Technical Reference Guide
Compaq Deskpro EXS and Workstation 300 Personal Computers
Featuring the Intel Pentium 4 Processor
First Edition - December 2000
3-5
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.
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.
SDRAM
Memory
Controller
SDRAM Interface
64-bit Path
8 bytes @ 133 MHz = 1.0 GB/s
DIMM
Byte 0
Byte 1
Byte 2
Byte 3
Byte 4
Byte 5
Byte 6
Byte 7
RDRAM Interface (Dual Channel)
32-bit Path
4 bytes @ 400 MHz = 3.2 GB/s (see text)
RDRAM
Memory
Controller
Byte 0
Byte 1
Ch 1
RIMM
Byte 2
Byte 3
Ch 2
RIMM
