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Register File [TOC]
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In this step, D flip-flops should be used to implement 4-bit-wide registers.
The "Register File" module should internally possess four such registers and
enable writing data into a register or reading data from two registers. A
possible layout for this module is depicted in the following diagram:

---- IMAGE (width=600) -----------
session04/s04_4.png
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Here's a brief description of how this module can be used:

- The input `addr_in` selects a destination register (`%0`, `%1`, `%2`, or
  `%3`). When there's a rising edge of the clock signal, the value of `data_in`
  is written into this register.
- `addr1_out` determines which register content is connected to `data1_out` and
  can thus be read.
- `addr2_out` determines which register content is connected to `data2_out` and
  can thus be read.

As a guide, here's a circuit showing a register file with two 4-bit registers,
where only one register can be read at a time. This demonstrates how
multiplexers and demultiplexers can be used in this task:

---- IMAGE (width=600) -----------
session04/s04_5.png
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