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159<h1>Nand2tetris - 触发器，寄存器，RAM</h1>
160
161<blockquote>
162<p>本文是 nand2tetris 课程的读书笔记，不会列举过多细节，仅精炼出有意思的概念，供日后查阅。</p>
163</blockquote>
164
165<hr/>
166
167<p>存储的意思是，维持曾经有的状态。<br/>
168一旦有了“曾经”，我们的系统就有了时间概念。<br/>
169时钟信号为系统带来时间，触发器使系统能看一眼“曾经”。</p>
170
171<blockquote>
172<p>\(out(t) = in(t - 1)\)</p>
173</blockquote>
174
175<p>如果将输出端接回输入端，我们可以让 \(out(t)\) 保持无限长的时间：</p>
176
177<blockquote>
178<p>\(out(t) = out(t-1)\)</p>
179</blockquote>
180
181<p>如果给这个函数加上一个控制位，我们就得到了 1-bit 寄存器：</p>
182
183<blockquote>
184<p>\(out(t)=\begin{equation}<br/>
185\begin{cases}<br/>
186out(t-1)&amp;  load = 0\\<br/>
187in&amp; load = 1\\<br/>
188\end{cases}<br/>
189\end{equation}\)</p>
190</blockquote>
191
192<p>如果将它与加法器连在一起，我们就得到了计数器：</p>
193
194<blockquote>
195<p>\(out(t) = out(t-1) + 1\)</p>
196</blockquote>
197
198<p>基于触发器与反馈回路的结合，我们可以构造任意复杂的，具有状态的系统。</p>
199
200<p><img src="media/15471318129470/%E5%B1%8F%E5%B9%95%E5%BF%AB%E7%85%A7%202019-01-10%20%E4%B8%8B%E5%8D%8811.10.58.png" alt="屏幕快照 2019-01-10 下午11.10.58"/></p>
201
202<hr/>
203
204<p>用 1-bit 寄存器并联，我们得到了 n-bit register。<br/>
205用 k 个 n-bit register 并联，加上寻址逻辑电路，我们得到了 RAM 的基本架构。<br/>
206由简单到复杂演变，全依赖组件间的抽象架构。register 提供单个字的存储 <strong>服务</strong>，RAM 调用了这一服务，并对更上层模块提供了随机存储 <strong>服务</strong>。</p>
207
208<p>计算机技术日新月异，程序员很容易跟不上时代，这是真的吗？：）</p>
209
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