![]() ![]() SDRAM uses a clock to pipeline memory accesses. DRAM technologies such as synchronous DRAM ( SDRAM) and double data rate ( DDR) SDRAM have been developed to overcome this problem. DRAM also fundamentally has lower throughput than SRAM, because it must refresh data periodically and after a read. DRAM must wait for charge to move (relatively) slowly from the capacitor to the bitline. DRAM latency is longer than that of SRAM because its bitline is not actively driven by a transistor. Generally, the more transistors a device has, the more area, power, and cost it requires. But flip-flops take at least 20 transistors to build. The data bit stored in a flip-flop is available immediately at its output. ![]() Table 5.6 shows a comparison of these three types of volatile memory. Flip-flops, SRAMs, and DRAMs are all volatile memories, but each has different area and delay characteristics. ![]()
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