HP DL360 Memory technology evolution: an overview of system memory technologie - Page 12

Advanced ECC memory To improve memory protection beyond standard ECC, HP introduced Advanced ECC technology in 1996. HP and most other server manufacturers use this solution in industry-standard products. Advanced ECC can correct a multi-bit error that occurs within one DRAM chip; thus, it can correct a complete DRAM chip failure. In Advanced ECC with 4-bit (x4) memory devices, each chip contributes four bits of data to the data word. The four bits from each chip are distributed across four ECC devices (one bit per ECC device), so that an error in one chip could produce up to four separate single-bit errors. Figure 9 shows how one ECC device receives four data bits from four DRAM chips. Figure 9. Advanced ECC Since each ECC device can correct single-bit errors, Advanced ECC can actually correct a multi-bit error that occurs within one DRAM chip. As a result, Advanced ECC provides device failure protection (Table 1). Table 1. Comparison of ECC and Advanced ECC error protection Error Condition Single-bit Double-bit DRAM failure ECC Outcome Correct Detect Detect Advanced ECC Outcome Correct Correct or detect Correct Although Advanced ECC provides failure protection, it can reliably correct multi-bit errors only when they occur within a single DRAM chip. Advanced ECC does not provide failover capability. As a result, if there is a memory failure, the system must be shut down before the memory can be replaced. The latest generation of HP ProLiant servers offers two levels of Advanced Memory Protection that provide increased fault tolerance for applications requiring higher levels of availability. 12