Memory Subsystem - Compaq Deskpro EXS Technical Reference Manual

3.3

MEMORY SUBSYSTEM

The 82850 MCH features Direct Rambus technology and supports two Rambus channels, each
channel supporting up to two Rambus DRAM modules (RIMMs). Direct Rambus technology
provides a significant improvement in performance over DRAM/SDRAM memory designs and
allows the system memory to keep pace with increasing processor performance. Rambus
technology implements RDRAM devices accessed over a channel specifically designed for high
speed operations.
As shown in Figure 3-3, the conventional DRAM-based memory interface with a transfer rate of
66, 100, or 133 MHz increases bandwidth by widening the data bus. With the current top speed of
133 MHz, a 64-bit SDRAM interface achieves a maximum transfer rate of 1.0 GB/s.
SDRAM
Memory
Controller
SDRAM Interface
8 bytes @ 133 MHz = 1.0 GB/s
Figure 3–3. SDRAM/RDRAM Bandwidth Comparison
Although a Rambus channel handles only two bytes per transfer, data is clocked on both the rising
and falling edges of the clock signal, allowing a 400-MHz clock to provide an effective speed of
800 MHz and resulting a transfer rate of 1.6 GB/s. Doubling the number of channels doubles the
throughput. These systems feature dual-channel RDRAM architecture that provides a maximum
transfer rate of 3.2 GB/s.
Byte 0
Byte 1
Byte 2
Byte 3
DIMM
Byte 4
Byte 5
Byte 6
Byte 7
64-bit Path
Compaq Deskpro EXS and Workstation 300 Personal Computers
First Edition - December 2000
Technical Reference Guide
Byte 0
Byte 1
RDRAM
Memory
Controller
Byte 2
Byte 3
RDRAM Interface (Dual Channel)
32-bit Path
4 bytes @ 400 MHz = 3.2 GB/s (see text)
Featuring the Intel Pentium 4 Processor
Ch 1
RIMM
Ch 2
RIMM
3-5