Lab 3: RISC-V Assembly
Deadline: Friday, September 24, 04:00:00 PM PT
Goals
- Get familiar with using the Venus simulator
- Practice running and debugging RISC-V assembly code.
- Get an idea of how to translate C code to RISC-V.
- Write RISC-V functions with the correct function calling procedure.
Setup
In your labs
directory, pull the files for this lab with:
If you get an error like the following:
fatal: 'starter' does not appear to be a git repository
fatal: Could not read from remote repository.
make sure to set the starter remote as follows:
and run the original command again.
Please note that it may take longer to pull this set of starter updates, since Venus, a RISC-V simulator, is included.
Introduction to Assembly
In this course so far, we have dealt mostly with C programs (with the .c
file extension), used the gcc
program to compile them to machine code,
and then executed them directly on your computer or hive machine.
Now, we're shifting our focus to the RISC-V assembly language, which is a
lower-level language much closer to machine code.
We can't execute RISC-V code directly because your computer and the hives
are built to run machine code from other assembly languages --- most likely
x86 or ARM.
For the next few labs, we will work with several RISC-V assembly files,
each of which has a .s
file extension. To run these, we will be using Venus, an educational RISC-V assembler and simulator.
You can run Venus locally from your own terminal or on the Venus website, and the following instructions will guide you through the steps to set it up. Though you may find using the web editor
easier to use for this lab, please go through these instructions for local setup
regardless: these steps will also set up other infrastructure needed for future
projects and labs.
Assembly/Venus Basics
To get started with Venus, please take a look at "The Editor Tab" and "The Simulator Tab" in the Venus reference. We recommend that you read this whole page at some point, but these sections should be enough to get started.
Warning: For the following exercises, please make sure your completed code is saved on a file on your local machine. Otherwise, we will have no proof that you completed the lab exercises.
Exercise 1: Connecting your files to Venus
You can "mount" a folder from your local device onto Venus's web frontend, so that edits you make within the browser Venus editor are reflected in your local file system, and vice versa. If you don't do this step, files created and edited in Venus will be lost each time you close the tab, unless you copy/paste them to a local file.
This exercise will walk you through the process of connecting your file system to Venus, which should save you a lot of trouble copy/pasting files between your local drive and the Venus editor.
If for some reason this feature ends up not working for you (it's relatively new, and there's a chance there might still be bugs), then for the rest of this assignment, wherever it says to open a file in Venus, you should copy/paste the contents into the Venus web editor, and manually copy/paste those changes back to your local machine.
Here's what you need to do:
- If you don't already have your labs repo cloned on your local machine, open a terminal on your local machine and clone it.
- Windows users should clone outside WSL (Git Bash is recommended). Note that Windows paths are also accessible from WSL (e.g.
C:/Users/oski/cs61c/labs/
in Windows is/mnt/c/Users/oski/cs61c/labs/
in WSL).
- Windows users should clone outside WSL (Git Bash is recommended). Note that Windows paths are also accessible from WSL (e.g.
cd
into your labs repo folder, and runjava -jar tools/venus.jar . -dm
. This will expose your lab directory to Venus on a network port.- You should see a big "Javalin" logo.
- If you see a message along the lines of "port unable to be bound", then you
can specify another port number explicitly by appending
--port PORT_NUM
to the command (for example,java -jar tools/venus.jar . -dm --port 6162
will expose the file system on port 6162).
- Open https://venus.cs61c.org in your web browser (Chrome or Firefox are recommended). In the Venus web terminal, run
mount local labs
(if you chose a different port, replace "local" with the full URL, such ashttp://localhost:6162
). This connects Venus to your file system.- In your browser, you may see a prompt saying
Key has been shown in the Venus mount server! Please copy and paste it into here.
. You should be able to see a key in the most recent line of your local terminal output; just copy and paste it into the dialog.
- In your browser, you may see a prompt saying
- Go to the "Files" tab. You should now be able to see your labs directory under
the
labs
folder. - Navigate to
lab03
, and make sure it works by hitting theEdit
button next toex1.s
. This should open in theEditor
tab.- If you make any changes to the file in the
Editor
tab, hitting command-s on a Mac and ctrl-s on Windows/Linux will update your local copy of the file. To check if the save was successful, open the file on your local machine to see if it matches what you have in the web editor (unfortunately no feedback message has been implemented yet). - Note: If you make any changes to a file in your local machine, if you had the same file open in the Venus editor, you'll need to reopen it from the "Files" menu to get the new changes.
- If you make any changes to the file in the
- To make it so that the file system will attempt to remount automatically whenever you close and reopen Venus, enable "Save on Close" in the Settings pane (again in the Venus tab). This will make the Venus web client attempt to locate the file system exposed by running Venus locally, and will pop up an error saying that it couldn't connect to the server if it doesn't see it running. If this happens, just follow the above steps to manually remount the file system.
Once you've got ex1.s
open, you're ready to move on to Exercise 2!
Exercise 2: Familiarizing yourself with Venus
Getting started:
- Open
ex1.s
into the Venus editor. If you were unable to mount the filesystem in Exercise 1, then you can copy/pasteex1.s
from your local machine into the Venus editor directly. - Click the "Simulator" tab and click the "Assemble & Simulate from Editor" button. This will prepare the code you wrote for execution. If you click back to the "Editor" tab, your simulation will be reset.
- In the simulator, to execute the next instruction, click the "step" button.
- To undo an instruction, click the "prev" button. Note that undo may or may not
undo operations performed by
ecall
, such as exiting the program or printing to console. - To run the program to completion, click the "run" button.
- To reset the program from the start, click the "reset" button.
- The contents of all 32 registers are on the right-hand side, and the console output is at the bottom.
- To view the contents of memory, click the "Memory" tab on the right. You can navigate to different portions of your memory using the dropdown menu at the bottom.
Action Item
Open ex1.s
in Venus and answers the following questions. Some of the questions
will require you to run the RISC-V code using Venus's simulator tab.
As with the last lab, since we're not autograding these answers, we've once again provided answers to some of these questions so you can verify your understanding.
- What do the
.data
,.word
,.text
directives mean (i.e. what do you use them for)? Hint: think about the 4 sections of memory.
Denotes where global variables are declared.data
Allocates and initializes space for a 4-byte variable in the data segment..word
Indicates the start of the code..text
- Run the program to completion. What number did the program output? What does this number represent?
- At what address is
n
stored in memory? Hint: Look at the contents of the registers. - Without actually editing the code (i.e. without going into the "Editor" tab), have the program calculate the 13th fib number (0-indexed) by manually modifying the value of a register. You may find it helpful to first step through the code. If you prefer to look at decimal values, change the "Display Settings" option at the bottom.
Exercise 3: Translating from C to RISC-V
Open the files ex2.c
and ex2.s
. The assembly code provided (.s file) is a
translation of the given C program into RISC-V.
In addition to opening a file in the "Editor" tab and then running in the "Simulator"
tab as described above, you can also run ex2.s
directly within the Venus terminal
by cd
ing into the appropriate folder, then running run ex2.s
or ./ex2.s
.
Typing vdb ex2.s
will also assemble the file and take you to the "Simulator" tab
directly.
Action Item
Find and identify the following components of this assembly file, and be able to explain how they work.
The register representing the variable `k`.
Thet0
register.The register representing the variable `sum`.
Thes0
register.The registers acting as pointers to the `source` and `dest` arrays.
Thes1
register is source,s2
is destThe assembly code for the loop found in the C code.
The loop label denotes the start of the loop; thejal
at the end of the label causes the loop to keep running, and thebeq
at the start checks the end condition.How the pointers are manipulated in the assembly code.
Theslli
instruction computes an offset from the start of the address stored ink
. This offset is added tos1
ands2
to find the address from which we want to load word.
Exercise 4: Factorial
In this exercise, you will be implementing the factorial
function in RISC-V.
This function takes in a single integer parameter n
and returns n!
. A stub of
this function can be found in the file factorial.s
.
The argument that is passed into the function is located at the label n
. You can modify n
to test different factorials. To implement, you will need to add instructions under the factorial
label. Note that you may find it helpful to add additional labels to simplify control flow. You may solve this
problem using either recursion or iteration and you can assume that the factorial
function will only be called on positive values with results that won't overflow a
32-bit two's complement integer.
Testing
As a sanity check, you should make sure your function properly returns
that 3! = 6
, 7! = 5040
and 8! = 40320
.
You can test this using the online version of Venus, but as promised, we've also provided
Venus for you to test locally! We'll be using this local version in the autograder, so make sure
to update your factorial.s
file and run the following command before you submit to verify
that the output is correct (You will need to run this from the labs directory).
Exercise 5: RISC-V function calling with map
This exercise uses the file list_map.s
.
In this exercise, you will complete an implementation of map
on linked-lists in RISC-V. Our function will be simplified to mutate the
list in-place, rather than creating and returning a new list with the
modified values.
You will find it helpful to refer to the RISC-V green card to complete this exercise. If you encounter any instructions or pseudo-instructions you are unfamiliar with, use this as a resource.
Our map
procedure will take two parameters; the first parameter will
be the address of the head node of a singly-linked list whose values are
32-bit integers. So, in C, the structure would be defined as:
;
Our second parameter will be the address of a function that takes
one int
as an argument and returns an int
. We'll use the jalr
RISC-V
instruction to call this function on the list node values (how does jalr
work?).
Our map
function will recursively go down the list, applying the
function to each value of the list and storing the value returned in
that corresponding node. In C, the function would be something like
this:
void
If you haven't seen the int (*f)(int)
kind of declaration before,
don't worry too much about it. Basically it means that f
is a pointer
to a function that takes an int
as an argument (you may recall that Philphix makes use of function pointers as well). We can call this function f
just like any other.
There are exactly ten (10) markers (1 in done
, 7 in map
, and 2 in main
) in the
provided code where it says YOUR CODE HERE
.
Action Item
Complete the implementation of map
by filling out each of these ten markers
with the appropriate code. Furthermore, provide a call to map
with square
as the function argument. There are comments in the code that explain what should
be accomplished at each marker. When you've filled in these instructions, running
the code should provide you with the following output:
9 8 7 6 5 4 3 2 1 0
81 64 49 36 25 16 9 4 1 0
80 63 48 35 24 15 8 3 0 -1
The first line is the original list, and the second line is the list with all elements
squared after calling map(head, &square)
, and the third is the list with all elements
decremented after now calling map(head, &decrement)
.
The autograder will check that your code satisfies calling convention, so make sure you are saving and loading where necessary.
Testing
To test this in the Venus web simulator, run list_map.s
and examine the output.
To test this locally, run the following command in your labs directory (much
like the one for factorial.s
):
Transitioning to More Complex RISC-V Programs
In the future, we'll be working with more complex RISC-V programs that require multiple files of assembly code. To prepare for this, we recommend looking over the Venus reference.
Submission
Save, commit, and push your work, then submit to the Lab 3 assignment on Gradescope.
Checkoff
Be prepared to answer the Action Item questions in Exercises 2 and 3.