Instruction fetch unit with early instruction fetch mechanism

Abstract

An instruction fetch unit in which an early instruction fetch is initiated to access a main memory simultaneously with checking a cache for the desired instruction. On a slow path to main memory is a large main translation lookaside buffer (TLB) that holds address translations. On a fast path is a smaller translation write buffer (TWB), a mini-TLB, that holds recently used address translations. A guess fetch access in initiated by presenting an address to the main memory in parallel with presenting the address to the cache. The address is compared with the contents of the TWB for a hit and with the contents of the cache for a hit. The guess access is allowed to proceed upon the condition that there is a hit in the TWB (the TWB is able to translate the logical address into a physical address) and a miss in the I-cache (the data are not available in the I-cache and hence the guess access of main memory is necessary to get the data). The guess access is canceled upon the condition that there is either a miss in the TWB (the TWB is unable to translate the logical address into a physical address) or a hit in the I-cache (the data are available in the I-cache and hence the guess access of main memory is not necessary). In this case a fetch access is reissued on the 'slow' path that goes through the large main TLB.