## 未分配的寄存器(Unassigned register)

### 一、简介

SpinalHDL将会检查所有设计中有用的寄存器都被分配了。

### 二、例子

下述代码：
```Scala
class TopLevel extends Component {
  val result = out(UInt(8 bits))
  val a = Reg(UInt(8 bits))
  result := a
}
```

会报错：
```
UNASSIGNED REGISTER (toplevel/a :  UInt[8 bits]), defined at
  ***
  Source file location of the toplevel/a definition via the stack trace
  ***
```
可以修复为：
```Scala
class TopLevel extends Component {
  val result = out(UInt(8 bits))
  val a = Reg(UInt(8 bits))
  a := 42
  result := a
}
```
### 三、只有初始化的寄存器(Register with only init)

在某些情况下, 可能由于硬件参数化, 生成只有`init`描述但无分配的寄存器也是有意义的。
```Scala
class TopLevel extends Component {
  val result = out(UInt(8 bits))
  val a = Reg(UInt(8 bits)) init(42)

  if(something)
    a := somethingElse  //有问题，这个可综合
  result := a
}
```
会报错：
```
UNASSIGNED REGISTER (toplevel/a :  UInt[8 bits]), defined at
  ***
  Source file location of the toplevel/a definition via the stack trace
  ***
```
为了修复它, 可以让SpinalHDL转换该只有`init`描述但未分配的寄存器为一个组合逻辑：
```Scala
class TopLevel extends Component {
  val result = out(UInt(8 bits))
  val a = Reg(UInt(8 bits)).init(42).allowUnsetRegToAvoidLatch

  if(something)
    a := somethingElse
  result := a
}
```