Sysbus

Sysbus is a term used primarily in QEMU to describe a generic system bus that connects emulated peripherals to the host machine’s address space. It provides a flexible, software-defined way to place devices that are not part of specialized buses like PCI, USB, or I2C into a virtual machine. The sysbus serves as a simple conduit for devices that should be visible as memory-mapped or I/O-mapped components within the system.

In the QEMU device model, devices on the sysbus are implemented as SysBusDevice objects. These devices register one or more memory regions on the bus, and the bus performs address decoding and forwards read and write operations to the device’s handlers. Sysbus devices can also expose interrupt lines to the CPU, enabling standard inter-device communication patterns through interrupts. This arrangement allows developers to model a wide range of SoC peripherals, custom controllers, or prototype hardware without requiring a dedicated bus protocol.

Typical usage of the sysbus includes emulating system-on-chip peripherals or other generic hardware components that do not rely on PCI, ISA, or other established buses. Examples might include real-time clocks, memory-mapped timers, simple controllers, or experimental peripherals defined within a platform’s SoC model. Because the sysbus is an abstract, software-level construct, it provides a convenient way to attach and test a variety of devices in a virtual platform, independently of physical bus standards.

Overall, the sysbus represents a flexible mechanism within QEMU for modeling and connecting non-specialized hardware components to the emulated system, complementing more structured buses while remaining distinct from real hardware bus architectures.