Simulator

The Simulator is the main component of venus.

Simulator Controls

The following controls are available:

• Run / Stop: runs the program until it terminates or a breakpoint is hit. While running, the program can be stopped by clicking it again.
• Step: executes the highlighted instruction.
• Prev: undo the previously executed instruction.
• Reset: resets the simulator to its initial state (equivalent to reassembling the program).

Program View

The program view displays the assembled program. It contains three columns:

• Machine Code: the machine code of the instruction in hexadecimal.
• Basic Code: the assembly code after pseudoinstruction expansion and linking.
• Original Code: the original code in the program.

Debugging

The instruction which is about to be executed is highlighted in green.

In order to add a breakpoint, simply click on the instruction. This will cause the row to be highlighted red, indicating that a breakpoint has been set. To remove the breakpoint, click it again.

Another option is if you always want to break at a certain point in the code, you can just put an ebreak where you want the debugger to stop at. The debugger will stop just before it executes the instruction after the ebreak.

Debugging Tips

Debugging RISC-V can be hard! Hopefully Venus can help you step though your code. If you are struggling with understanding what is actually happening, I recommend that you first analyze your code and write out what you expect it to do. Then go into the debugger and step through to see if your program matches what you expected it to do. This can be hard and time-consuming but I found for myself, this was the most helpful. Using breakpoints can be helpful but make sure you are looking at the right time to break. Currently Venus does not support conditional breakpoints but you can always make a branch like this to force a conditional breakpoint:

beq a0 x0 nobreak
    ebreak
nobreak:
    nop # This is is the next line of your code which should be executed if the break is or is not taken.

Make sure you have custom labels for all of the breakpoints you may set since Venus does not allow for duplicate labels in a file. What this code is doing is a conditional where we want to break when a0 != 0 since if the condition is such that a0 == 0 We will jump to nobreak which means the ebreak will never be executed thus not executing that breakpoint. If that condition is not true, we will execute the ebreak which will cause Venus to break on the nop (which is just a placeholder for the instruction which you want to conditionally stop before it is executed).

Note that ebreak only works on the Webversion at the moment.

Sidebar View

Registers

The register tab displays the values of the current registers. The most recently changed register is highlighted in blue. In addition, the registers values can be edited.

Memory

The memory tab shows the bytes currently in memory.

Memory Controls

The Jump to select box allows jumping to a memory segment.

The Up / Down buttons scroll the memory view. Up moves towards higher addresses, and down moves towards lower addresses.

The Address box allows you to jump to a specific address of memory.

Display Settings

The display settings can be used to change how values are displayed. By default, values are displayed in unsigned hexadecimal.

• Hex (default) - display as unsigned hexadecimal.
• Decimal - display as signed decimal.
• Unsigned - display as unsigned decimal.
• ASCII - display as ASCII character, surrounded by single quotes.
  • If the number is too big (not between 0 and 255), it is displayed as hex instead.
  • Anything which is in ASCII but not printable is displayed as the unicode replacement glyph: �

Console Output

The console output is used for ecalls and displaying any errors which occur.

Exit Code

The web version will now display an exit code when the program finishes. This may be due to the program calling an ecall or the PC counter going beyond the loaded text section. If your program goes beyond the program’s text, it will determine the exit code from the register a0. You can think of it as returning an int from main. The JV version will actually cause Venus to exit with that error code. Note that Venus jvm currently exits with a -1 error code when an internal error occurs (aka. Assembly error).