System Cost: Memory Granularity
An RDRAM system has a very compelling memory advantage over
competing SDRAM or DDR-based systems. Since a single RDRAM device can act as an
entire memory subsystem, the granularity of the memory footprint is a single
device: 32 MB for today’s 256 Mb densities of RDRAM and 64 MB for tomorrow’s
512 M bit generation. 64-bit DDR systems on the other hand, have a minimum
granularity of 128 MB using x16 devices in 256 M bit technology. The
granularity problem is illustrated in the table. Applications such as networking,
communications and consumer electronics can take advantage of RDRAM memory’s
single device performance to optimize the total memory footprint while
providing the highest system performance.
Packaging- An Overview
Rambus systems can be
classified either as short Channel systems or as long Channel systems depending
on the electrical length of the Rambus Channel. Long Channel systems typically
have eight or more RDRAMs per Channel and consist of a motherboard and one or
more memory modules. Such an arrangement allows the user to expand the memory
size either by increasing the number of modules plugged into the motherboard or
by using higher capacity modules. The impedance of long Channel systems is
generally constrained to 28_ because of the need to conform to the impedance of
available memory modules. Short Channel systems typically have fewer than five
RDRAMs per Channel and are implemented on a common motherboard. An example of a
short Channel system is a graphics controller which uses RDRAMs for the frame
buffer.
Introduction
In this quest for speed, most of
the attention is focused on the microprocessor. But a PC’s memory is equally
important in supporting the new capabilities of visual computing. And commodity
Dynamic RAMs (DRAMs), the mainstay of PC memory architecture, have fallen
behind the microprocessor in their ability to handle data in the volume
necessary to support complex graphics. While device densities have increased by
nearly six orders of magnitude, DRAM access times have only improved by 10.
Over the same time, microprocessor performance has jumped by a factor of 100.
In other words, while bus frequency has evolved from 33 MHz for EDO to the
current standard of 100 Mhz for SDRAMs and up to 133 MHz for the latest PC-133 specification,
memory speed has been out spaced by the operation frequency of the
microprocessor which reached 600 MHz plus by the turn of the century. Thus, the
memory subsystem risked to become a bottleneck for overall system performance
or had created a significant performance gap between computing elements and
their associated memory devices.
The High-Speed Memory Interface
The Direct Rambus™ ASIC Cell (Direct RAC) is a library
macrocell used in ASIC designs to interface the core logic of a CMOS ASIC device
to a high-speed Direct Rambus Channel. The Direct RAC incorporates all of the
high-speed interface circuitry and logic necessary to provide the ASIC designer
full access and control over the Direct Rambus Channel without forcing any
particular design implementation. The Direct RAC is flexible enough to be used
for implementations ranging from simple memory controllers, complex multi-port
memory controllers, or as a communication path for high speed chip-to-chip
interface.
Performance
Memory performance is usually summarized by two measures:
bandwidth and latency. Peak system bandwidth can be defined as the maximum
transfer rate of a particular memory technology given ideal conditions. Effects
particular to each memory system degrade this peak bandwidth to a lower number
called effective bandwidth. Effective bandwidth can be expressed as the product
of peak bandwidth and efficiency, a measure of the ability of the memory system
to deliver on its peak bandwidth given real world memory transaction patterns.
In addition to examining the pitfalls of bandwidth measures, it is also
important to look at latency. Devices are usually characterized by component
latency, which is the time between row strobe and the delivery of the first bit
of data on the output pin for an individual component.
Abstract
During the last two decades,
there has been an exponential growth in the
operational speed of microprocessors. Also RAM capacities have been
improving at more than fifty percent per year. However the speed and access
time of the memory have been improving at slower rate. In order to keep up in
performance and reliability with processor technology it is necessary to make
considerable improvements in the memory access time.
Conclusion
RDRAM memory ensures that the computing experience is
unbounded by memory and increases the lifecycle of systems.
No comments:
Post a Comment