GNU MCU Eclipse

A family of Eclipse CDT extensions and tools for GNU ARM & RISC-V development

GNU MCU Eclipse OpenOCD v0.10.0-3-20170826* released GNU MCU Eclipse OpenOCD v0.10.0-4-20171004* released

GNU MCU Eclipse RISC-V Embedded GCC v7.1.1-2-20170912-* released

Version v7.1.1-2-20170912-2255 is a new release of GNU MCU Eclipse RISC-V Embedded GCC; after the initial version of the riscv64-unknown-elf toolchain was released, it was discovered that it was intended more to applications that include a kernel-like environment, and less to bare-metal environments. This second release is intended to provide a temporary fix to this problem and make the toolchain more suitable for embedded bare-metal applications; for this it was necessary to to update to the latest newlib 2.5 release which uses ‘underscore’ syscall functions and to remove the mandatory use of libgloss. Hopefully the official toolchain distribution will follow shortly and use the same approach.

In addition, based on users request, support for a new march=rv32imaf/mabi=ilp32f library was added.

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All GNU MCU Eclipse RISC-V Embedded GCC releases are based on the official RISC-V source files maintained by SiFive.

The current version is based on the following commits:

The supported architectures (-march=) are:

  • rv32i[m][a][f[d]][c]
  • rv32g[c]
  • rv64i[m][a][f[d]][c]
  • rv64g[c]

The supported ABIs (-mabi=) are:

  • ilp32 (32-bits, soft-float)
  • ilp32f (32-bits with single-precision in registers and double in memory, niche use only)
  • ilp32d (32-bits, hard-float)
  • lp64 (64-bits long and pointers, soft-float)
  • lp64f (64-bits long and pointers, with single-precision in registers and double in memory, niche use only)
  • lp64d (64-bits long and pointers, hard-float).

The supported libraries are (in parenthesis are combinations that reuse simpler libraries):

march=rv32i/mabi=ilp32 (march.rv32ic/mabi.ilp32)
march=rv32im/mabi=ilp32 (march.rv32imc/mabi.ilp32)
march=rv32imaf/mabi=ilp32f <-- New
march=rv32imafc/mabi=ilp32f (march.rv32imafdc/mabi.ilp32f, march.rv32gc/mabi.ilp32f)
march=rv64imafdc/mabi=lp64d (march.rv64gc/mabi.lp64d)

Please note that, although all combinations of march/mabi are supported by the compiler, not all of them have libraries.


Compared to the original RISC-V riscv64-unknown-elf toolchain, the following improvements can be noted:

  • a newer newlib was included, which supports the ‘underscore’ syscall functions
  • the mandatory reference to libgloss in the linker configuration was removed
  • the march=rv32imaf/mabi=ilp32f libary was added to the list of multilibs
  • support for newlib-nano was added
  • the standard documentation, in PDF and HTML, was added


Currently GNU MCU Eclipse RISC-V Embedded GCC is the only RISC-V toolchain that provides support for newlib-nano, using the --specs=nano.specs option.

If no syscalls are needed, --specs=nosys.specs can be used at link time to provide empty implementations for the POSIX system calls.

The nano versions of the libraries are configured with simplified implementations for printf() and malloc(); in addition they are compiled with -Os, while the regular versions are compiled with -O2.

‘Underscore’ newlib syscalls

The initial RISC-V newlib was erroneously configured to directly call system functions via their direct names (like write()), assuming they are implemented via kernel traps in libgloss.

The latest newlib 2.5.0 for RISC-V fixed this, and switched to the usual newlib configuration, which uses ‘underscore’ functions (like _write()) that must be defined by the application to implement the system calls.

This change is welcome, since it brings the RISC-V toolchain in line with other toolchains, like arm-none-eabi.

Unfortunatley this change breaks the builds for the initial SiFive SDK samples, which implements the direct function names in the libwrap library. To fix them, libwrap should no longer be used, the --wrap options should no longer be passed to the linker, and several functions (like _write(), _istty(), …) must be implemented by the application.


For RISC-V, this library implements all syscalls via kernel traps; on other platforms (like ARM) it is used for semihosting.

The GNU MCU Eclipse RISC-V Embedded GCC toolchain no longer links automatically libgloss; however libgloss is still available in the distribution and applications that need to access a kernel can include it using the usual -lgloss link option.


Another addition compared to the official distribution is the presence of the documentation files, including the PDF manuals for all tools.


Binaries for Windows, macOS and GNU/Linux are provided. For Windows and macOS, separate installable and plain archives are provided. For Windows and GNU/Linux, both 32/64-bits binaries are provided.

Instructions on how to install them are available in the How to install the RISC-V toolchain? page.

For better control and repetability, the build scripts use Docker containers; all files required during builds are available as a separate gnu-mcu-eclipse/riscv-none-gcc-build project.


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