Memory

不同的方法来组织内存，如：ROM和RAM。

• Registers can store bits temporarily.

– But it is costly to use registers to store large amount of data.

– Key advantages of registers for data storage are speed and easy-of-use.

use memory storage devices to store data in higher density (capacity) and over longer time (volatility：断电也能保留存储内容).

Memory Sizes

内存大小通常使用2的幂来指定

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尽管通常在SI单位中使用10的幂

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IEC -国际电工技术委员会-建议对2的幂使用单独的术语，以避免通信中的混淆。

General Model of Memory Device

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control用来控制读写或者电源

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读/写头必须从当前位置传输到新地址，中间要经过其他地址，这很耗时。

适合于大量数据存储而不适合快速读写

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Read/Write Memory

读/写内存(RWM)的特点是能够随时支持存储和检索

Most RWMs are random-access memory (RAM)

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Most RAMs are volatile, meaning that they lose their memory when power is removed

Two main types:

– Static RAM (SRAM) – once word written at a location it remains stored until chip power removed or bit overwritten.

– Dynamic RAM (DRAM) – data stored at each location must be refreshed periodically by reading it and writing it back again or else it disappears.

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Dynamic RAM (DRAM)

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为了构建具有更高密度(每个芯片有更多比特)的ram，芯片设计师发明了每比特只使用一个晶体管的存储单元。基于dram的内存系统使用刷新周期定期更新每个内存单元。这涉及到将每个单元中有些退化的内容依次读入D-latch

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写5V(‘1’):先column设为5v,然后row设为5V, 最后等电容器电压到达5V后row设为0v

读取数据：

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Bit line (i.e. column) is first pre-charged to a voltage half-wave between HIGH and LOW (e.g. 2.5 V).

A sense amplifier detects this small change and recovers a ‘1‘ or ‘0’, respectively

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• set column to 2.5 V

• assert row to turn transistor ON

• sense voltage raise ‘1’ stored; otherwise ‘0‘

• bit latched

• write back: set column = Q

• output data OE = ‘1’

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Write down the sequence of data and control signals to

  1. write “10” to cells w and x;

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  2. read cells y and z;

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  3. copy cells y and z to cells w and x

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