
441
Chapter 8 Selecting a Processor
The LAPD benchmark results indicate that a 2/1 DRAM–only system is 27%
faster than a 3/2 DRAM–only system. The interleaved 3/1 DRAM–only system is
14% faster than the non–interleaved 3/2 DRAM–only system, and 23% faster for the
clock scaled systems based on the Am29040. The results for 3/1 and 3/2
DRAM–only systems are highlighted on Figure 8-8. Scalable Clocking enables the
Am29030 to be replaced with an Am29040 which improves the performance of the
3/2 system by 110%. With a 3/1 system, the performance is improved by 128%.
Note also that a system based on an Am29030 using 3/2 DRAM–only at 25
MHz, has 2% lower performance than an Am29030 using 2/1 DRAM–only at 20
MHz. Consequently, in this case, there is a performance advantage obtainable by
reducing the memory system operating frequency.
33 MHz Memory Systems
The performance of processors operating with 33 MHz memory systems is
shown in Figure 8-9. Scalable Clocking is no longer available with 33 MHz memory
systems. This places 33 MHz systems in competition with slower memory systems
using clock scaled processors. It is not likely that construction of a memory system
faster than 33 MHz would be practical. Clock rates such as 40 MHz would require
additional memory wait states. Consequently, the system performance would be no
faster than a slower memory system operating with fewer memory access wait states.
Only the very fastest DRAMs and support circuitry could achieve 1–cycle
burst–mode access without using an interleaved DRAM design. Achieving 2–cycle
initial access is also unlikely given the 31.25 ns Clock period. A 3–cycle (93.75 ns)
first access seems more likely. The results for 3/1 and 3/2 DRAM–only systems are
highlighted on Figure 8-9.
The complexity of building a 3/1 rather than a 3/2 memory system at 33 Mhz
must be justified by a sufficient performance gain. Using an Am29030 processor a
performance gain of 13.6% is shown. With an Am29040, the performance gain is
17.2%. However, using Scalable Clocking, an Am29040 (50 MHz) using 3/1 DRAM
at 25 MHz is 41% faster than an Am29040 using 3/2 DRAM at 33 MHz; and 20%
faster than an Am29040 using faster 3/1 DRAM at 33 MHz. An Am29040 operating
at 50 MHz with a 2/1 DRAM system operating at 25 MHz is the fastest DRAM–only
system benchmarked.
Additional Observations.
Setting the Am29050 aside due to its enhanced floating–point processing
capability, system performance is likely to be the main influence on processor
selection. However, if low power consumption, 3.3–volt operation, or
multi–processor cache consistency are important issues, then the Am29040
processor is the clear choice. Scalable Clocking technology clearly offers a major
performance enhancement, but it does require the purchase of a more expensive
processor. This may be offset by a simpler memory system design which uses less