JG82855GMESL7VN Datasheet, PDF (140/213 Pages) Intel Corporation – Intel® 855GM/855GME Chipset Graphics and Memory Controller Hub (GMCH)

Functional Description
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6.3.3
6.3.3.1
DDR SDRAM Performance Description
The overall system memory performance is controlled by the DDR SDRAM timing register,
pipelining depth used in GMCH, system memory speed grade and the type of DDR SDRAM used
in the system. Besides this, the exact performance in a system is also dependent on the total
system memory supported, external buffering and system memory array layout. The most
important contribution to overall performance by the system memory controller is to minimize the
latency required to initiate and complete requests to system memory, and to support the highest
possible bandwidth (full streaming, quick turn-arounds). One measure of performance is the total
flight time to complete a cache line request. A true discussion of performance really involves the
entire chipset, not just the system memory controller.
Data Integrity (ECC)
The GMCH supports single-bit Error Correcting Code (or Error Checking and Correcting) and
multiple-bit EC (Error Checking) on the Main Memory interface. The GMCH generate an 8-bit
code word for each 64-bit Qword of memory and performs two Qword writes at a time so two 8-
bit codes are sent with each write. Since the code word covers a full Qword, writes of less than a
Qword require a read-merge-write operation. Consider a Dword write to memory. In this case,
when in ECC mode, GMCH will read the Qword where the addressed Dword will be written,
merge in the new Dword, generate a code covering the new Qword and finally write the entire
Qword and code back to memory. Any correctable (single-bit) errors detected during the initial
Qword read are corrected before merging the new Dword. GMCH also supports another data
integrity mode, EC (Error Checking) mode. In this mode, the GMCH generates and stores a code
for each Qword of memory. It then checks the code for reads from memory but does not correct
any errors that are found.
6.4
Integrated Graphics Overview
The Intel 855GM/855GME GMCH provides a highly integrated graphics accelerator and PCI set
while allowing a flexible Integrated System Graphics solution.
Note: Intel 855GME GMCH can support an AGP discrete graphics controller.
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Datasheet