Logisim Register File


In this assignment, you will build a register file with four 4-bit registers. This register file will reappear along with your ALU from the previous assignment later in the semester when we construct an entire processor datapath in Logisim.


Your grade for this assignment will be determined by how many tests your register file implementation can pass.

Additional consideration will be given to the clarity of organization within your circuits. Wires, gates, and pins should be laid out in a fashion that facilitates comprehension.

Getting Started

Download the starter file registers.circ and complete all of your implementation in this file.

Please do not move or rename any input and output pins in the starter file—this makes it difficult for us to test your implementation.

Problem 1: A 4-bit register

In the “4-bit register” subcircuit, build a four-bit register from four D flip-flops. Use Logisim’s built-in D flip-flop components, which are available in the explorer pane’s “Memory” category. Your four bit register must have the following inputs and outputs:

output (4 bits)
The four bits currently stored in the register
data (4 bits)
The four bits that should be stored in the register
write_enable (1 bit)
When true, the value on the data input should be written to the register on a falling clock edge.
clock (1 bit)
The clock input

Writing to the register should be controlled by the clock signal with write_enable connected to the flip-flop’s “Enable” pin en (on the bottom of the flip-flop). The value from data should be stored on the falling clock edge, only when write_enable is high. Logisim defaults to rising-edge flip-flops, so you will need to change this in the flip-flop’s attribute panel.

Problem 2: A 4x4 register file

In the “4x4 register” subcircuit, build a register file consisting of four copies of your “4-bit register” subcircuit. The register file has the following inputs and outputs:

data (4 bits)
The values to store in the register specified by the write index
output (4 bits)
The values stored in the register specified by the read index
read (2 bits)
The index of the register to display on output
write (2 bits)
The index of the register that should be written when write_enable is set
write_enable (1 bit)
A control line to determine whether the value on data should be written to the register specified by the write index.
clock (1 bit)
The clock signal

Be sure to thoroughly test your logic circuit before submitting your work. Unfortunately, because this is a sequential logic element, the simple truth-table based test mechanisms used previously no longer work.

Copyright © 2019, 2020, 2022 Janet Davis, Charlie Curtsinger, and Jerod Weinman

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