Virtual memory in contemporary microprocessors

Virtual memory is a technique for managing the resource of physical memory. It gives an application the illusion of a very large amount of memory, typically much larger than what is actually available. It protects the code and data of user-level applications from the actions of other programs but also allows programs to share portions of their address spaces if desired. It supports the execution of processes partially resident in memory. Only the most recently used portions of a process’s address space actually occupy physical memory—the rest of the address space is stored on disk until needed. For a primer on virtual memory, see our companion article in Computer magazine. Most contemporary general-purpose processors support virtual memory through a hardware memory management unit (MMU) that translates virtual addresses to physical addresses. Unfortunately, the various microarchitectures define the virtual-memory interface differently, and, as explained in the next section, this is becoming a significant problem. Here, we consider the memory management designs of a sampling of six recent processors, focusing primarily on their architectural differences, and hint at optimizations that someone designing or porting system software might want to consider. We selected examples from the most popular commercial microarchitectures: the MIPS R10000, Alpha 21164, PowerPC 604, PA-8000, UltraSPARC-I, and Pentium II. Table 1 points out a few of their similarities by comparing their support for some core virtual-memory functions.