A complete overview of the SunOS virtual memory (VM) system is beyond the scope of this book, but two virtual memory terms of special importance are used when discussing device drivers: virtual address and address space.
Virtual address - A virtual address is an address that is mapped by the memory management unit (MMU) to a physical hardware address. All addresses directly accessible by the driver are kernel virtual addresses; they refer to the kernel address space.
Address space - An address space is a set of virtual address segments, each of which is a contiguous range of virtual addresses. Each user process has an address space called the user address space. The kernel has its own address space called the kernel address space.