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History of RAM
Different types of RAM have been in common use during different time periods. It is important to understand them all, because many companies use a mix of legacy systems and more current ones. DRAM, or dynamic RAM, is one of the oldest types. It requires frequent refreshing of contents. DRAM stores each bit of data or program code in a storage cell consisting of a capacitor and a transistor, and is typically organized in a rectangular configuration of storage cells. A DRAM storage cell is dynamic in that it needs to be refreshed or given a new electronic charge every few milliseconds to compensate for charge leaks from the capacitor.
SRAM, or static RAM, solved the issue of the need for constant refreshing, but it is quite expensive to manufacture. Therefore, SRAM is only used for very high-speed needs, such as level 1, level 2, and level 3 CPU caches, hard drive buffers, and LCD screen buffers.
SDRAM, or synchronous DRAM, was the first memory that operated at the same speed as the motherboard’s bus. The initial chips were 168-pin dual in-line memory modules, which have their speed rated in MHz. Common naming conventions include PC-66 (which is equivalent to 66 Mhz), or PC-133, or PC-266.
While SDRAM performs at the same speed as the motherboard’s bus, the CPU operates many times faster than the motherboard’s bus. DDR SDRAM stands for double data rate synchronous dynamic random access memory. This type of memory can transfer data two times per clock cycle, thereby doubling the speed over traditional SDRAM. These chips come in 184-pin varieties, rated in MBps (throughput). A good example of this is something like PC-1600, which is 1600 MBps throughput.
DDR2 SDRAM is double double data rate SDRAM, which has a higher latency but faster access to the external bus. These chips have 240 pins to connect their modules to the slot, and are also rated in MBps, such as PC2-4200, which is 4200 MBps (or 4.2 GBps).
DDR3 SDRAM is known as double data rate 3 SDRAM, and is the latest generation of SDRAM in use today. It runs at a lower voltage and is faster than DDR2, but again the latency is increased over the previous DDR2 modules. These modules also have 240 pins, but they are “keyed” differently than DDR2. The “keyed” portion is where the gap in the pins is located, so only a DDR2 will fit in a DDR2 slot, and a DDR3 will fit only in a DDR3 slot. The common notation for these is PC3-10600, where the 10600 is the speed in MBps.
Rambus direct ram (RDRAM) is also known as RIMM (Rambus in-line memory module). These modules come in a 184-pin variant (16-bit version) and a 232-pin variant (32-bit version). Each empty slot in the motherboard has to have a continuity module (CRIMM) placed inside it, or the motherboard won’t recognize the memory properly. This is not used in recent systems, but was used heavily in Pentium 4 systems. The RDRAM is measured in MBps, such as RIMM 4200 (or 4200 MBps).
在不同時間段內，通常使用不同類型的RAM。了解所有這些信息非常重要，因為許多公司使用的都是舊系統和最新系統。 DRAM或動態RAM是最古老的類型之一。它需要經常刷新內容。 DRAM將數據或程序代碼的每一位存儲在由電容器和晶體管組成的存儲單元中，並且通常以矩形配置的存儲單元來組織。 DRAM存儲單元是動態的，因為它需要每幾毫秒刷新一次或給其提供新的電荷，以補償電容器的電荷洩漏。 SRAM或靜態RAM解決了持續刷新的問題，但製造成本很高。因此，SRAM僅用於非常高速的需求，例如1級，2級和3級CPU高速緩存，硬盤驅動器緩衝區和LCD屏幕緩衝區。 SDRAM或同步DRAM是第一個以與主板總線相同的速度運行的內存。最初的芯片是168針雙列直插式內存模塊，其速度額定為MHz。常見的命名約定包括PC-66（相當於66 Mhz），PC-133或PC-266。 儘管SDRAM的性能與主板總線相同，但CPU的運行速度比主板總線快許多倍。 DDR SDRAM代表雙倍數據速率同步動態隨機存取存儲器。這種類型的存儲器可以在每個時鐘週期兩次傳輸數據，從而使速度比傳統SDRAM快一倍。這些芯片有184引腳的型號，額定為MBps（吞吐量）。一個很好的例子就是PC-1600，吞吐量為1600 MBps。DDR2 SDRAM是雙倍數據速率SDRAM，具有更高的延遲，但對外部總線的訪問速度更快。這些芯片具有240個引腳以將其模塊連接到插槽，並且還以MBps額定，例如PC2-4200，即4200 MBps（或4.2 GBps）。 DDR3 SDRAM被稱為雙倍數據速率3 SDRAM，是當今使用的最新一代SDRAM。它在較低的電壓下運行，並且比DDR2更快，但是與之前的DDR2模塊相比，延遲又增加了。這些模塊還具有240針，但是它們的“鍵控”與DDR2不同。 “鍵控”部分是引腳之間的間隙所在的位置，因此只有DDR2可以插入DDR2插槽，而DDR3只能插入DDR3插槽。這些的常用符號是PC3-10600，其中10600是MBps的速度。 Rambus直接RAM（RDRAM）也被稱為RIMM（Rambus嵌入式存儲模塊）。這些模塊具有184針變體（16位版本）和232針變體（32位版本）。主板上的每個空插槽都必須在其中放置一個連續性模塊（CRIMM），否則主板將無法正確識別內存。在最近的系統中未使用此功能，但在Pentium 4系統中大量使用了此功能。 RDRAM以MBps為單位進行測量，例如RIMM 4200（或4200 MBps）。
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