# Instance launch behavior with Amazon EC2 boot modes
<a name="ami-boot"></a>

When a computer boots, the first software that it runs is responsible for initializing the platform and providing an interface for the operating system to perform platform-specific operations.

In Amazon EC2, two variants of the boot mode software are supported: Unified Extensible Firmware Interface (UEFI) and Legacy BIOS.

**Possible boot mode parameters on an AMI**  
An AMI can have one of the following boot mode parameter values: `uefi`, `legacy-bios`, or `uefi-preferred`. The AMI boot mode parameter is optional. For AMIs with no boot mode parameter, the instances launched from these AMIs use the default boot mode value of the instance type.

**Purpose of the AMI boot mode parameter**  
The AMI boot mode parameter signals to Amazon EC2 which boot mode to use when launching an instance. When the boot mode parameter is set to `uefi`, EC2 attempts to launch the instance on UEFI. If the operating system is not configured to support UEFI, the instance launch will be unsuccessful.

**UEFI Preferred boot mode parameter**  
You can create AMIs that support both UEFI and Legacy BIOS by using the `uefi-preferred` boot mode parameter. When the boot mode parameter is set to `uefi-preferred`, and if the instance type supports UEFI, the instance is launched on UEFI. If the instance type does not support UEFI, the instance is launched on Legacy BIOS.

**Warning**  
Some features, like UEFI Secure Boot, are only available on instances that boot on UEFI. When you use the `uefi-preferred` AMI boot mode parameter with an instance type that does not support UEFI, the instance will launch as Legacy BIOS and the UEFI-dependent feature will be disabled. If you rely on the availability of a UEFI-dependent feature, set your AMI boot mode parameter to `uefi`.

**Default boot modes for instance types**
+ Graviton instance types: UEFI
+ Intel and AMD instance types: Legacy BIOS

**Zone support**  
UEFI boot is not supported in Wavelength Zones.

**Topics**
+ [Requirements to launch an EC2 instance in UEFI boot mode](launch-instance-boot-mode.md)
+ [Determine the boot mode parameter of an Amazon EC2 AMI](ami-boot-mode.md)
+ [Determine the supported boot modes of an EC2 instance type](instance-type-boot-mode.md)
+ [Determine the boot mode of an EC2 instance](instance-boot-mode.md)
+ [Determine the boot mode of the operating system for your EC2 instance](os-boot-mode.md)
+ [Set the boot mode of an Amazon EC2 AMI](set-ami-boot-mode.md)
+ [UEFI variables for Amazon EC2 instances](uefi-variables.md)
+ [UEFI Secure Boot for Amazon EC2 instances](uefi-secure-boot.md)