Project 3: Pipeline Solution

    • Why Pipelining?

The datapath design that we implemented for Project 1 was, in fact, grossly ine cient. By focusing on increasing throughput, a pipelined processor can get more instructions done per clock cycle. In the real world, that means higher performance, lower power draw, and most importantly, happy customers!


    • Project Requirements

In this extra credit project, you will make a pipelined processor that implements the Bob-2200 ISA. There will be ve stages in your pipeline:

    1. IF - Instruction Fetch

    2. ID/RR - Instruction Decode/Register Read

    3. EX - Execute (ALU operations)

    4. MEM - Memory (both reads and writes with memory)

    5. WB - Writeback (writing to registers)

Before you move on, read Appendix A: Bob-2200 Instruction Set Architecture to understand the ISA that you will be implementing. Understanding the instructions supported by your ISA will make designing your pipeline much easier. We provide you with a Brandonsim le with the some of the structure laid out.


    • Building the Pipeline

First, you will have to build the hardware to support all of your instructions. You will have to make each stage such that it can accommodate the actions of all instructions passing through it. Use the book (Ch. 5) to get an idea of what the pipeline looks like and to understand the function of each stage before you start building your circuits.


1. IF Stage

The IF stage is responsible for:

Getting the instruction from I-MEM at location PC Updating the PC

For normal sequential execution, we would update the PC by incrementing it by 1. Notice, however, that this may not be the case when executing a BNE or JALR instruction. Hence, you will likely need to multiplex which value is used to update the PC.


2. ID/RR Stage

The ID/RR stage is responsible for:

Decoding the instruction

Reading the appropriate registers
Project 3: Pipeline    CS 2200 - Systems and Networks     



Please look at Appendix A: Bob-2200 Instruction Set Architecture in order to understand the instruction formats! You will have a dual ported register le (DPRF), which allows you to read from two registers and write one register all at the same time. As you will notice, the TAs have been very kind in making the DPRF and providing it to you.

Some of the instructions require both inputs into to the ALU to be values pulled from the DPRF. However, other instructions contain a value within the instruction, such as an immval20, o set20, or PCo set20 eld. You may either pass all of these possible values to the next stage (requires bigger bu er registers), or condense them into just the values needed to execute the instruction in the following cycles (requires more logic, but bu er size can be optimized).


3. EX Stage

The EX stage is responsible for:

Performing all necessary arithmetic and logic calculations Resolving any BNE or JALR instructions

In the Execute (EX) stage, you will perform any arithmetic computations required by the instruction. This stage should host a complete ALU to perform the actual adding or NANDing as required by the instruction. For memory access instructions, this stage will perform the Base + O set computation required to determine the memory address to access.


4. MEM Stage

The MEM stage is responsible for:

Reading from or writing a result to memory

All you need to do is to use the value calculated in the EX stage as the address for the RAM. Note that you must use the maximum address length for the RAM block - this is 24 bits. To accomplish this, simply take the lower 24 bits of the calculated address. Depending on the instruction, this stage will need to pass either the value read from memory or the value computed in EX to the WB stage.


5. WB Stage

The WB stage is responsible for:

Writing results back to the DPRF (dual-ported register  le)

Depending on the instruction, you may need to write a value back to a register. To do this, your WB stage will attach to the data in and write enable inputs of the DPRF in ID/RR. Remember that the DPRF can write and read di erent registers in the same clock cycle, which is why WB and ID/RR can share the same register le. For instructions that do not write a register, your WB stage may not do anything at all.
Project 3: Pipeline    CS 2200 - Systems and Networks     



    • General Advice

Subcircuits

For this project, we highly encourage using modular design and creating subcircuits when necessary. We strongly recommend using subcircuits when building your pipeline bu ers, stalling unit, and extra credit forwarding unit. A modular design will make it easier to debug and test your circuit during development.


Pipeline Bu ers

For deciding what to pass through bu ers, remember that we need to support the requirements of every possible instruction. Think of what each instruction needs to ful ll its duty, and pass a union of all those requirements. (By union we mean the mathematical union, for example say I1 needs PC and Rx, while I2 needs Rx and Ry, then you should pass PC, Rx and Ry through the bu er). You can also feel free to implement your hardware such that you re-use space in the bu er for di erent purposes depending on the instruction, but this is not required.


Control Signals

In the Project 1 datapath, recall that we had one main ROM that was the single source of all the control signals on the datapath. Now that we are spreading out our work across di erent stages of the pipeline, you have a choice of how to implement your signals!

There are two options:

    1. You can either have a single large main ROM in ID/RR which calculates all the control signals for every stage.

OR

    2. you can have a small(er) ROM in each stage which takes in the opcode and assert the proper signals for that operation.

Note that if you choose the rst method, you will need to pass all the signals needed for later stages through the earlier stages, and in the second method, you will need to pass the instruction opcode though all the stages so that you know which signals to assert during that stage.


Stalling the Pipeline

One must stall the pipeline when an instruction cannot proceed to the next stage because a value is not yet available to an instruction. This usually happens because of a data hazard. For example, consider two instructions in the following program:

    1. LW $t0, 5($t1)

    2. ADDI $t0, $t0, 1

Without stalling the ADDI instruction in the ID/RR stage, it will get an out of date value for $t0 from the reg le, as the correct value for $t0 isn’t known the LW reaches the MEM stage! Therefore, we must stall. Consult the textbook (or your notes) for more information on data hazards.

To stall the pipeline, the stages preceding the stalled stage should disable writes into their bu ers, i.e. they should continue to output the previous value into the next stage. The stalled stage itself will output NOOP (example, ADD $zero, $zero, $zero) instructions down the pipeline until the cause of the stall nishes.
Project 3: Pipeline    CS 2200 - Systems and Networks     



Note that you may eliminate a good deal of stalls by implementing data forwarding. Data forwarding is extra credit for this project, as described in the Data Forwarding section later in this document. You must implement the pipeline without data forwarding rst, as it will be turned in separately from the version with fowarding.

It is recommended that you make a stalling unit that implements various stock rules. The stalling unit should take the two register values you are reading, as well as the registers involved in stages further down the pipeline. Using these values you will need to determine if a data hazard has occurred, and stall the pipeline if necessary.


Branch Prediction

Since branch instructions (BNE/JALR) are resolved in EX, the pipeline may be unsure of which instructions are correct to fetch. We could stall fetching further instructions until resolution, but this is ine cient and naive. To better handle control hazards, we can \predict" which instruction could be correct.

For this project we will be predicting the branch is not taken, and so the pipeline will continue fetching sequentially. Upon resolving the branch, the pipeline should continue normally in the case of a correct prediction, or ush the incorrectly fetched instructions in the case of an incorrect prediction.


Flushing the Pipeline

For the BNE/JALR instructions, we calculate the target in the EX stage of the pipeline. However, the next two instructions the IF stage fetches while EX is computing the target may not be the next instructions we want to execute. When this happens, we must have a hardware mechanism to \cancel" or \ ush" the incorrectly-fetched instructions after we realize they are incorrect.

In implementing your ushing mechanism, it is highly recommended that you avoid the asynchronous clear feature of registers in Brandonsim, as this may cause timing issues. Instead, we suggest using a multiplexer to selectively send a NOOP into the bu er input.


    • Data Forwarding (Extra Credit)

Implementing data forwarding is extra credit for Project 3. In addition to your initial pipeline, you may create a second circuit with data forwarding capabilities for up to 35 points of extra credit. This is about 2 points added to your total course grade.

Even if you choose to complete the extra credit, you must still also submit a pipeline without data forwarding that handles hazards using only stalls and ushes. Additionally, you must have obtained a full score (100 points) on your basic pipeline to receive credit for the pipeline with data forwarding.

Forwarding is one way to increase the performance of the pipeline. This allows the ID/RR stage to retrieve values computed in later stages of the pipeline early so that stalling the instruction is not necessary. It is strongly recommended that you not use the busy bit/read pending bit strategy suggested in the book - this has some very nasty edge cases and requires much more logic than necessary.

It is recommended that you make a forwarding unit that implements various stock rules. The forwarding unit should take in the two register values you are reading, the output value from the EX stage, the output value from the MEM stage, and the output value from the WB stage. To forward a value from a future stage back to ID/RR, you must check to see if the destination register number from a particular stage is equal to your source register numbers in the ID/RR stage. If so, you must forward the value from that stage to your ID/RR stage.
Project 3: Pipeline    CS 2200 - Systems and Networks     



You should not update the value of the register when you forward the value back - writes to the register le should only occur in the WB stage. Note, forwarding cannot save you from one situation: when the destination register of a LW instruction is the source register of an instruction immediately after it. In this case, sometimes called \load-to-use", you must stall the instruction in the ID/RR stage. It is your job to esh out all of the stall rules.

Keep in mind: the zero register can never change, therefore it should not be considered for forwarding and stalling situations.


    • Testing

When you have constructed your pipeline, you should test it instruction by instruction to see if you have all the necessary components to ensure proper execution.

Be careful to only use the instructions listed in the appendix - there are some subtle points in having a separate instruction and data memory. Load the assembled program into both the instruction memory and the data memory and let your processor execute it. Any writes to memory will only a ect the data memory.


    • Deliverables

You can run make submit to automatically package your project for submission. Run make ec to package both the basic pipeline and the extra credit version. Submit the resulting tar.gz compressed folder on T-square.

The archive should contain the following    les:

BOB2200-Pipeline.circ: Your Brandonsim  le, without support for data fowarding

BOB2200-Pipeline-EC.circ (Optional): Extra credit Brandonsim le, with support for data for-warding

Always re-download your assignment from Canvas after submitting to ensure that all necessary les were properly uploaded. If what we download does not work, you will get a 0 regardless of what is on your machine.
Project 3: Pipeline    CS 2200 - Systems and Networks     



    • Appendix A: Bob-2200 Instruction Set Architecture

The Bob-2200 is a simple, yet capable computer architecture. The Bob-2200 combines attributes of both ARM and the LC-2200 ISA de ned in the Ramachandran & Leahy textbook for CS 2200.

The Bob-2200 is a word-addressable, 32-bit computer. All addresses refer to words, i.e. the rst word (four bytes) in memory occupies address 0x0, the second word, 0x1, etc.

All memory addresses are truncated to 24 bits on access, discarding the 8 most signi cant bits if the address was stored in a 32-bit register. This provides roughly 67 MB of addressable memory.


8.1    Registers

The Bob-2200 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 they jump back to the caller.
Project 3: Pipeline    CS 2200 - Systems and Networks     



    7. Register 12 is reserved for handling interrupts. While it should be implemented, it otherwise will not have any special use on this assignment.

    8. Register 13 is your anchor on the stack. It keeps track of the top of the activation record for a subroutine.

    9. Register 14 is used to point to the rst 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  nished executing.


8.2    Instruction Overview

The Bob-2200 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: Bob-2200 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
o set20

SW






0100
SR
BaseR
o set20

BNE






0101
SR1
SR2
o set20

JALR






0110
RA
AT
unused

HALT






0111


unused

SLT






1000
DR
SR1
unused
SR2
LEA






1001
DR
unused
o set20










8.2.1    Conditional Branching

Conditional branching in the Bob-2200 ISA is provided via the BNE (\branch if not equal") instruction. BNE will branch to address "incrementedPC + o set20" only if SR1 and SR2 are not equal
Project 3: Pipeline    CS 2200 - Systems and Networks     



8.3    Detailed Instruction Reference

8.3.1    ADD

Assembler Syntax

ADD    DR, SR1, SR2

Encoding

31302928272625242322212019181716151413121110 9    8    7    6    5    4    3    2    1    0

0000
DR
SR1
unused
SR2






Operation

DR = SR1 + SR2;

Description

The ADD instruction obtains the rst source operand from the SR1 register. The second source operand is obtained from the SR2 register. The second operand is added to the rst source operand, and the result is stored in DR.


8.3.2    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 3: Pipeline    CS 2200 - Systems and Networks     



8.3.3    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 rst source operand from the SR1 register. The second source operand is obtained by sign-extending the immval20 eld to 32 bits. The resulting operand is added to the rst source operand, and the result is stored in DR.


8.3.4    LW

Assembler Syntax

LW    DR, offset20(BaseR)

Encoding

31302928272625242322212019181716151413121110 9    8    7    6    5    4    3    2    1    0

0011
DR
BaseR
o set20





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 speci ed by bits [23:20]. The 32-bit word at this address is loaded into DR.
Project 3: Pipeline    CS 2200 - Systems and Networks     



8.3.5    SW

Assembler Syntax

SW    SR, offset20(BaseR)

Encoding

31302928272625242322212019181716151413121110 9    8    7    6    5    4    3    2    1    0

0100
SR
BaseR
o set20





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 speci ed by bits [23:20]. The 32-bit word obtained from register SR is then stored at this address.


8.3.6    BNE

Assembler Syntax

BNE    SR1, SR2, offset20

Encoding

31302928272625242322212019181716151413121110 9    8    7    6    5    4    3    2    1    0

0101
SR1
SR2

o set20





Operation

if (SR1 != SR2) {

PC = incrementedPC + offset20

}





Description

A branch is taken if SR1 and SR2 are not equal. 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 o set[19:0].
Project 3: Pipeline    CS 2200 - Systems and Networks     



8.3.7    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.


8.3.8    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.


8.3.9    SLT

Assembler Syntax

SW    DR, SR1, SR2

Encoding

31302928272625242322212019181716151413121110 9    8    7    6    5    4    3    2    1    0

1000
DR
SR1
unused
SR2






Operation

if (SR1 < SR2) {

DR=1

} else { DR=0

}

Description

If SR1 is less than SR2, a 1 should be stored into DR. Otherwise a 0 should be stored in DR.
Project 3: Pipeline    CS 2200 - Systems and Networks     



8.3.10    LEA

Assembler Syntax

LEA    DR, label

Encoding

31302928272625242322212019181716151413121110 9    8    7    6    5    4    3    2    1    0

1001
DR
unused
PCo set20





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.