In order to make a correct design in a somewhat expedient fashion you need to be methodical!
This means you should have a good idea of how your processor should work before you start writing code. While chisel is more pleasent to work with than other HDLs the bricoleur approach is not recommended.
My recommended approach is therefore to create an RTL sketch of your processor design. Start with an overall sketch showing all the components, then drill down. In your sketch you will eventually add a box for registers, IMEM and DMEM, which should make it clear how the already finished modules fit into the grander design, making the skeleton-code less mysterious.
To give you an idea of how a drill down looks like, here is my sketch of the ID stage: Note that this sketch does not contain everything an ID stage needs to contain, this is simply an example. First draft was made on paper.
I would generally advice to do these on paper, but don’t half-ass them. I advise you to use squared paper unless you are exceptionally talented at freehand drawing.
In order to get started designing your processor the following steps guide you to
implementing the necessary functionality for adding two integers by implementing the
ADDI operation which you can read about in the ISA overview.
Info will be progressively omitted in the later steps in order to not bog you down in repetitive details. After all brevity is wit.
In order to verify that the project is set up properly, open sbt in your project root
by typing ./sbt.sh (or simply sbt if you already use scala).
sbt, which stands for scala build tool will provide you with a repl where you can
compile and test your code. This should be familiar from the chisel introduction exercise.
If you have not done this I advise you to complete it first, it can be found here: Chisel Intro
The initial run might take quite a while to finish as all the necessary stuff is downloaded.
In your sbt console, type compile to verify that everything compiles correctly.
In your console, type test to verify that the tests run, and that chisel can correctly
build your design.
This command will unleash the full battery of tests on you, so be prepared for a lot of
console outpute.
To reduce the amount of tests being run you need to modify the test manifest.
In the very top of the Manifest object, change the value of singleTest from "forward2.s"
to "addi.s". It is not necessary to deal with filepaths, all tests are globally visible and
should preferrably not share names even if they are in different directories.
The full battery of tests can be found in ./src/test/resources/tests/ but for now focus on
./src/test/resources/tests/basic/immediate/addi.s which as you can see is an assembly file
consisting only of addi instructions.
When running the following in the sbt console: testOnly FiveStage.SingleTest
only the test pointed at in Manifest.singleTest will be run, and the log will be more
thorough.
For now the log will be rather confusing since your processor is doing nothing.
As you follow the guide you will see the output log making more sense!
Now that only one test is running it is time to take it from red to green.
In order to execute instructions your processor must be able to fetch them.
In ./src/test/main/IF.scala you can see that the IMEM module is already set to fetch
the current program counter address (line 41), however since the current PC is stuck
at 0 it will fetch the same instruction over and over. Rectify this by commenting in
// PC := PC + 4.U at line 48.
You can now verify that your design fetches new instructions each cycle by running
the test as in the previous step. The log should now be much shorter since the tester
will be able to synchronize the clock and correctly deduce that the DUT (device under test)
is not doing anything.
Next, the instruction must be forwarded to the ID stage, so you will need to add the instruction to the io interface of the IF module as an output signal. In ./src/test/main/IF.scala at line 21 you can see how the program counter is already defined as an output. You should do the same with the instruction signal.
Note
Even though an instruction is just a 32 bit signal it is very useful to treat it as
a more refined type.
In ./src/main/scala/ToplevelSignals.scala you can see the Instruction class which
comes with many useful helper methods.
When defining an output for the instruction you need to define the type as follows:
Output(new Instruction)
This is also explained in the comments in the file itself.
As you defined the instruction as an output for your IF module, declare it as an input
in your ID module (./src/test/main/ID.scala line 21).
This input should be defined nearly identical to step 2, only substituting Output with
Input like the following: Input(new Instruction)
Next you need to ensure that the registers and decoder gets the relevant data from the
instruction.
This is made more convenient by the fact that Instruction is a class, allowing you
to access methods defined on it.
Your IDE should give you some hints as to what these methods are, but if you want to look
at the definition you can check out Instruction in TopLevelSignals.scala.
Keep in mind that it is only a class during compile and build time, it will be
indistinguishable from a regular UInt(32.W) in your finished circuit.
The methods can be accessed like this:
// Drive funct6 of myModule with the 26th to 31st bit of instruction
myModule.io.funct6 := io.instruction.funct6Your IF should now have an instruction as an OUTPUT, and your ID as an INPUT, however they are not connected. This must be done in the CPU class where both the ID and IF are instantiated. In the overview sketch you probably noticed the barriers between IF and ID. In accordance with the overview, it is incorrect to directly connect the two modules, instead you must connect them using a barrier.
A barrier is responsible for keeping a value inbetween cycles, facilitating pipelining. There is however one complicating matter: It takes a cycle to get the instruction from the instruction memory, thus we don’t want to delay it in the barrier!
It is not very conductive to learning to introduce a rule and then break it right away, however it is a good idea to highlight the importance of RTL sketches! If you look at the ID stage sketch at the top you can see that the Instruction memory block is overlapping with the IFID barrier register, reminding you that the instruction should not be stored in the barrier.
In order to make code readable I suggest adding a new file for your barriers, containing four different modules for the barriers your design will need. I prefer one file per barrier rather than one large file for all, but you can do as you please.
Start with implementing your IF barrier module, which should contain the following:
- An input and output for PC where the output is delayed by a single cycle.
- An input and output for instruction where the output is wired directly to the input with no delay.
The sketch for your barrier looks like this
Hints
The instruction signal can be wired straight from input to output.
The PC must be saved in a register. You can use RegInit(0.U(32.W)) to define this register.
By driving the register with the input PC and the output with the register you will attain
a one cycle delay.
You can now verify that the correct control signals are produced, either with printf or gtkwave. ensure that:
- The program counter is increasing in increments of 4
- The instruction in ID is as expected
- The decoder output is as expected
- The correct operands are fetched from the registers
I advise you to use gtkwave first and foremost since it has a learning curve and is very useful. Unlike previous exercise the outputs are now located in the waveform directory and is automatically produced each time you run a test.
The following image shows gtkwave output with some formatting showing the desired results:
As you can see, this isn’t very helpful, there’s a little too much data, however it does verify that something is going on. If you followed the introduction you might have wondered how the bootloader works, which is what you are seeing here. While a program is being loaded the setup signal in the testHarness is true (1), thus you should zoom in on what happens as soon as the setup signal is set to false, which is when your processor starts working.
By zooming in on this region you should see something similar (I’ve set data format to decimal in this image to make the output
more readable).
As you can see, the PC signal that ID receives is one cycle delayed compared to IF, whereas the instruction signal is not since
it is one cycle delayed anyways.
You should also verify that registers get the correct signals.
You will now have to create the EX stage. Use the structure of the IF and ID modules to guide you here. In your EX stage you should have an ALU, preferrable in its own module a la registers in ID. While the ALU is hugely complex, it’s very easy to describle in hardware design languages! Using the same approach as in the decoder should be sufficient:
val ALUopMap = Array(
ADD -> (io.op1 + io.op2),
SUB -> (io.op1 - io.op2),
...
)
// MuxLookup API: https://github.com/freechipsproject/chisel3/wiki/Muxes-and-Input-Selection#muxlookup
io.aluResult := MuxLookup(io.aluOp, 0.U(32.W), ALUopMap)As with the ID stage, you will need a barrier between ID and EX stage. In this case, as the overview sketch indicates, all values should be delayed one cycle.
When you have finished the barrier, instantiate it and wire ID and EX together with the barrier in the same fashion as IF and ID. You don’t need to add every single signal for your barrier, rather you should add them as they become needed, i.e if you need a signal in EX, wire it from ID.
Your MEM stage does very little when an ADDI instruction is executed, so implementing it should be easy. All you have to do is forward signals.
From the overview sketch you can see that the same trick used in the IF/ID barrier is utilized
here, bypassing the data memory read value since it is already delayed by a cycle, however addi
does not interact with the data memory so this can be omitted.
You now need to actually write the result back to your register bank. This should be handled at the CPU level. If you sketched your processor already you probably made sure to keep track of the control signals for the instruction currently in WB, so writing to the correct register address should be easy for you ;)
Did you just realize that you had been driving registers.writeEnable and registers.writeAddress
with the instruction from the IFID barrier?
If so the signal is at the correct spot but at the wrong time!
It is only when the instruction is fully computed that it should be written back, therefore the control signals for register write enable and address are propagated through the pipeline at the same pace as the instruction itself so that they reach the register module when the result is ready!
Ensure that the simplest addi test works, and give yourself a pat on the back! You’ve just found the corner pieces of the puzzle, so filling in the rest is “simply” being methodical.