Systems and Networks Project 1 Solution

    • 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



    • 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.

    • 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 proces-sor. 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 in-formation. 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).

    • 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 “as-sembly” 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.

    • 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



    • 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

31302928272625242322212019181716151413121110 9    8    7    6    5    4    3    2    1    0

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

31302928272625242322212019181716151413121110 9    8    7    6    5    4    3    2    1    0


0000
DR
SR1
unused





Operation

DR = SR1 + SR2;

Description

SR2

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

31302928272625242322212019181716151413121110 9    8    7    6    5    4    3    2    1    0


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

31302928272625242322212019181716151413121110 9    8    7    6    5    4    3    2    1    0


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

31302928272625242322212019181716151413121110 9    8    7    6    5    4    3    2    1    0


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

31302928272625242322212019181716151413121110 9    8    7    6    5    4    3    2    1    0


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

31302928272625242322212019181716151413121110 9    8    7    6    5    4    3    2    1    0

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

31302928272625242322212019181716151413121110 9    8    7    6    5    4    3    2    1    0


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

31302928272625242322212019181716151413121110 9    8    7    6    5    4    3    2    1    0


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

31302928272625242322212019181716151413121110 9    8    7    6    5    4    3    2    1    0


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

31302928272625242322212019181716151413121110 9    8    7    6    5    4    3    2    1    0


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

31302928272625242322212019181716151413121110 9    8    7    6    5    4    3    2    1    0


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



    • 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