Computer Design Project
ELEC 5200/6200-Computer Architecture and Design
Spring 2008
Vishwani D. Agrawal
James J.Danaher Professor
Department of Electrical and Computer Engineering
Auburn University, Auburn, AL 36849.
http://www.eng.auburn.edu/~vagrawal
vagrawal@eng.auburn.edu
Mar. 10, 2008
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CPU Design Project (Assigned Monday,
Mar. 10, 2008)
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A small RISC CPU is to be designed in the VHDL modeling
language, verified via Mentor Graphics "ModelSim EE" simulator
and implemented on the DE2 FPGA board from Altera using
Quartus II software.
Project can be executed individually or in a team of two.
The project consists of five parts:
 Part 1 - ISA, report due Monday, 3/24/08
 Part 2 - Datapath, report due Monday, 3/31/08
 Part 3 - Datapath Verification, report due Monday, 4/7/08
 Part 4 - Control Unit, report due Monday, 4/14/08
 Part 5 - FPGA Implementation, demo and report due Monday,
4/21/08
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CPU Design Project- Part 1 - ISA
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An instruction set architecture (ISA) for a new
microprocessor (μP) is to be designed.
ISA is to be designed using RISC design
principles, with primary design goals being
low cost and a minimal number of clock
cycles per instruction.
ISA may contain no more than 16 unique
instructions. However, you may have multiple
formats for a given type of instruction, if
necessary.
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Of the 16 instructions, one instruction
should be HALT instruction.
Need not use the pointers like stack pointer,
global pointer, etc.
The ISA is to support 16-bit data words only.
(No byte operands.)
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All operands are to be 16-bit signed integers (2’s
complement).
Each instruction must be encoded using one 16-bit
word.
The ISA is to support linear addressing of
8K, 16-bit words memory. The memory is to
be word-addressable only - not byteaddressable.
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The ISA should contain appropriate numbers and
types of user-programmable registers to support it.
The ISA must “support” the following C
Programming Language constructs:
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Assignment operator: variable = expression;
Expressions must support only two arithmetic operators: add
(+) and subtract (-). Multiply(*) and divide(/) not necessary
to implement.
Expressions must support Logical operators: And and Or.
Data are limited to: a) 16-bit two’s-complement integers
(Example: int a;) , b) One-dimensional integer arrays
(Example: int a[10];)
Control flow structures: “if-else” structures, “while” loops, “for”
loops
These should support the six standard relational operators:
==, !=, >, <=, <, >=
Functions (call and return), with parameters able to be
passed by value or by reference.
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Report:
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List and description of the user-programmable registers.
List and description of the different instruction formats used.
For each instruction in your instruction set, list the following:
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Assembly language for each form of the instruction - mnemonic
and operands.
Machine language for each form of the instruction - instruction code
format, op-code, and operand encoding
Justification for including each form of the instruction in your ISA
For each C construct listed, provide an example showing how
the construct would be “compiled”, i.e. implemented with your
instruction set, by writing an example of the C construct and
the corresponding assembly language (AL) implementation.
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CPU Design Project- Part 2 - Datapath
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Include the following in your report:
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A block diagram (register level) of the datapath, with all
components and control signals clearly labeled.
A description of the function of each component in the
datapath.
For each instruction of your ISA, list the register transfers, or
sequence of register transfers, required to fetch and execute
the instruction. Register names should correspond to
components in your datapath diagram. For example, add
instruction will have following register transfers:
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Fetch : IR <= Memory [PC], PC <= PC + 1;
Decode: A <= Register (IR[11:8]), B <= Register (IR[7:4]), ALUOut
<= PC + Sign Extend [IR(3-0)]
Execute: ALUOut <= A + B
Write Register: Register (IR[11:8]) <= ALUOut
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Part 2- Datapath Report (cont.)
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A discussion of the tradeoffs and other design decisions
made in developing your datapath. This should include:
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Cost vs. speed tradeoffs that you considered.
Why you chose a single-cycle or multi-cycle design.
Decisions related to “shared” and/or “dedicated”
components.
Selection of edge-triggered vs. latching registers.
Other decisions that were considered.
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CPU Design Project- Part 3 - Datapath
Verification
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Develop and verify a VHDL model of the datapath of your
CPU, as described in the block diagram and register transfers
defined in Part 2.
The CPU must be capable of working with a single memory
outside the CPU; the memory for Logic Simulation in
ModelSim will be added in Part 4, which will be the RAM block
in the Altera Megafunctions Library.
This is to be a register-transfer-level (RTL) design (not gate
level).
The datapath must have following external and internal ports:
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External ports: 16-bit bidirectional databus and address bus,
various control and status signals, 16-bit port named “outvalue”
which is used to display the contents of the particular register of
the 16 registers in the final part of the project,
Internal port: 4-bit port named “inr” which is the register number
of the particular register you want to select to display its content.
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Ports “outvalue” and “inr” along with clock and reset
should be the only ports in your top-level of the design
which will then be connected to the FPGA board.
The datapath should be tested thoroughly by forcing
the control inputs to selected values to mimic the
operation of a control unit.
Major Datapath Components likely to be needed:
 ALU: The ALU must provide all arithmetic and logic
functions required to support your instruction set. It
should not provide unnecessary functions.
 Register file: Design as a multi-port “memory array”.
DO NOT instantiate individual registers !
 Sign/zero extension logic, as appropriate, for ALU
inputs.
 Program counter (PC).
 Instruction register (IR) (if required).
 Assorted multiplexers for data paths and register
address inputs.
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Report:
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Control Signal Table
VHDL codes and List Format of the
Simulations of each unique component used
in the datapath
VHDL code and List format of the Simulation
of the top-level design of the datapath.
Please annotate and explain your
simulations.
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CPU Design Project- Part 4 - Control Unit
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Design and test a VHDL “behavioral” model of the control
unit to realize the behavior described in your Control Signal
Table from the previous part of the project.
The test program will be provided to you. Hand compile the
program into binary code and then use this code and modify
the given RAM_init.mif file according to your program code.
Using this .mif file create a 16 bit memory module from
Altera’s Megafunction Library as explained in MegaWizard
Plug-In Manager Manual. A .vhd file will be created in your
working directory.
Include this memory.vhd file in your datapath.
Create a CPU component by instantiating and connecting
your control unit and datapath components. CPU I/O ports
should be limited to a clock, reset, inr as the input ports and
outvalue as the output port.
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Report:
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VHDL code and simulation results of the Control
Unit.
Your assembly language code and binary code of
the test program given to you.
Final simulation of the test program of the top-level
design.
Show a sufficient set of control signals to
demonstrate correct operation of each instruction
(control unit state, address bus, data bus, ALU
output, register file outputs, register file input,
memory control signals, etc.)
On the simulation listing, annotate by writing the
corresponding assembly language instruction next
to each execute cycle and highlighting the
“significant” result register or bus value.
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CPU Design Project- Part 5 - Hardware
(FPGA) Implementation
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Follow the Quartus II and DE2 Manuals given to you
for designing and implementing your circuit on the
FPGA.
Clock may be connected to any of the two freerunning clock frequencies available on the board,
27MHz and 50MHz. But for debugging your design
using the manual clock by connecting it to one of 4
keys on the board will be an advantage and will make
debugging easier.
Run the program given to you, or any other program
suitable for a demo, and verify the results with your
simulation in Part 4.
You will demonstrate a working implementation of
your design on a DE2 Board.
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Part 5 Demo
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Briefly describe what is implemented, what
program you will run and what result is
expected.
Run the program pointing to the functions of
the buttons you press. Let the observer
examine the result.
Offer to make a change to some parameter
to an observer-selected value and rerun the
demo.
Total duration of demo: FIVE MINUTES.
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Part 5 Report
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Write a final report (no longer than one
page) on:
A. What you learned from the project.
B. What would you do differently next time.
C. Your advice to others doing such a project.
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