Reorganize website docs (#1806)

Updates to chisel3 documentation for website:
* guard code examples with mdoc and fix errors encountered along the way
* move some website content here vs splitting the content across two repos
* Bring in the interval-types and loading memories content so that it will be visible from the website
* remove all references to the wiki (deprecated)
* Remove reference to Wiki from the README
* fix tabbing and compile of chisel3-vs-chisel2 section
* Appendix: faqs now guarded and compile
* FAQs: move to resources section

1 files changed, 62 insertions, 0 deletions

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+---
+layout: docs
+title:  "Muxes and Input Selection"
+section: "chisel3"
+---
+
+# Muxes and Input Selection
+
+Selecting inputs is very useful in hardware description, and therefore Chisel provides several built-in generic input-selection implementations.
+
+### Mux
+The first one is `Mux`. This is a 2-input selector. Unlike the `Mux2` example which was presented previously, the built-in `Mux` allows
+the inputs (`in0` and `in1`) to be any datatype as long as they are the same subclass of `Data`.
+
+By using the functional module creation feature presented in the previous section, we can create multi-input selector in a simple way:
+
+```scala
+Mux(c1, a, Mux(c2, b, Mux(..., default)))
+```
+
+### MuxCase
+
+The nested `Mux` is not necessary since Chisel also provides the built-in `MuxCase`, which implements that exact feature.
+`MuxCase` is an n-way `Mux`, which can be used as follows:
+
+```scala
+MuxCase(default, Array(c1 -> a, c2 -> b, ...))
+```
+
+Where each selection dependency is represented as a tuple in a Scala
+array [ condition -> selected_input_port ].
+
+### MuxLookup
+Chisel also provides `MuxLookup` which is an n-way indexed multiplexer:
+
+```scala
+MuxLookup(idx, default,
+          Array(0.U -> a, 1.U -> b, ...))
+```
+
+This is the same as a `MuxCase`, where the conditions are all index based selection:
+
+```scala
+MuxCase(default,
+        Array((idx === 0.U) -> a,
+              (idx === 1.U) -> b, ...))
+```
+
+Note that the conditions/cases/selectors (eg. c1, c2) must be in parentheses.
+
+### Mux1H
+Another ```Mux``` utility is the one-hot mux, ```Mux1H```. It takes a sequence of selectors and values and returns the value associated with the one selector that is set. If zero or multiple selectors are set the behavior is undefined.  For example:
+
+```scala
+  val hotValue = chisel3.util.Mux1H(Seq(
+    io.selector(0) -> 2.U,
+    io.selector(1) -> 4.U,
+    io.selector(2) -> 8.U,
+    io.selector(4) -> 11.U,
+  ))
+```
+```Mux1H``` whenever possible generates *Firrtl* that is readily optimizable as low depth and/or tree.  This optimization is not possible when the values are of type ```FixedPoint``` or an aggregate type that contains ```FixedPoint```s and results instead as a simple ```Mux``` tree.  This behavior could be sub-optimal.  As ```FixedPoint``` is still *experimental* this behavior may change in the future.