Keyboard shortcuts

Press ← or → to navigate between chapters

Press S or / to search in the book

Press ? to show this help

Press Esc to hide this help

Writing Programs

This chapter covers how to write and run RISC-V assembly programs in QtRvSim.

Using the Built-in Assembler

QtRvSim includes a built-in assembler that can compile RISC-V assembly code directly within the application:

  1. Write your assembly code in the editor
  2. Click Compile Source (Ctrl+E) to assemble the code
  3. The program is loaded into simulated memory and ready to execute

This is the recommended workflow for learning and experimenting with RISC-V assembly.

RISC-V Register Reference

RISC-V has 32 general-purpose registers. The simulator supports both numeric names (x0–x31) and ABI mnemonic names:

RegisterABI NameDescription
x0zeroHard-wired zero
x1raReturn address
x2spStack pointer
x3gpGlobal pointer
x4tpThread pointer
x5–x7t0–t2Temporary registers
x8s0/fpSaved register / Frame pointer
x9s1Saved register
x10–x11a0–a1Function arguments / Return values
x12–x17a2–a7Function arguments
x18–x27s2–s11Saved registers
x28–x31t3–t6Temporary registers

Toggle between numeric and mnemonic names via Machine → Mnemonic Registers.

Simple Assembly Program Template

Here is a minimal RISC-V assembly template for use with the built-in assembler:

.globl _start
.text

_start:
    # Your code here
    
    # Example: Load immediate values
    li a0, 42          # Load 42 into a0
    li a1, 10          # Load 10 into a1
    add a2, a0, a1     # a2 = a0 + a1

end:
    ebreak             # Stop execution
    j end              # Loop forever if continued

The ebreak instruction triggers a breakpoint exception, which causes the simulator to pause execution.

Breakpoints and Debugging

Software Breakpoints

The ebreak instruction causes the simulator to pause execution. This is useful for stopping at specific points in your program:

    # ... some code ...
    ebreak           # Pause here
    # ... more code ...

When ebreak is encountered, the simulator stops. You can then inspect registers and memory, and continue execution with Step or Run.

Hardware Breakpoints

You can also set breakpoints directly in the QtRvSim interface:

  1. In the Program panel, click on the address or instruction where you want to break
  2. The simulator will pause when that instruction is fetched

Loading Programs

There are several ways to load programs into QtRvSim:

  1. Built-in editor: Write assembly in the editor and compile with Ctrl+E
  2. Open source file: Use File → Open Source to load a .s file, then compile
  3. Load ELF: In the launch dialog, use Browse to select a precompiled .elf file
  4. Build Executable: Use Machine → Build Executable (Ctrl+B) to compile using an external toolchain

Next Steps

Once you’re comfortable writing basic assembly programs, you may want to explore: