Project 1 - Systems and Networks 

Project 1 CS2200 - Systems and Networks 
1 Requirements
• Download the proper version of CircuitSim. The proper version is version 1.8.2. A copy of CircuitSim is available under Files on Canvas. You may also download it from the CircuitSim website
(https://ra4king.github.io/CircuitSim/). In order to run CircuitSim, Java must be installed. If you
are a Mac user, you may need to right-click on the JAR file and select “Open” in the menu to bypass
Gatekeeper restrictions.

• The LC-22 assembler is written in Python. If you do not have Python 2.6 or newer installed on your
system, you will need to install it before you continue.
2 Project Overview and Description
Project 1 is designed to give you a good feel for exactly how a processor works. In Phase 1, you will design
a datapath in CircuitSim to implement a supplied instruction set architecture. You will use the datapath
as a tool to determine the control signals needed to execute each instruction. In Phases 2 and 3, you are
required to build a simple finite state machine (the “control unit”) to control your computer and actually
run programs on it.
Note: You will need to have a working knowledge of CircuitSim. Make sure that you know how to make
basic circuits as well as subcircuits before proceeding. The TAs are always here if you need help.
Project 1 CS2200 - Systems and Networks Spring 2022
3 Phase 1 - Implement the Datapath
Figure 1: Datapath for the LC-22 Processor
In this phase of the project, you must learn the Instruction Set Architecture (ISA) for the processor we will
be implementing. Afterwards, we will implement a complete LC-22 datapath in CircuitSim using what you
have just learned.
You must do the following:
1. Learn and understand the LC-22 ISA. The ISA is fully specified and defined in Appendix A: LC-22
Instruction Set Architecture. Do not move on until you have fully read and understood the
ISA specification. Every single detail will be relevant to implementing your datapath in the next
step.
2. Using CircuitSim, implement the LC-22 datapath. To do this, you will need to use the details of the
LC-22 datapath defined in Appendix A: LC-22 Instruction Set Architecture. You should model your
datapath on Figure 1.
3. Put your name on your CircuitSim data path in a comment box so we know it is your work.
3.1 Hints
3.1.1 Subcircuits
CircuitSim enables you to create reusable components in the form of subcircuits. We highly recommend
that you break parts of your design up into subcircuits. At a minimum, you will want to implement
your ALU in a subcircuit. The control unit you implement in Phase 2 is another prime candidate for a
subcircuit.
Project 1 CS2200 - Systems and Networks Spring 2022
3.1.2 Debugging
As you build the datapath, you should consider adding functionality that will allow you to operate the whole
datapath by hand. This will make testing individual operations quite simple. We suggest your datapath
includes devices that will allow you to put arbitrary values on the bus and to view the current value of the
bus. Feel free to add any additional hardware that will help you understand what is going on.
3.1.3 Memory Addresses
Because of CircuitSim limitations, the RAM module is limited to no more than 16 address bits. Therefore,
per our ISA, any 32-bit values used as memory addresses will be truncated to 16 bits (with the 16 most
significant bits disregarded). If you use the RAM subcircuit we provide, this truncation has already been
handled, and you can simply attach the 32-bit value from the MAR (Memory Address Register) to our
custom RAM circuit. Otherwise, you will need to truncate the most significant bits of the the address value
from the MAR before feeding it into the RAM.
3.1.4 Comparison Logic
The “comparison logic” (cmp logic) box in Figure 1 is responsible for performing the comparison logic
associated with the BLT and BGT instructions. The comparison logic should read the current value on the
bus plus the BrSel bit from the FSM. When executing BLT and BGT, you should compute A − B using the
ALU. While this result of the ALU is being driven on the bus, the comparison logic should read the result
A − B and output a single “true” or “false” bit for either the condition A > B or A < B depending on the
BrSel input.
Your comparison logic should be purely combinational. Feel free to use any CircuitSim components you wish
to aid in your implementation.
3.1.5 Register File
You must implement your own register file. That is to say, you cannot use CircuitSim’s built-in RAM to
create the register file. Consider what logic components you may want to use to implement addressing
functionality (multiplexers, demultiplexers, decoders, etc). Your zero register must be implemented such
that writes to it are ineffective, i.e., attempting to write a non-zero value to the zero register will do nothing.
Do not forget to do this or you will lose points!
3.1.6 Register Select
From lecture and the textbook, you should be familiar with the “register select” (RegSel) multiplexer. The
mux is responsible for feeding the register number from the correct field in the instruction into the register
file. See Table 4 for a list of inputs your mux should have.
4 Phase 2 - Implement the Microcontrol Unit
In this phase of the project, you will use CircuitSim to implement the microcontrol unit for the LC-22 processor. This component is referred to as the “Control Logic” in the images and schematics. The microcontroller
will contain all of the signal lines to the various parts of the datapath.
You must do the following:
1. Read and understand the microcontroller logic:
• Please refer to Appendix B: Microcontrol Unit for details.
• Note: You will be required to generate the control signals for each state of the processor in
the next phase, so make sure you understand the connections between the datapath and the
microcontrol unit before moving on.
Project 1 CS2200 - Systems and Networks Spring 2022
2. Implement the Microcontrol Unit using CircuitSim. The appendix contains all of the necessary information. Take note that the input and output signals on the schematics directly match the signals
marked in the LC-22 datapath schematic (see Figure 1).
5 Phase 3 - Microcode and Testing
In this final stage of the project, you will write the microcode control program that will be loaded into the
microcontrol unit you implemented in Phase 2. Then, you will hook up the control unit you built in Phase 2
of the project to the datapath you implemented in Phase 1. Finally, you will test your completed computer
using a simple test program and ensure that it properly executes.
You must do the following:
1. Open and fill out microcode.xlsx file we’ve provided you (note: the formulas in the provided file are
only tested to work with MS Excel). You will need to mark which control signal is high (that is 1) for
each of the states. If you can’t use the Microsoft Excel due to your operating system, you can use the
browser version of Excel that is provided to you with your Gatech Office 356 account.
2. After you have completed all the microstates, convert the binary strings you just computed into hex
and move them into the main ROM. You can just copy and paste the hex column (highlighted yellow)
from the spreadsheet directly into the ROM component in Circuitsim. Do the same for the sequencer
and condition ROMs.
3. Connect the completed control unit to the datapath you implemented in Phase 1. Using Figures 1 and
2, connect the control signals to their appropriate spots.
4. Finally, it is time to test your completed computer. Use the provided assembler (found in the “assembly” folder) to convert a test program from assembly to hex. For instructions on how to use the
assembler and simulator, see README.txt in the “assembly” folder. Note: The simulator does
not test your project, it simply provides a model. To test your design, you must load
the assembled HEX into CircuitSim. We recommend using test programs that contain a single
instruction since you are bound to have a few bugs at this stage of the project. Once you have built
confidence, test your processor with the provided pow.s program as a more comprehensive test case.
6 Deliverables
To submit your project, you need to upload the following files to Gradescope:
• CircuitSim datapath file (LC-22.sim)
• Microcode file (microcode.xlsx)
If you are missing one or both of those files, you will get a 0 so make sure that you have
uploaded both of them.
Always re-download your assignment from Gradescope after submitting to ensure that all
necessary files were properly uploaded. If what we download does not work, you will get a 0
regardless of what is on your machine.
This project will be demoed. In order to receive full credit, you must sign up for a demo slot and
complete the demo. We will announce when demo times are released.
Project 1 CS2200 - Systems and Networks Spring 2022
7 Appendix A: LC-22 Instruction Set Architecture
The LC-22 is a simple, yet capable computer architecture. The LC-22 combines attributes of both ARM
and the LC-2200 ISA defined in the Ramachandran & Leahy textbook for CS 2200.
The LC-22 is a word-addressable, 32-bit computer. All addresses refer to words, i.e. the first word
(four bytes) in memory occupies address 0x0, the second word, 0x1, etc.
All memory addresses are truncated to 16 bits on access, discarding the 16 most significant bits if the address
was stored in a 32-bit register. This provides 256 KB of addressable memory.
7.1 Registers
The LC-22 has 16 general-purpose registers. While there are no hardware-enforced restraints on the uses of
these registers, your code is expected to follow the conventions outlined below.
Table 1: Registers and their Uses
Register Number Name Use Callee Save?
0 $zero Always Zero NA
1 $at Assembler/Target Address NA
2 $v0 Return Value No
3 $a0 Argument 1 No
4 $a1 Argument 2 No
5 $a2 Argument 3 No
6 $t0 Temporary Variable No
7 $t1 Temporary Variable No
8 $t2 Temporary Variable No
9 $s0 Saved Register Yes
10 $s1 Saved Register Yes
11 $s2 Saved Register Yes
12 $k0 Reserved for OS and Traps NA
13 $sp Stack Pointer No
14 $fp Frame Pointer Yes
15 $ra Return Address No
1. Register 0 is always read as zero. Any values written to it are discarded. Note: for the purposes of
this project, you must implement the zero register. Regardless of what is written to this register, it
should always output zero.
2. Register 1 is used to hold the target address of a jump. It may also be used by pseudo-instructions
generated by the assembler.
3. Register 2 is where you should store any returned value from a subroutine call.
4. Registers 3 - 5 are used to store function/subroutine arguments. Note: registers 2 through 8 should
be placed on the stack if the caller wants to retain those values. These registers are fair game for the
callee (subroutine) to trash.
5. Registers 6 - 8 are designated for temporary variables. The caller must save these registers if they
want these values to be retained.
6. Registers 9 - 11 are saved registers. The caller may assume that these registers are never tampered
with by the subroutine. If the subroutine needs these registers, then it should place them on the stack
and restore them before it jumps back to the caller.
7. Register 12 is reserved for handling interrupts. While it should be implemented, it otherwise will not
have any special use on this assignment.
Project 1 CS2200 - Systems and Networks Spring 2022
8. Register 13 is the everchanging top of the stack; it keeps track of the top of the activation record for
a subroutine.
9. Register 14 is the anchor point of the activation frame. It is used to point to the first address on the
activation record for the currently executing process.
10. Register 15 is used to store the address a subroutine should return to when it is finished executing.
7.2 Instruction Overview
The LC-22 supports a variety of instruction forms, only a few of which we will use for this project. The
instructions we will implement in this project are summarized below.
Table 2: LC-22 Instruction Set
012345678910111213141516171819202122232425262728293031
ADD 0000 DR SR1 unused SR2
NAND 0001 DR SR1 unused SR2
ADDI 0010 DR SR1 immval20
LW 0011 DR BaseR offset20
SW 0100 SR BaseR offset20
BR 0101 unused offset20
JALR 0110 RA AT unused
HALT 0111 unused
BLT 1000 SR1 SR2 offset20
BGT 1001 SR1 SR2 offset20
LEA 1010 DR unused PCoffset20
7.2.1 Conditional Branching
Branching in the LC-22 ISA is a bit different than usual. We have a set of branching instructions including
BR, an unconditional branch, as well as BLT and BGT, which offer the ability to branch upon a certain
condition being met. The BLT and BGT instructions use comparison operators, comparing the values of
two source registers. If the comparisons are true (for example, with the BGT instruction, if SR1 > SR2),
then we will branch to the target destination of incrementedPC + offset20.
Note: The conditional branch instructions make use of the BrSel signal. Think about ways that you can use
this signal to implement conditional logic.
Project 1 CS2200 - Systems and Networks Spring 2022
7.3 Detailed Instruction Reference
7.3.1 ADD
Assembler Syntax
ADD DR, SR1, SR2
Encoding
012345678910111213141516171819202122232425262728293031
0000 DR SR1 unused SR2
Operation
DR = SR1 + SR2;
Description
The ADD instruction obtains the first source operand from the SR1 register. The second source operand is
obtained from the SR2 register. The second operand is added to the first source operand, and the result is
stored in DR.
7.3.2 NAND
Assembler Syntax
NAND DR, SR1, SR2
Encoding
012345678910111213141516171819202122232425262728293031
0001 DR SR1 unused SR2
Operation
DR = ~(SR1 & SR2);
Description
The NAND instruction performs a logical NAND (AND NOT) on the source operands obtained from SR1
and SR2. The result is stored in DR.
HINT: A logical NOT can be achieved by performing a NAND with both source operands the same.
For instance,
NAND DR, SR1, SR1
...achieves the following logical operation: DR ← SR1.
Project 1 CS2200 - Systems and Networks Spring 2022
7.3.3 ADDI
Assembler Syntax
ADDI DR, SR1, immval20
Encoding
012345678910111213141516171819202122232425262728293031
0010 DR SR1 immval20
Operation
DR = SR1 + SEXT(immval20);
Description
The ADDI instruction obtains the first source operand from the SR1 register. The second source operand is
obtained by sign-extending the immval20 field to 32 bits. The resulting operand is added to the first source
operand, and the result is stored in DR.
7.3.4 LW
Assembler Syntax
LW DR, offset20(BaseR)
Encoding
012345678910111213141516171819202122232425262728293031
0011 DR BaseR offset20
Operation
DR = MEM[BaseR + SEXT(offset20)];
Description
An address is computed by sign-extending bits [19:0] to 32 bits and then adding this result to the contents
of the register specified by bits [23:20]. The 32-bit word at this address is loaded into DR.
7.3.5 SW
Assembler Syntax
SW SR, offset20(BaseR)
Encoding
012345678910111213141516171819202122232425262728293031
0100 SR BaseR offset20
Operation
MEM[BaseR + SEXT(offset20)] = SR;
Description
An address is computed by sign-extending bits [19:0] to 32 bits and then adding this result to the contents
of the register specified by bits [23:20]. The 32-bit word obtained from register SR is then stored at this
address.
Project 1 CS2200 - Systems and Networks Spring 2022
7.3.6 BR
Assembler Syntax
BR offset20
Encoding
012345678910111213141516171819202122232425262728293031
0101 unused offset20
Operation
PC = incrementedPC + offset20
Description
A branch is unconditionally taken. The PC will be set to the sum of the incremented PC (since we have
already undergone fetch) and the sign-extended offset[19:0].
7.3.7 JALR
Assembler Syntax
JALR RA, AT
Encoding
012345678910111213141516171819202122232425262728293031
0110 RA AT unused
Operation
RA = PC;
PC = AT;
Description
First, the incremented PC (address of the instruction + 1) is stored into register RA. Next, the PC is loaded
with the value of register AT, and the computer resumes execution at the new PC.
7.3.8 HALT
Assembler Syntax
HALT
Encoding
012345678910111213141516171819202122232425262728293031
0111 unused
Description
The machine is brought to a halt and executes no further instructions.
Project 1 CS2200 - Systems and Networks Spring 2022
7.3.9 BLT
Assembler Syntax
BLT SR1, SR2, offset20
Encoding
012345678910111213141516171819202122232425262728293031
1000 SR1 SR2 offset20
Operation
if (SR1 < SR2) {
PC = incrementedPC + offset20
}
Description
A branch is taken if SR1 is less than SR2. If this is the case, the PC will be set to the sum of the incremented
PC (since we have already undergone fetch) and the sign-extended offset[19:0].
7.3.10 BGT
Assembler Syntax
BGT SR1, SR2, offset20
Encoding
012345678910111213141516171819202122232425262728293031
1001 SR1 SR2 offset20
Operation
if (SR1 > SR2) {
PC = incrementedPC + offset20
}
Description
A branch is taken if SR1 is greater than SR2. If this is the case, the PC will be set to the sum of the
incremented PC (since we have already undergone fetch) and the sign-extended offset[19:0].
7.3.11 LEA
Assembler Syntax
LEA DR, label
Encoding
012345678910111213141516171819202122232425262728293031
1010 DR unused PCoffset20
Operation
DR = PC + SEXT(PCoffset20);
Description
An address is computed by sign-extending bits [19:0] to 32 bits and adding this result to the incremented
PC (address of instruction + 1). It then stores the computed address into register DR.
Project 1 CS2200 - Systems and Networks Spring 2022
8 Appendix B: Microcontrol Unit
You will make a microcontrol unit which will drive all of the control signals to various items on the datapath.
This Finite State Machine (FSM) can be constructed in a variety of ways. You could implement it with
combinational logic and flip flops, or you could hard-wire signals using a single ROM. The single ROM
solution will waste a tremendous amount of space since most of the microstates do not depend on the opcode
or the conditional test to determine which signals to assert. For example, since the condition line is an input
for the address, every microstate would have to have an address for condition = 0 as well as condition = 1,
even though this only matters for one particular microstate.
To solve this problem, we will use a three ROM microcontroller. In this arrangement, we will have three
ROMs:
• the main ROM, which outputs the control signals,
• the sequencer ROM, which helps to determine which microstate to go to at the end of the FETCH
state,
• and the condition ROM, which helps determine whether or not to branch during the BLT and BGT
instructions.
Examine the following:
Figure 2: Three ROM Microcontrol Unit
As you can see, there are three different locations that the next state can come from: part of the output
from the previous state (main ROM), the sequencer ROM, and the condition ROM. The mux controls which
of these sources gets through to the state register. If the previous state’s “next state” field determines where
to go, neither the OPTest nor ChkCmp signals will be asserted. If the opcode from the IR determines the
next state (such as at the end of the FETCH state), the OPTest signal will be asserted. If the comparison
circuitry determines the next state (such as in the BLT and BGT instructions), the ChkCmp signal will be
Project 1 CS2200 - Systems and Networks Spring 2022
asserted. Note that these two signals should never be asserted at the same time since nothing is input into
the “11” pin on the MUX.
The sequencer ROM should have one address per instruction, and the condition ROM should have one
address for condition true and one for condition false.
Before getting down to specifics you need to determine the control scheme for the datapath. To do this
examine each instruction, one by one, and construct a finite state bubble diagram showing exactly what
control signals will be set in each state. Also determine what are the conditions necessary to pass from one
state to the next. You can experiment by manually controlling your control signals on the bus you’ve created
in Phase 1 - Implement the Datapath to make sure that your logic is sound.
Once the finite state bubble diagram is produced, the next step is to encode the contents of the Control Unit
ROM. Then you must design and build (in CircuitSim) the Control Unit circuit which will contain the three
ROMs, a MUX, and a state register. Your design will be better if it allows you to single step and ensure
that it is working properly. Finally, you will load the Control Unit’s ROMs with the hexadecimal generated
by your filled out microcode.xlsx.
Note that the input address to the ROM uses bit 0 for the lowest bit of the current state and 5 for the
highest bit for the current state.
Table 3: ROM Output Signals
Bit Purpose Bit Purpose Bit Purpose Bit Purpose Bit Purpose
0 NextState[0] 6 DrREG 12 LdIR 18 WrMEM 24 ChkCmp
1 NextState[1] 7 DrMEM 13 LdMAR 19 RegSelLo 25 BrSel
2 NextState[2] 8 DrALU 14 LdA 20 RegSelHi
3 NextState[3] 9 DrPC 15 LdB 21 ALULo
4 NextState[4] 10 DrOFF 16 LdCmp 22 ALUHi
5 NextState[5] 11 LdPC 17 WrREG 23 OPTest
Table 4: Register Selection Map
RegSelHi RegSelLo Register
0 0 RX (IR[27:24])
0 1 RY (IR[23:20])
1 0 RZ (IR[3:0])
1 1 unused
Table 5: ALU Function Map
ALUHi ALUlLo Function
0 0 ADD
0 1 SUB
1 0 NAND
1 1 A + 1