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RTOS Demo for RISC-V QEMU sifive_e Model
[RTOS Ports]


This page documents a pre-configured SiFive Freedom Studio project that builds and runs a FreeRTOS RISC-V demo in the sifive_e QEMU model using GCC and GDB.

IMPORTANT! Notes on using the SiFive RISC-V port

Please read all the following points before using this RTOS port.

  1. Instructions on using FreeRTOS on RISC-V cores
  2. Source code organisation
  3. The demo application functionality
  4. Building the RTOS demo application
  5. Running/debugging the RTOS demo in the QEMU emulator
  6. RTOS configuration and usage details
Also see the FAQ My application does not run, what could be wrong?.

Instructions on using FreeRTOS on RISC-V cores

If you want to go beyond just running the demo described on this page, or if you want to create your own RISC-V FreeRTOS project, then please also read the documentation page that provides generic information on running the FreeRTOS kernel on RISC-V cores.

Source Code Organization

The FreeRTOS zip file download contains the source code for all the FreeRTOS ports, and every demo application. That means it contains many more files than are required to use the FreeRTOS sifive_e QEMU RISC-V demo. See the Source Code Organization page for information on the zip file’s directory structure. The sifive_e RISC-V QEMU Freedom Studio project is located in the /Demo/RISC-V-Qemu-sifive_e-FreedomStudio directory. More information is provided in the build instructions section below.

On RISC-V architectures the additional freertos_risc_v_chip_specific_extensions.h header file is used to extend the base RISC-V RTOS port to any chip specific extensions the target RISC-V chip may implement. The QEMU sive5_e model does not implement any registers over and above those defined by the base RISC-V architecture, and does emulate a CLINT. Therefore this project uses the freertos_risc_v_chip_specific_extensions.h header file from the /FreeRTOS/Source/portable/GCC/RISC-V/chip_specific_extensions/RV32I_CLINT_no_extensions directory.

The sifive_e QEMU RISC-V Demo Application


The constant mainCREATE_SIMPLE_BLINKY_DEMO_ONLY, which is defined at the top of main.c, is used to switch between a simple ‘blinky’ style getting started project and a more comprehensive test and demo application.


When mainCREATE_SIMPLE_BLINKY_DEMO_ONLY is set to 1 main() calls main_blinky(). main_blinky() creates a basic example that uses two tasks and one queue.
  • The Queue Send Task:

    The queue send task is implemented by the prvQueueSendTask() function. It sits in a loop, sending the value 100 to the queue every 1000 emulated milliseconds (1 emulated second).

  • The Queue Receive Task:

    The queue receive task is implemented by the prvQueueReceiveTask() function. It sits in a loop that blocks on attempts to read from the queue (no CPU cycles are consumed while the task is blocked), writing ‘blink’ to the QEMU console each time the value 100 is received from the queue send task.

    As the queue send task writes to the queue every 1000 emulated milliseconds the queue receive task unblocks and writes to the QEMU console every 1000 emulated milliseconds.


When mainCREATE_SIMPLE_BLINKY_DEMO_ONLY is set to 0 main() calls main_full(). main_full() implements a comprehensive test and demo application that demonstrates and/or tests (among other things): The created tasks are from the set of standard demo tasks. Standard demo tasks are used by all FreeRTOS port demo applications. They have no specific functionality, and are created just to demonstrate how to use the FreeRTOS API, and test the RTOS port.

A ‘check’ task is created that periodically inspects the standard demo tasks (which contain self monitoring code) to ensure all the tasks are functioning as expected. The check task outputs a ‘.’ character or error message to the QEMU console each time it executes. This gives visual feedback of the system health. If the ‘.’ appears on the console every 3 emulated seconds (which may be different to real seconds), then the check task has not discovered any problems. If the console displays an error message then the check task has discovered a problem in one or more tasks.

Building the RTOS demo application

Important note: The project will not build if the directory structure is different to the directory structure used in official FreeRTOS zip file releases. Ensure the ‘copy projects into workspace’ check box is not checked when importing the project into the Eclipse workspace.

To open and build the Freedom Studio RISC-V project:

  1. Download and install the Freedom Studio development tools (scroll down to see software downloads).

  2. Start Freedom Studio and either select an existing or create a new workspace when prompted.

  3. Select “Import…” from the Freedom Studio ‘File’ menu. The Import dialog box will open.

  4. In the Import dialog box, select “General->Existing Project into Workspace”. The Import Projects dialog box will open.

    Importing an existing project into the workspace

  5. In the Import Projects dialog box, navigate to and select the FreeRTOS/Demo/RISC-V-Qemu-siFive_e-FreedomStudio directory, and ensure the ‘copy projects into workspace’ check box is not checked.

    Selecting the directory and project in the Import Project dialog box. Click to enlarge.

  6. In the ‘Projects’ window of the Import Projects dialog box, select the RTOSDemo project, and click finish.

  7. Select “Build all” from the Freedom Studio ‘Project’ menu. The project should build without any errors or warnings, and create a file called RTOSDemo.elf.

Running the RTOS demo application in the QEMU emulator

  1. Download QEMU. The project was created and tested using the pre-built Windows binaries.

  2. Use the command line below to start QEMU, replacing “path/to” with the real path to the RTOSDemo.elf output by the following the build instructions above:
    qemu-system-riscv32 -kernel path/to/RTOSDemo.elf -S -s -machine sifive_e
  3. Finally, right click the “Hardware_QEMU.launch” file in the Eclipse project explorer, then select “Debug As->Hardware_QEMU” from the pop up menu. The debugger should start and connect to QEMU (assuming the previous step left QEMU running).

    Creating a debug launch configuration. Click to enlarge.

Configuration and Usage Details

RTOS port specific configuration

This section relates to the information provided on the Running FreeRTOS on RISC-V Cores documentation page:
  • Configuration items specific to this demo are contained in FreeRTOS/Demo/RISC-V-Qemu-sifive_e-FreedomStudio/FreeRTOSConfig.h. The constants defined in that file can be edited to suit your application. In particular, as the emulated SiFive core includes a machine timer (MTIMER)configMTIME_BASE_ADDRESS and configMTIMECMP_BASE_ADDRESS are defined to ( CLINT_CTRL_ADDR ) + 0xBFF8 and ( CLINT_CTRL_ADDR ) + 0x4000 respectively.

  • The emulated SiFive core does not include any registers over and above those defined by the base RISC-V architecture. The project therefore uses the freertos_risc_v_chip_specific_extensions.h header file located in the /FreeRTOS/Source/portable/GCC/RISC-V/chip_specific_extensions/RV32I_CLINT_no_extensions directory, so that directory is in the assembler’s include path.

  • The interrupt handler provided in the SiFive software development kit (SDK) is called trap_handler, so the assembler’s command line options include -DportasmHANDLE_INTERRUPT=handle_trap.

  • The file flash.lds is a version of the linker script provided with the development tools, edited to add the __freertos_irq_stack_top linker variable necessary to ensure the stack that was used by main before the scheduler starts is reused as the interrupt stack after the scheduler starts.

Other notes:

  • vPortEndScheduler() has not been implemented.

  • Source/Portable/MemMang/heap_4.c is included in the RISC-V project to provide the memory allocation required by the RTOS kernel. Please refer to the Memory Management section of the API documentation for full information.

  • At the time of writing, the demo does not support interrupt nesting.

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