Verifying advanced microarchitectures that support speculation and exceptions

Abstract

In this paper, we discuss the verification of a microprocessor involving a reorder buffer, a store buffer, speculative execution and exceptions at the microarchitectural level. We extend the earlier proposed Completion Functions Approach [HSG98] in a uniform manner to handle the verification of such microarchitectures. The key extension to our previous work was in systematically extending the abstraction map to accommodate the possibility of all the pending instructions being squashed. An interesting detail that arises in doing so is how the commutativity obligation for the program counter is proved despite the program counter being updated by both the instruction fetch stage (when a speculative branch may be entertained) and the retirement stage (when the speculation may be discovered to be incorrect). Another interesting detail pertains to how store buffers are handled. We highlight a new type of invariant in this work-one which keeps correspondence between store buffer pointers and reorder buffer pointers. All these results, taken together with the features handled using the completion functions approach in our earlier published work [HSG98,HSG99,HGS99], demonstrates that the approach is uniformly applicable to a wide variety of pipelined designs.