# Arm Server Base System Architecture Reference board (`sbsa-ref`)

While the `virt` board is a generic board platform that doesn’t match any real hardware the `sbsa-ref` board intends to look like real hardware. The Server Base System Architecture defines a minimum base line of hardware support and importantly how the firmware reports that to any operating system.

It is intended to be a machine for developing firmware and testing standards compliance with operating systems.

## Supported devices

The `sbsa-ref` board supports:

> - A configurable number of AArch64 CPUs

> - GIC version 3

> - System bus AHCI controller

> - System bus XHCI controller

> - CDROM and hard disc on AHCI bus

> - E1000E ethernet card on PCIe bus

> - Bochs display adapter on PCIe bus

> - A generic SBSA watchdog device

## Board to firmware interface

`sbsa-ref` is a static system that reports a very minimal devicetree to the firmware for non-discoverable information about system components. This includes both internal hardware and parts affected by the qemu command line (i.e. CPUs and memory). As a result it must have a firmware specifically built to expect a certain hardware layout (as you would in a real machine).

### DeviceTree information

The devicetree provided by the board model to the firmware is not intended to be a complete compliant DT. It currently reports:

> - CPUs

> - memory

> - platform version

> - GIC addresses

### Platform version

The platform version is only for informing platform firmware about what kind of `sbsa-ref` board it is running on. It is neither a QEMU versioned machine type nor a reflection of the level of the SBSA/SystemReady SR support provided.

The `machine-version-major` value is updated when changes breaking fw compatibility are introduced. The `machine-version-minor` value is updated when features are added that don’t break fw compatibility.

Platform version changes:

0.0

  Devicetree holds information about CPUs, memory and platform version.

0.1

  GIC information is present in devicetree.

0.2

  GIC ITS information is present in devicetree.

0.3

  The USB controller is an XHCI device, not EHCI