CS152 Computer Architecture and Engineering

Lab reports for Lab 4 due Tuesday 10/14 at 11:59pm via the submit program.  You will demonstrate your lab to your TA on Monday, 10/13 at an arranged time.  During this demo, the TA will provide you with secret test code. If you are able to pass these tests on your first try, you will receive bonus points. You can also receive bonus points if you fix your processor to pass the tests within your section time. Otherwise, the TA will not give you the source code to the tests until after section, which will leave you the weekend and Monday to fully debug your processor. Ideally, you will have already tested your processor thoroughly, so that you will be able to pass these tests on the first try. 

A formal design document and a division of labor is due via e-mail to your TA Thursday 10/2 by 9pm. This will allow the TAs time to go over your design document with you during section on Friday 10/3. You are encouraged to finish your design document and turn it in as early as possible, so that the TAa can review it earlier, which will allow you to begin working on this lab earlier. 

Problem 0: (Team Evaluation) is also due by Thursday 10/2 at 9pm via e-mail to your TA.

Like all the previous labs, this lab is rather long, so get started early!!!

This lab assignment is to be completed with your project partners. Once again, please select a spokesperson for this lab (they should be named in the design document). The responsibility of the spokesperson is to communicate questions to the TA and reply to questions from the TA. Please choose a different spokesperson than the one you had for Lab 3.

Problem 0: Team Evaluation for Lab 3.

To help us understand how your team is functioning we require you to evaluate yourself and each of your team members individually.

To evaluate yourself, give us a list of the portions of Lab 4 that you were originally assigned to work on, as well as the set of things that you eventually ended up doing (these are not necessarily the same, and we realize this).

Next, based on your own observations, evaluate the performance of the other members of your team during the last lab assignment.  You do not evaluate yourself. Assume an average team member would receive a score of 20 points. Top performers would receive more, sliders less.

The maximum score any one person can get is 40.

Note that each evaluation should have a one or two sentence justification for the evaluation.

For example, suppose you were on a 5 person team with the following other members: Sue Superstar, Teddy Tryhard, Annie Average, and Ned Neverthere. Your distribution might look like the following:
 

Name	Score	Reasoning
Sue Superstar	30	Sue really helped along the group.  Sue figured out how to handle interlocks in the pipeline, and stayed extra long last week to fix out last bug.
Teddy Tryhard	15	Teddy was always at our meetings, and despite lacking some design experience, had a very positive attitude and did everything the group asked him to.
Annie Average	20	Annie did a good job.
Ned Neverthere	5	Ned never showed up to group meetings.  We ended up reimplementing the one piece that he did give us.
total	80

You should reevaluate your team members after every lab assignment and base your evaluation only on their performance during that lab assignment. These scores will be used for grading. Be honest and fair as you would hope others will be.

Please email this information to your TA by Thursday, October 2 by 9pm. Each team member must turn a report in (separately).

Problem 1: Pipelining Your Design

Please read this section through completely before starting any of it!

Problem 1a: pipeline it.

For this design, you must use a new RAM file for your instruction and data memories.  Note that verilog files that we refer to in this LAB are all in the m:\lab4\ directory. Start by reading the Readme in m:\lab4\Lab4Help.pdf. The new RAM (called sramblock2048.v) is fully synchronous. This means that you must set up the address and any data to be written before the edge of the clock.  Both reads and writes are synchronous in this way.  Keep this in mind when you work on your pipeline.  One way to view the result, is that some of the registers that you see in the pipeline diagrams that we show you (for instance, the PC or the S and D registers) are partially duplicated in the RAM block.  This means, for instance, that you still need to keep a separate PC register, but that you also need to pipe the value of the address before the PC register to the actual RAM block; on a clock edge, the new address will be clocked into both the PC register and the internal address registers of the RAM.

You have to implement the following five-stage pipeline:

IF Instruction Fetch:    Access the instruction cache for the instruction to be executed.

ID Instruction Decode:    Decode the instruction and read the operands from the register file. For branch instructions, calculate the branch target instruction address and compare the registers.

EX Execute:    Bypass operands from other pipeline stages. One of the following activities can occur:

• For computational instructions, the ALU or the shifter performs the arithmetic or logical instructions.
• For load or store instructions, the data address is calculated.

WB Write Back:    Write result to register file.

Add the appropriate pipeline registers to your single cycle design. Assume that memory accesses take only one cycle in your implementation.

Here are the instructions you must implement:
 

Type	Instructions
arithmetic	addu, subu, addiu
logical	and, andi, or, ori, xor, xori, lui
shift	sll, sra, srl
compare	slt, slti, sltu, sltui
control	beq, bne, bgez, bltz, j, jr, jal
data transfer	lw, sw
coprocessor*	multu, mfhi, mflo
Other:	break

*Problem 2 will discuss the multiplier further.

You should implement the components you need in verilog. All verilog models should correspond to realistic components (e.g. register, comparator, etc).  No super-composite components, e.g. a branch unit that takes in the opcode, operands, and PC, and outputs a new PC, or something like that. 

All modules should be rising-edge triggered.

All control instructions should have a single delay slot (i.e. the following instruction is always executed after the control instruction).

Make sure to verify the coding of instructions such as bgez in Appendix A of P&H.  Note that the "rt" field is actually used to distinguish bgez and bltz.

These are the only operations that the ALU should support.  As in Lab 3, there should an arithmetic/logical multibit (31) shifter external to the ALU.  Furthermore, instructions such as SLT should be handled outside the ALU as well.  SLT should subtract the two operands, and then use the ALU status flags (Zero, Neg, Ovf) to compute the output, and then put the correct value back in the destination register.

The break instruction is special.  See A-75 of P&H(3rd ed.) for its coding.  Although this is normally an exception-causing instruction, you should treat it more like a halt instruction.  After being decoded, the break instruction should freeze the pipeline from advancing further.  This means that the PC will not advance further, the break instruction will stay in the decode stage, and later instructions will drain from the pipeline.  The proper terminology for this is that the break instruction will "stall" in the decode stage.  Assume that there will be a single input signal called "release" that comes from outside.  When it is high, you should release a blocked break instruction exactly once (you need to build a small circuit that generates a single, one-clock-cycle pulse when release is high, then ignores release until it goes low again).  When we map our pipeline to the board (Problem 4), the break instruction will stop the pipeline and potentially display its code on the LEDs.  Further, we will have the option to "unfreeze" the pipeline with a debounced switch.

Make sure to produce an 8-bit output signal (STAT): that is as follows: if the processor is not stopped, STAT=0.  Otherwise, the high bit (bit 7) of STAT = 1 and the low 7-bits = low 7 bits of break code (which is in bits 6-25 of the break instruction).  Whenever this signal changes, make sure that you have a monitor output that prints "STATUS Changed: 0xvalue" on the console.

Problem 1b: Memory-Mapped Input/Output

Any processor is useless without I/O, and your processor is no exception. Therefore you must build a memory-mapped I/O module.  All writes to addresses 0x80000000-0xFFFFFFFC will be considered writes to I/O space.  All reads and writes to I/O space should not be mapped to your data memory.  Instead these operations should be handled by your memory I/O module.

The specifications of the I/O module are as follows.  It should have a 32-bit address, 32-bit data input, and a 32-bit data output for the processor, just like memory.  It should also have 2 I/O buses: a 32-bit; input data bus and a 32-bit output data bus and a 1-bit output selector.  Other control signals are probably necessary as well.  Internally, this I/O module should have two 32-bit I/O registers.

Behavior is as follows: Reads and writes to and from 0xFFFFFFF0 go to one 32-bit register (call it DP0).  Reads and writes to and from 0xFFFFFFF4 go to the other (call it DP1).  Reads from 0xFFFFFFF8 will come from the input I/O bus. Writes to 0xFFFFFFF8 are ignored. Reads and writes to and from addresses 0x80000000-0xFFFFFFEC and 0xFFFFFFFC are ignored.  The output I/O bus will be DP0 if the output selector is 0 and DP1 if the selector is 1.

The input I/O bus will be connected to the dipswitches on the board.  The output I/O bus will be connected to the hexadecimal LEDs on the board.

Note that you can read/write I/O space with normal loads and stores with negative offsets:

        lw $r1, -8($r0)    ; Read input => $r1
      sw $r7, -16($r0)    ; Write $r7 to DP0
      sw $r8, -12($r0)    ; Write $r8 to DP1

This works because offsets are sign-extended.  Thus, for instance, -12($r0) means address 0xFFFFFFF4.

Finally, within this module (marked to be non-synthesized -- see synplify manual), output a message to the console whenever a change is written (i.e. something like: "I/O Write to DP0: 0x44455523"). This message should also be written to a file called "iooutput.trace".  Further, whenever the module inputs a value, arrange to have the value to come as the next value from an input file called ("ioinput.trace").

Problem 1c: update your monitor module

Make sure to update your disassembly monitoring module from Lab3:

• Output to a file ("trace.txt") rather than to the display module. This will make it easier to read, debug, and search.
• Add the new instructions
• Make sure to account for pipeline effects.

        EXCinstruction = instruction;
        MEMinstruction = instruction;

In this way, you have the value of the instruction word when it is at the end of the memory stage.  This is like a mini pipeline.  Values of input registers wouldn't have to be kept as long, etc.  Think through this very carefully...

Problem 1d: Top-level module integration: chip mapping

As with Lab 4, you will have a top-level schematic module that ties everything together.  Now, however, you will have several I/O pins left over: a clock net, 1 reset signal, 1 release signal (for break instructions), 1 output select signal (from the I/O module), 1 8-bit output (from the break logic), 1 32-bit output (I/O) and 32-bit input (I/O).

Use the TopLevel.v module in m:\lab4 as the top-level integration for your design.  You may want to briefly read the description of the Calinx boards (manual off the handouts page) to see what the various pins mean. You will be modifying the verilog for this top-level module to integrate all the pieces you need.

You should assume that the following is true:

There are 2 sets of 4 pushbuttons. We will only use group 1 (although you are free to use the others if you wish). 

• Switch 5 should be the RESET signal. 
• Switch 8 should be used to release the break instruction. 
• Switch 7 should be used to select the output of your I/O module.  
• Switch 4 will be a special "SINGLE_CLOCK" signal.  We will mention this below. 

Since these are buttons, they will be naturally bouncy, so you should include the debouncer module from m:\lab4, just as you did in lab 2.

The break instruction outputs an 8-bit signal called STAT.  This should be mapped to the 8 individual LEDs (on the side of the board).

The output from your memory-mapped I/O module should go to the HEX leds.  To drive these properly, you need to bin2Hex module.

The input of the I/O module should come from the first group of 8-bit dip switches (switch9).  Assume that the value on these switches goes to the lowest 8-bits of the input bus and that the top 24-bits are set to zero.  If you like (possibly a good idea), you can consider switching in the current PC or the instruction being executed to the HEX leds when a switch is set.  Consider the second switch of the second set of 8 dipswitches as controlling this.  You can use other switches to indicate what is going to be displayed other than the normal I/O.

Finally, the CLOCK net for your pipeline should be connected either to the clk from the DLL off of the XILINX board or to your debounced SINGLE_CLOCK signal.  Let the first switch of the second set of 8 dipswitches (switch10) be the choice (call this signal "CLK_SOURCE"):

    processor_clock = CLK_SOURCE ? LAB_CLK: SINGLE_CLOCK;

Please read the documentation on DLLs, available in the M:\lab4\DLL_Examples for more information on using DLLs. TopLevel.v is currently setup so that you will receive a 27 Mhz clock. This may be too fast.

Problem 1e: Initial testing

Build one or more test-bench modules around the FPGA top-level module above that provides clock (as in LAB 3), and prints output to the console when I/O changes, and perhaps "pushes" the buttons for testing.  Note that debouncing of the switches is tricky when interacting with the clock.  Think carefully before trying to test the single-stepping clock feature.  You may also want to design test-benches to wrap around your processor module (inside the FPGA top-level module) just to make things easier to test.

To test your processor, you should write diagnostic programs, similar to those that you wrote in lab 1 (broken spim).  You might even want to reuse some of your test code from lab 1....  

Think carefully about the I/O features.  How will you test these?   For demonstrating your pipeline in simulation, create a test module that recognizes when break has been asserted, waits 10 cycles, then asserts the release line -- printing something to the console in the process.  This will allow us to run programs that utilize the I/O features to output results.

Keep in mind that you haven't handled hazards yet (Problem 3), so you must be careful that your test programs don't try to use values too soon after they are generated.

Problem 2: Add a multiplier unit

Add a multiplier unit to your design.  This means adding the following three new instructions: multu, mflo, mfhi. You may simply use one of the multipliers you created in lab 2, and insert it into your processor and modify it as necessary.

In keeping with the original MIPS multiplier, your multiplier is a "coprocessor." By this, we mean that the multiplier will calculate it's result independent of the rest of the processor. When you receive a mulu, you rmultiplier should take the multiplier and multiplicand during the execute stage and starts executing.  Sometime later (independent of the rest of the pipeline, it will finish).  This means that

• Your HI and LO registers should be contained in your multiplier
• In the decode stage, your should stall the pipeline on multu, mflo, or mfhi if the multiplier is still running.  (Actually, if you are clever, you can stall if it will still be executing on the next cycle, since that is when we would try to start another multiply/need to bypass the ALU with the value of the HI or LO register). You may want to add a control signal coming out of your multiplier signaling whether or not it's busy.

How many cycles does your multiplier take? What's the penalty of a 64 cycle multiplier vs. a 32 cycle multiplier now?

Make sure to test this with a number of different distances between the multu instruction and mflo or mfhi instructions.
 

Problem 3: Handling Hazards

Problem 3a: Handle hazards

Here is a list of the hazards you must handle:

• Data Hazards: Data hazards occur when the data produced by an instruction is used as an operand by subsequent instructions.
 
• The ALU is one source of data hazards. Add the necessary forwarding logic and buses so that the result of an ALU operation can immediately be used by the following three instructions without waiting for the data to be written in the register file.
• Load instructions also present a data hazard because the data is not available until the end of the MEM phase. The MIPS instruction set specifies that load instructions have a one cycle delay. That means that the compiler cannot generate code sequences where the data of a load will be used during the following cycle. Implement your datapath so that it has one load delay slot.

• Control Hazards: Branch instructions present a common case of control hazards. Comply with the MIPS instruction set definition and implement your processor so that it has one branch delay slot that is always executed regardless of the result of the branch.
• Structural hazards: are there any in this design?  If so, explain what they are and how you are handling them.  If not, why not?

ADD    $1, $2, $3
SLL    $5, $6
SUB    $6, $1, $7

Problem 3b: Pipeline interlocks/Interlocking Loads

Now that you have basic hazards dealt with,  you should figure out how to handle pipeline stalls.  Your current processor deals with the load delay slot in the same way as the original version of MIPS: If the compiler generates a code sequence in which a value is loaded from memory and used by the next instruction, the following instruction gets the wrong value.  Of course, the instruction specification explicitly disallowed such code sequences; if no other options were available, the compiler would have to introduce a noop in the load delay slot to avoid getting the wrong answer.

As your final exercise, introduce a pipeline stall so that a value can be used by the compiler in the very next cycle after it is loaded from memory. This feature was added to later versions of the MIPS instructions set.  To be clear, we want the following code sequence to do the "obvious" thing, i.e. the add should use the value loaded from memory:

    LW     $1, 4($2)
    ADD    $2, $1, $3

Make sure to rerun all of your tests from part 3a to verify that you haven't broken anything.  Write tests that try several different distances between loads and their following values.  Hint: the mechanism for this single-cycle stall is very similar to what you need for the break instruction...
 

Problem 4: Map it to the board

As  a last step, map your processor design directly down to the boards.  Make sure to read the information in Lab5Help.pdf about changes to get versions of the RAMs that map to the on-chip XILINX boards.  You should be using the same design flow as you did in lab 2.

Note that you should be able to put the processor in single-step mode (first dipswitch of second set put to zero).  Then you should be able to use push-button #4 as a single-stepping clock.  You should also be able to spread break instructions in your code and debug code this way.  Note also that you should be able to use a loop at the very end of your execution with a combination of break instructions and writes to the I/O (address to 0xFFFFFFF0, data to 0xFFFFFFF4) to dump the contents of your memory to the HEX display when you are done.  Make sure that this works!

Make sure that the RESET line causes important processor state to be reset!  Remember that "initial" blocks in Verilog will be ignored by the synthesizer.  Many bugs can be introduced when registers contain random initial state!  One obvious thing that must be reset is the PC.  Are there other things?

Don't try to debug everything at once.  Start with extremely simple examples.  Possibly divert the HEX LEDs to display PC information during debugging (feel free to divert other things as well).  For instance, what about simple program with break as the first instruction and a bunch of nops.  Can you get that to work?  What about simple I/O examples?  If you make sure that your simplest debugging mechanisms work, then you can move on to more expensive examples.

Please include information in your writeup about the total number of FPGA slices used for your design and the fraction of the Xilinx part that has been used for your design.  This information should be availble in the log files post place-and-route.
 

Problem 5: Pipelining Gain

    Calculate the cycle time for your pipelined processor.  We will post some information on using the Xilinx timing analysis tools soon. In your writeup, discuss what your critical path is, and what steps you could do to reduce it.  If we just took our lab 3 single-cycle processor, and added pipeline registers at key points, we would expect the cycle time to be the inverse of the delay through the longest block (ALU? Next PC? Memory?).  Is this the perfromance that you were able to achieve?   Why or why not?

Wrap it up:  Turn in a copy of your verilog code, processor schematic, diagnostic program(s) and your on-line logs.  Also turn in simulation logs that show correct operation of the processor.  These logs should show the operations that were performed, and then the contents of memory with the correct values in it.

As part of your writeup, explain to us your testing philosophy.  Why do you think that you have a working processor?

How much time did your team spend on this assignment?