HP 117755-003 740 Vue d'ensemble - Page 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

Col

NOP

Data

64b

NOP

64b