JOS Lab 2: Memory Management

代码说明：

The first component is a physical memory allocator for the kernel, so that the kernel can allocate memory and later free it. Your allocator will operate in units of 4096 bytes, called pages. Your task will be to maintain data structures that record which physical pages are free and which are allocated, and how many processes are sharing each allocated page. You will also write the routines to allocate and free pages of memory. The second component of memory management is virtual memory, which maps the virtual addresses used by kernel and user software to addresses in physical memory. The x86 hardware memory management unit (MMU) performs the mapping when instructions use memory, consulting a set of page tables. You will modify JOS to set up the MMU page tables according to a specification we provide.