An area model for on-chip memories and its application

An area model suitable for comparing data buffers of different organizations (e.g. caches versus register files) and arbitrary sizes is described. The area model considers the supplied bandwidth of a memory cell and includes such buffer overhead as control logic, driver logic and tag storage. The model gave less than 10% error when verified against real caches and register files. It is shown that, comparing caches and register files in terms of area for the same storage capacity, caches generally occupy more area per bit than register files for small caches because the overhead dominates the cache area at these sizes. For larger caches, the smaller storage cells in the cache provide a smaller total cache area per bit than the register set. Studying cache performance (traffic ratio) as a function of area, it is shown that, for small caches (less than the area occupied by 256 registers bits-r.b.e.-or 32 b), direct-mapped caches perform significantly better than four-way set-associative caches and, for caches of medium areas (between 256 r.b.e. and 4096 r.b.e.), both direct-mapped and set-associative caches perform better than fully associative caches.>