The Double Data Rate (DDR2) memory interface is a dual-channel interface which supports up to 4GB of memory and supports memory transfer rates of 400MHz, 533MHz and 667MHz. Each of the channels (A and B) terminates at a dual in-line memory module (DIMM) socket. The System BIOS automatically detects memory type, size and speed.

The SHB uses industry standard gold finger memory modules, which must be PC2-3200, PC2-4200 or PC2-5300 compliant and have the following features:

• Gold-plated contacts
• Non-ECC (64-bit) DDR2 memory
• Unbuffered configuration

The following DIMM sizes are supported:

FSB	DIMM Type	Width	Component Density
1066	PC2-4200	x8, x16	256MB, 512MB, 1GB
1066	PC2-5300	x8, x16	256MB, 512MB
800	PC2-3200	x8, x16	256MB, 512MB, 1GB
800	PC2-4200	x8, x16	256MB, 512MB, 1GB
800	PC2-5300	x8, x16	256MB, 512MB
533	PC2-3200	x8, x16	256MB, 512MB, 1GB
533	PC2-4200	x8, x16	256MB, 512MB, 1GB

NOTE: To maximize system performance and reliability, Chassis Plans recommends using DIMMs that support the Serial Presence Detect (SPD) data structure. All memory modules must have gold contacts.

Memory modules can be installed in one or both DIMM sockets. If only one DIMM module is used, it may be populated in either DIMM socket (BK1 or BK2). To operate at maximum bandwidth, two DIMMs of the same size must be installed, but the DIMMs may differ in technology (component density) and/or device width.

The SHB provides two types of memory operation, depending on how the DIMMs are populated. Asymmetric mode occurs when only one DIMM is installed or when two DIMMs are installed but differ in size. In the latter case, memory addressing begins with channel A, and when the top of channel A is reached, addressing starts at the bottom of channel B. The total system memory is the total installed in both channels, but accesses occur only at a width of 64 bits.

Interleave mode enables the highest memory interface speed and bandwidth throughput capacity. This is achieved by using two DIMM modules of the same memory size, although the DIMMs may vary in technology and/or device width. If the DIMMs are of different speeds, the slower memory module determines the memory interface speed.

For example, with a single PC2-5300 DIMM installed, the memory interface operates with a theoretical memory bandwidth of up to 5.4GB/s. If two PC2-5300 DIMMs which are identical in size are installed, they operate in interleave mode, where both DIMM channels are accessed simultaneously for a 128-bit wide memory subsystem access. This theoretically doubles the memory interface bandwidth to 10.7GB/s.

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