9.18. x86_64

9.18.1. amd64

This BSP offers only one variant, amd64. The BSP can run on UEFI-capable systems by using FreeBSD’s bootloader, which then loads the RTEMS executable (an ELF image).

Currently only the console driver and context initialization and switching are functional (to a bare minimum), but this is enough to run the hello.exe sample in the RTEMS testsuite. Build Configuration Options

There are no BSP configuration options available at build time. Testing with QEMU

To test with QEMU, we need to:

  • Build / install QEMU (most distributions should have it available on the package manager).

  • Build UEFI firmware that QEMU can use to simulate an x86-64 system capable of booting a UEFI-aware kernel, through the --bios flag. Building TianoCore’s UEFI firmware, OVMF

Complete detailed instructions are available at TianoCore’s Github’s wiki.

Quick instructions (which may fall out of date) are:

$ git clone git://github.com/tianocore/edk2.git
$ cd edk2
$ make -C BaseTools
$ . edksetup.sh

Then edit Conf/target.txt to set:

ACTIVE_PLATFORM       = OvmfPkg/OvmfPkgX64.dsc
TARGET                = DEBUG
TARGET_ARCH           = X64
# You can use GCC46 as well, if you'd prefer

Then run build in the edk2 directory - the output should list the location of the OVMF.fd file, which can be used with QEMU to boot into a UEFI shell.

You can find the OVMF.fd file like this as well in the edk2 directory:

$ find . -name "*.fd"
./Build/OvmfX64/DEBUG_GCC5/FV/OVMF.fd # the file we're looking for
./Build/OvmfX64/DEBUG_GCC5/FV/OVMF_VARS.fd Boot RTEMS via FreeBSD’s bootloader

The RTEMS executable produced (an ELF file) needs to be placed in the FreeBSD’s /boot/kernel/kernel’s place.

To do that, we first need a hard-disk image with FreeBSD installed on it. Download FreeBSD’s installer “memstick” image for amd64 and then run the following commands, replacing paths as appropriate.

$ qemu-img create freebsd.img 8G
$ OVMF_LOCATION=/path/to/ovmf/OVMF.fd
$ FREEBSD_MEMSTICK=/path/to/FreeBSD-11.2-amd64-memstick.img
$ qemu-system-x86_64 -m 1024 -serial stdio --bios $OVMF_LOCATION \
    -drive format=raw,file=freebsd.img \
    -drive format=raw,file=$FREEBSD_MEMSTICK

The first time you do this, continue through and install FreeBSD. FreeBSD’s installation guide may prove useful if required.

Once installed, build your RTEMS executable (an ELF file), for eg. hello.exe. We need to transfer this executable into freebsd.img’s filesystem, at either /boot/kernel/kernel or /boot/kernel.old/kernel (or elsewhere, if you don’t mind user FreeBSD’s loader’s prompt to boot your custom kernel).

If your host system supports mounting UFS filesystems as read-write (eg. FreeBSD), go ahead and:

  1. Mount freebsd.img as read-write

  2. Within the filesystem, back the existing FreeBSD kernel up (i.e. effectively cp -r /boot/kernel /boot/kernel.old).

  3. Place your RTEMS executable at /boot/kernel/kernel

If your host doesn’t support mounting UFS filesystems (eg. most Linux kernels), do something to the effect of the following.

On the host

# Upload hello.exe anywhere accessible within the host
$ curl --upload-file hello.exe https://transfer.sh/rtems

Then on the guest (FreeBSD), login with root and

# Back the FreeBSD kernel up
$ cp -r /boot/kernel/ /boot/kernel.old
# Bring networking online if it isn't already
$ dhclient em0
# You may need to add the --no-verify-peer depending on your server
$ fetch https://host.com/path/to/rtems/hello.exe
# Replace default kernel
$ cp hello.exe /boot/kernel/kernel
$ reboot

After rebooting, the RTEMS kernel should run after the UEFI firmware and FreeBSD’s bootloader. The -serial stdio QEMU flag will let the RTEMS console send its output to the host’s stdio stream. Paging

During the BSP’s initialization, the paging tables are setup to identity-map the first 512GiB, i.e. virtual addresses are the same as physical addresses for the first 512GiB.

The page structures are set up statically with 1GiB super-pages.


Page-faults are not handled.


RAM size is not detected dynamically and defaults to 1GiB, if the configuration-time RamSize parameter is not used. Interrupt Setup

Interrupt vectors 0 through 32 (i.e. 33 interrupt vectors in total) are setup as “RTEMS interrupts”, which can be hooked through rtems_interrupt_handler_install.

The Interrupt Descriptor Table supports a total of 256 possible vectors (0 through 255), which leaves a lot of room for “raw interrupts”, which can be hooked through _CPU_ISR_install_raw_handler.

Since the APIC needs to be used for the clock driver, the PIC is remapped (IRQ0 of the PIC is redirected to vector 32, and so on), and then all interrupts are masked to disable the PIC. In this state, the PIC may _still_ produce spurious interrupts (IRQ7 and IRQ15, redirected to vector 39 and vector 47 respectively).

The clock driver triggers the initialization of the APIC and then the APIC timer.

The I/O APIC is not supported at the moment.


IRQ32 is reserved by default for the APIC timer (see following section).

IRQ255 is reserved by default for the APIC’s spurious vector.


Besides the first 33 vectors (0 through 32), and vector 255 (the APIC spurious vector), no other handlers are attached by default. Clock Driver

The clock driver currently uses the APIC timer. Since the APIC timer runs at the CPU bus frequency, which can’t be detected easily, the PIT is used to calibrate the APIC timer, and then the APIC timer is enabled in periodic mode, with the initial counter setup such that interrupts fire at the same frequency as the clock tick frequency, as requested by CONFIGURE_MICROSECONDS_PER_TICK. Console Driver

The console driver defaults to using the COM1 UART port (at I/O port 0x3F8), using the NS16550 polled driver.