Branch Search / Instruction Pre-decoding
Eight-stage superpipeline structure of SH-X2
Figure 3.19 illustrates the pipeline structure of the SH-X2. The I3 stage was
added and performs branch search and instruction predecoding. Then the ID stage
timing was relaxed, and the achievable frequency increased.
Another critical timing path was in first-level (L1) memory access logic. SH-X
had L1 memories of a local memory and I- and D-caches, and the local memory was
unified for both instruction and data accesses. Since all the memories could not be
placed closely, a memory separation for instruction and data was good to relax the
critical timing path. Therefore, the SH-X2 separated the unified L1 local memory of
the SH-X into instruction and data local memories (ILRAM and OLRAM).
With the other various timing tuning, the SH-X2 achieved 800 MHz using a
90-nm generic process from the SH-X's 400 MHz using a 130-nm process. The
improvement was far higher than the process porting effect.
Low-Power Technologies of SH-X2
The SH-X2 enhanced the low-power technologies from that of the SH-X explained in
Sect. 220.127.116.11 . Figure 3.20 shows the clock-gating method of the SH-X2. The D-drivers
also gate the clock with the signals dynamically generated by hardware, and the leaf
F/Fs requires no CCP. As a result, the clock tree and total powers are 14% and 10%
lower, respectively, than in the SH-X method.
The SH-X2 adopted a way prediction method to the instruction cache. The SH-X2
aggressively fetched the instructions using branch prediction and early-stage branch
techniques to compensate branch penalty caused by long pipeline. The power con-
sumption of the instruction cache reached 17% of the SH-X2, and the 64% of the
instruction cache power was consumed by data arrays. The way prediction misses
were less than 1% in most cases and were 0% for the Dhrystone 2.1. Then the 56%
of the array access was eliminated by the prediction for the Dhrystone. As a result,
the instruction cache power was reduced by 33%, and the SH-X2 power was reduced