead