HP 153636-001 - NeoServer - 150 Übersicht - Seite 6
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additional data sections are accessed with every clock cycle after the first access (6-1-1-1) before the
memory controller has to send another CAS.
Figure 4. Burst mode access. NOP is a "No Operation" instruction.
Clock
Command
Active
Address
Row
Data
SDRAM technology
FPM and EDO DRAMs are controlled asynchronously, that is, without a memory bus clock. The
memory controller determined when to assert signals and when to expect data based on absolute
timing. The inefficiencies of transferring data between a synchronous system bus and an
asynchronous memory bus resulted in longer latency.
Consequently, JEDEC—the electronics industry standards agency for memory devices and modules—
developed the synchronous DRAM standard to reduce the number of system clock cycles required to
read or write data. SDRAM uses a memory bus clock to synchronize the input and output signals on
the memory chip. This simplified the memory controller and reduced the latency from CPU to memory.
In addition to synchronous operation and burst mode access, SDRAM has other features that
accelerate data retrieval and increase memory capacity—multiple memory banks, greater bandwidth,
and register logic chips. Figure 5 shows SDRAM DIMMs with two key notches that prevent incorrect
insertion and indicate a particular feature of the module.
Figure 5. SDRAM DIMM with two notches
Read
NOP
NOP
NOP
Col
NOP
NOP
NOP
NOP
Data
Data
Data
Data
64b
64b
64b
NOP
64b
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