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+[\<- Mealy machines](22.md)
+
+---
+
+# Decoders and register files
+
+## Decoders
+
+### Decoding a multi-bit value
+
+- An AND gate to detect each possible encoding
+	- Output is 1-hot: exactly 1 output is asserted
+
+![diagram](23.1.png)
+
+### Decoder abstraction
+
+- As with other abstractions, once we understand/agree on the behavior, we don't need to draw the details
+- N inputs => 2^N outputs
+	- Each output is being driven by an AND gate
+
+![diagram](23.2.png)
+
+### Decoders w/ enables
+
+- Some applications may want to control if any output is asserted
+- Can be masked via an enable/disable
+
+![diagram](23.3.png)
+
+### A Mux built with a Decoder
+
+- Decode the select signals
+- Use the 1-hot outputs to "pass" the selected input
+
+![diagram](23.4.png)
+
+---
+
+## Register file concepts
+
+### Storing a value in a register
+
+- With a load enable, we can control how long a value stays in a register
+	- The register "remembers" the value until we assert the load enable to cause a new value to be captured in the register
+- If we have multiple values that we want to store, we need multiple registers
+- Having some strucuture/organization around these multiple registers can be very useful => a register file
+
+### Register file concepts
+
+- A single input bus for "writing", along with a control signal to control when to "write"
+- An output bus (sometimes two) for "reading"
+- A means for selecting which register
+	- Called an "address"
+	- Sometimes there can be separate addresses for reading and writing
+
+![diagram](23.5.png)
+
+---
+
+## Implementation fo a register file
+
+### Selecting a value to "read"
+
+- Four registers => 2-bit address
+
+![diagram](23.6.png)
+
+### Writing to a register
+
+- Assert load enable, but how?
+
+![diagram](23.7.png)
+
+### Decoders in a register file
+
+- In general, we only ever want to write to one register at any given time
+	- Load enables want to be 1-hot
+- Decoders convert an encoded input to a 1-hot output
+	- Addresses are encodings
+- Load enables must also account for a global write enable
+	- Use as an enable to the decoder
+
+---
+
+## Complex example register file
+
+- Four 8-bit registers, separate rd/wr addresses
+
+![diagram](23.8.png)
+
+### Summary
+
+- Input and output data buses are same size
+- Need at least one address bus and a write enable
+- Address and data bus widths are \*independent\* parameters (A, D)
+	- Address size depends on how many registers
+
+![diagram](23.9.png)