Such use of the pc is not often beneficial so the arm

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Unformatted text preview: inst is constant for a given program (compiled 5-stage pipeline ARM organization 79 with a given compiler using a given set of optimizations, and so on) there are only two ways to increase performance: Increase the clock rate, fclk. This requires the logic in each pipeline stage to be simplified and, therefore, the number of pipeline stages to be increased. Reduce the average number of clock cycles per instruction, CPI. This requires either that instructions which occupy more than one pipeline slot in a 3-stage pipeline ARM are re-implemented to occupy fewer slots, or that pipeline stalls caused by dependencies between instructions are reduced, or a combination of both. Memory bottleneck The fundamental problem with reducing the CPI relative to a 3-stage core is related to the von Neumann bottleneck - any stored-program computer with a single instruction and data memory will have its performance limited by the available memory bandwidth. A 3-stage ARM core accesses memory on (almost) every clock cycle either to fetch an instruction or to transfer data. Simply tightening up on the few cycles where the memory is not used will yield only a small performance gain. To get a significantly better CPI the memory system must deliver more than one value in each clock cycle either by delivering more than 32 bits per cycle from a single memory or by having separate memories for instruction and data accesses. As a result of the above issues, higher performance ARM cores employ a 5-stage pipeline and have separate instruction and data memories. Breaking instruction execution down into five components rather than three reduces the maximum work which must be completed in a clock cycle, and hence allows a higher clock frequency to be used (provided that other system components, and particularly the instruction memory, are also redesigned to operate at this higher clock rate). The separate instruction and data memories (which may be separate caches connected to a unified instruction and data main memory) allow a significant reduction in the core's CPI. A typical 5-stage ARM pipeline is that employed in the ARM9TDMI. The organization of the ARM9TDMI is illus...
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