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Unformatted text preview: fetch instructions as far ahead as the capacities of the various pipeline buffers allow. Address non-determin ism Instructions that need to generate a new memory address, such as data transfer instructions and unpredicted branches, calculate the new address in the execution pipeline and then send it to the address interface. Since it arrives with arbitrary timing relative to the internal incrementing loop within the address interface, the point of insertion of the new address into the address stream is non-deterministic, so the processor's depth of prefetch beyond a branch instruction is fundamentally non-deterministic. AMULET1 379 Register coherency If the execution pipeline is to work efficiently, the register file must be able to issue the operands for an instruction before the result of the previous instruction has returned. However, in some cases an operand may depend on a preceding result (this is a read-after-write hazard), in which case the register file cannot issue the operand until the result has returned unless there is a forwarding mechanism to supply the correct value further down the pipeline. The register forwarding mechanism used on many RISC processors (including ARMS and StrongARM) is based upon the characteristics of a synchronous pipeline since it involves comparing the source operand register number in one pipeline stage with the destination register number in another stage. In an asynchronous pipeline the stages are all moving at different times and such a comparison can only be made by introducing explicit synchronization between the stages concerned, thereby losing most of the benefits of asynchronous operation. On AMULET 1 register coherency is achieved through a novel form of register locking, based on a register lock FIFO (first-in-first-out queue). The destination register numbers are stored, in decoded form, in a FIFO, until the associated result is returned from the execution or memory pipeline to the register bank. The organization of the lock FIFO is illustrated in Figure 14.4. Each stage of the FIFO holds a ' 1' in the...
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This document was uploaded on 10/30/2011 for the course CSE 378 380 at SUNY Buffalo.
- Spring '09