A virtual cache-based workstation architecture

The virtual cache-based architecture of the Apollo DN4000 workstation is presented. The architecture is discussed in detail, including the main factors that influenced its final design. The influences of the operating system on the cache architecture are discussed, as well as the subsequent influences of the cache on the operating system to maintain cache coherency. It is shown how the method of assessing objects in Apollo´s system has led to a write-allocate cache update policy to solve the virtual address synonym problem. It is also shown how maintaining cache coherency requires special consideration by the operating system during virtual-to-physical address mapping changes, context switches, modification of data without using virtual addresses (e.g. DMA), and processor modification of page-table used bits. The primary design goal of the DN4000 was to double the CPU performance of the existing Apollo DN3000. Performance measurements indicate that use of a virtually tagged cache and a virtual write buffer lead to achievement of this goal