2、 Digital logic functional unit

（ One ） Introduction to functional units
1. Buffer door
Equivalent to a wire . But in the real world , Its function is to improve the fan out ability of the input signal .

2. Not gate
3. Tristate gate （ Equivalent to a buffer door that can control on-off ）
Two inputs , An output ;

4. A little Wait and door , Or gate , NAND gate ……

（ Two ） Small scale integration portfolio

1. multiplexer
According to the input of the control port , Select one of the multiple data input ports to connect with the output port , Time division multiplexing data channel .

2. Encoder
Concept ： The completion is to integrate multiple input signals , The output is to convert them into a set of binary code . Unlike multiplexing , The multiplexed output is selected from one of the inputs ; But the encoder is converted into other binary outputs according to the combination of inputs .

The above figure is common 4 Input 2 Output ordinary encoder . It can be used to encode the address of the storage unit .
So what is the priority encoder we often mention ？
It is to encode according to the priority sequence of pre-set events ：

Apply to the state of the event handled by the encoder ： Interrupt response , Process scheduling .
3. Decoder
The decoder is the opposite of the encoder , The encoder mentioned above is used for the process of converting multiple inputs into binary code . The decoder is a process of binary code restoration . For example, a two input four output decoder ：（ function ： Address the task of getting chip selection signal ）

4. adder
A. Half adder ：
Input port ：2 Output ： Carry and result . The combination of XOR gate and and gate .
B. Full adder
Consider the carry of status .
Input port : 3 Output ： Carry and result .

Because the full adder takes into account the carry of the low order operation , So we realize the addition of two bus data by cascading multiple full adders .

This is a relatively inefficient implementation .
Therefore, carry selection adder and carry ahead adder are introduced .
C. Carry select adder

D… Carry select adder
First generate the circuit through the circuit .
5. Lookup table （ There are many inputs , Think of it as an address line composed of several inputs ）

Look up table is actually omnipotent , Because it can be used to realize arbitrary combinatorial logic . such as N Input ,M Output , We can decompose it into M individual N Input 1 This kind of combinational logic output .
however LUT It is a method of combinatorial logic that consumes a lot of resources .

How to use it? ？ After finding the address line , We can multiplex （ Multiple input select one of the inputs ）, Select one of the address units in this way .

But if there are many table items , We'll just use it RAM.