Boot options in config.txt

start_file, fixup_file

These options specify the firmware files transferred to the VideoCore GPU prior to booting.

start_file specifies the VideoCore firmware file to use. fixup_file specifies the file used to fix up memory locations used in the start_file to match the GPU memory split. Note that the start_file and the fixup_file are a matched pair - using unmatched files will stop the board from booting. This is an advanced option, so we advise that you use start_x and start_debug rather than this option.

start_x, start_debug

These provide a shortcut to some alternative start_file and fixup_file settings, and are the recommended methods for selecting firmware configurations.

start_x=1 implies start_file=start_x.elf fixup_file=fixup_x.dat

On the Pi 4, if the files start4x.elf and fixup4x.dat are present, these files will be used instead.

start_debug=1 implies start_file=start_db.elf fixup_file=fixup_db.dat

There is no specific handling for the Pi 4, so if you wish to use the Pi 4 debug firmware files, you need to manually specify start_file and fixup_file.

start_x=1 should be specified when using the camera module. Enabling the camera via raspi-config will set this automatically.

disable_commandline_tags

Set the disable_commandline_tags command to 1 to stop start.elf from filling in ATAGS (memory from 0x100) before launching the kernel.

cmdline

cmdline is the alternative filename on the boot partition from which to read the kernel command line string; the default value is cmdline.txt.

kernel

kernel is the alternative filename on the boot partition to use when loading the kernel. The default value on the Pi 1, Pi Zero, and Compute Module is kernel.img, and on the Pi 2, Pi 3, and Compute Module 3 it is kernel7.img. On the Pi4, it is kernel7l.img.

arm_64bit

If set to non-zero, forces the kernel loading system to assume a 64-bit kernel, starts the processors up in 64-bit mode, and sets kernel8.img to be the kernel image loaded, unless there is an explicit kernel option defined in which case that is used instead. Defaults to 0 on all platforms. NOTE: 64-bit kernels must be uncompressed image files.

Note that the 64-bit kernel will only work on the Pi4, Pi3, and Pi2B rev1.2 boards with latest firmware.

arm_control

DEPRECATED - use arm_64bit to enable 64-bit kernels.

Sets board-specific control bits.

armstub

armstub is the filename on the boot partition from which to load the ARM stub. The default ARM stub is stored in firmware and is selected automatically based on the Pi model and various settings.

The stub is a small piece of ARM code that is run before the kernel. Its job is to set up low-level hardware like the interrupt controller before passing control to the kernel.

arm_peri_high

Set arm_peri_high to 1 to enable "High Peripheral" mode on the Pi 4. It is set automatically if a suitable DTB is loaded.

NOTE: Enabling "High Peripheral" mode without a compatible device tree will make your system fail to boot. Currently ARM stub support is missing, so you will also need to load a suitable file using armstub.

kernel_address

kernel_address is the memory address to which the kernel image should be loaded. 32-bit kernels are loaded to address 0x8000 by default, and 64-bit kernels to address 0x80000. If kernel_old is set, kernels are loaded to the address 0x0.

kernel_old

Set kernel_old to 1 to load the kernel to the memory address 0x0.

ramfsfile

ramfsfile is the optional filename on the boot partition of a ramfs to load. N.B. Sufficiently new firmware supports the loading of multiple ramfs files - separate the multiple file names with commas, taking care not to exceed the 80-character line length limit. All the loaded files are concatenated in memory and treated as a single ramfs blob. More information is available here.

ramfsaddr

ramfsaddr is the memory address to which the ramfsfile should be loaded.

initramfs

The initramfs command specifies both the ramfs filename and the memory address to which to load it. It performs the actions of both ramfsfile and ramfsaddr in one parameter. The address can also be followkernel (or 0) to place it in memory after the kernel image. Example values are: initramfs initramf.gz 0x00800000 or initramfs init.gz followkernel. As with ramfsfile, newer firmwares allow the loading of multiple files by comma-separating their names. NOTE: This option uses different syntax from all the other options, and you should not use a = character here.

init_uart_baud

init_uart_baud is the initial UART baud rate. The default value is 115200.

init_uart_clock

init_uart_clock is the initial UART clock frequency. The default value is 48000000 (48MHz). Note that this clock only applies to UART0 (ttyAMA0 in Linux), and that the maximum baudrate for the UART is limited to 1/16th of the clock. The default UART on the Pi 3 and Pi Zero is UART1 (ttyS0 in Linux), and its clock is the core VPU clock - at least 250MHz.

bootcode_delay

The bootcode_delay command delays for a given number of seconds in bootcode.bin before loading start.elf: the default value is 0.

This is particularly useful to insert a delay before reading the EDID of the monitor, for example if the Pi and monitor are powered from the same source, but the monitor takes longer to start up than the Pi. Try setting this value if the display detection is wrong on initial boot, but is correct if you soft-reboot the Pi without removing power from the monitor.

boot_delay

The boot_delay command instructs to wait for a given number of seconds in start.elf before loading the kernel: the default value is 1. The total delay in milliseconds is calculated as (1000 x boot_delay) + boot_delay_ms. This can be useful if your SD card needs a while to get ready before Linux is able to boot from it.

boot_delay_ms

The boot_delay_ms command means wait for a given number of milliseconds in start.elf, together with boot_delay, before loading the kernel. The default value is 0.

disable_splash

If disable_splash is set to 1, the rainbow splash screen will not be shown on boot. The default value is 0.

enable_gic (Pi 4B only)

On the Raspberry Pi 4B, if this value is set to 0 then the interrupts will be routed to the ARM cores using the legacy interrupt controller, rather than via the GIC-400. The default value is 1.

force_eeprom_read

Set this option to 0 to prevent the firmware from trying to read an I2C HAT EEPROM (connected to pins ID_SD & ID_SC) at powerup.

os_prefix

os_prefix is an optional setting that allows you to choose between multiple versions of the kernel and Device Tree files installed on the same card. Any value in os_prefix is prepended to (stuck in front of) the name of any operating system files loaded by the firmware, where "operating system files" is defined to mean kernels, initramfs, cmdline.txt, .dtbs and overlays. The prefix would commonly be a directory name, but it could also be part of the filename such as "test-". For this reason, directory prefixes must include the trailing / character.

In an attempt to reduce the chance of a non-bootable system, the firmware first tests the supplied prefix value for viability - unless the expected kernel and .dtb can be found at the new location/name, the prefix is ignored (set to ""). A special case of this viability test is applied to overlays, which will only be loaded from ${os_prefix}${overlay_prefix} (where the default value of overlay_prefix is "overlays/") if ${os_prefix}${overlay_prefix}README exists, otherwise it ignores os_prefix and treats overlays as shared.

(The reason the firmware checks for the existence of key files rather than directories when checking prefixes is twofold - the prefix may not be a directory, and not all boot methods support testing for the existence of a directory.)

N.B. Any user-specified OS file can bypass all prefixes by using an absolute path (with respect to the boot partition) - just start the file path with a /, e.g. kernel=/my_common_kernel.img.

See also overlay_prefix and upstream_kernel.

overlay_prefix

Specifies a subdirectory/prefix from which to load overlays - defaults to overlays/ (note the trailing /). If used in conjunction with os_prefix, the os_prefix comes before the overlay_prefix, i.e. dtoverlay=disable-bt will attempt to load ${os_prefix}${overlay_prefix}disable-bt.dtbo.

Note that unless ${os_prefix}${overlay_prefix}README exists, overlays are shared with the main OS (i.e. os_prefix is ignored).

uart_2ndstage

Setting uart_2ndstage=1 causes the second-stage loader (bootcode.bin on devices prior to the Raspberry Pi 4, or the boot code in the EEPROM for Raspberry Pi 4 devices) and the main firmware (start*.elf) to output diagnostic information to UART0.

Be aware that output is likely to interfere with Bluetooth operation unless it is disabled (dtoverlay=disable-bt) or switched to the other UART (dtoverlay=miniuart-bt), and if the UART is accessed simultaneously to output from Linux then data loss can occur leading to corrupted output. This feature should only be required when trying to diagnose an early boot loading problem.

upstream_kernel

If upstream_kernel=1 is used, the firmware sets os_prefix to "upstream/", unless it has been explicitly set to something else, but like other os_prefix values it will be ignored if the required kernel and .dtb file can't be found when using the prefix.

The firmware will also prefer upstream Linux names for DTBs (bcm2837-rpi-3-b.dtb instead of bcm2710-rpi-3-b.dtb, for example). If the upstream file isn't found the firmware will load the downstream variant instead and automatically apply the "upstream" overlay to make some adjustments. Note that this process happens after the os_prefix has been finalised.

This article uses content from the eLinux wiki page RPiconfig, which is shared under the Creative Commons Attribution-ShareAlike 3.0 Unported license