RAM

RAM：Radom Access Memory 随机存取存储器

Unlike the computer's processing chips, which are based on combinational logic, the computer's memory logic requires a clock-based sequential logic.。

这是课程的原话，也就是说对于存储器首先要建立时钟的概念。最简单的元素是flip-flop，即out(t) = out(t-1)，在课程里用的是DFF。

利用DFF就可以构建1-Bit Register，主要是加入了一个Load。其HDL实现有个点是，可以创建一个circle，并对默认元素dffout进行使用并赋值。

/**
 * 1-bit register:
 * If load[t] == 1 then out[t+1] = in[t]
 *                 else out does not change (out[t+1] = out[t])
 */

CHIP Bit {
    IN in, load;
    OUT out;

    PARTS:
    // Put your code here:
    Mux(a = dffout,b = in,sel = load,out = muxout);
    DFF(in = muxout,out =dffout,out = out);
}

创建了1-Bit Register就可以创建m-Bit Register，这里的m表示该寄存器的长度。

一个计算机可以有N个寄存器，那如果选择一个寄存器进行读写操作呢，这就用到了地址的概念。比如有8个寄存器，即RAM8，就可以利用3个字节的address[3]来选择寄存器，课程里强调，这里的赋值不是选择一个来赋值，而是全都赋值，但只有一个寄存器会被读入正确的值。其实现思路就是，先确认8个寄存器是否读入正确值，即定义8个load值loadA/.../loadH，这个可以用DMux8Way，然后依次用Register进行操作，如果涉及到输出，就再用Mux8Way16（假设m=16）就可以啦

/**
 * Memory of 8 registers, each 16 bit-wide. Out holds the value
 * stored at the memory location specified by address. If load==1, then 
 * the in value is loaded into the memory location specified by address 
 * (the loaded value will be emitted to out from the next time step onward).
 */

CHIP RAM8 {
    IN in[16], load, address[3];
    OUT out[16];

    PARTS:
    // Put your code here:
    
    DMux8Way(in=load, sel=address, a=loadA, b=loadB, c=loadC, d=loadD, e=loadE, f=loadF, g=loadG, h=loadH);

    Register(in = in,load = loadA,out = outA);
    Register(in = in,load = loadB,out = outB);
    Register(in = in,load = loadC,out = outC);
    Register(in = in,load = loadD,out = outD);
    Register(in = in,load = loadE,out = outE);
    Register(in = in,load = loadF,out = outF);
    Register(in = in,load = loadG,out = outG);
    Register(in = in,load = loadH,out = outH);
    Mux8Way16(a=outA, b=outB, c=outC, d=outD, e=outE, f=outF, g=outG, h=outH, sel = address,out = out);
}

如果是更多的寄存器数目，比如RAM64，其地址为address[6]，就可以用地址为address[3]的RAM8来操作了

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