Enigma

Most modern microprocessors provide hardware support for rapidly translating a program logical address to a system physical address (PA). Translation typically sits on the critical path of every memory access, since an access cannot usually be performed until after it has been translated. Enigma is a novel approach to address translation that defers the bulk of the work associated with address translation until data must be retrieved from physical memory. Enigma replaces the address translation unit that exists in each conventional core with a simpler unit to translate from the logical address space to a new intermediate address (IA) space. Intermediate addresses are unique across the entire system except where sharing is required or desired, and their use sidesteps the "synonym" problem present in logically tagged caches. All cache addressing, as well as I/O and coherence traffic, is carried out using IA. Enigma translates an IA to a PA only when no cache in the entire CMP can satisfy the request and memory or I/O must be accessed. A central translation unit attached to the system bus performs translations on IA that must be resolved to a PA. Deferring the bulk of address translation work and removing it from each individual processor core in this manner affords many benefits.