Device-Extension Model

The Device Extension Model is a special case of the Function-Offload Model in which the SPEs act like I/O devices.

SPEs can also act as intelligent front ends to an I/O device. Mailboxes can be used as command and response FIFOs between the PPE and SPEs.

The SPEs can interact with I/O devices because:

all I/O devices are memory-mapped, and
the SPEs DMA transfers support transfer sizes of a single byte.

I/O devices can use an SPE's signal-notification facility (described in Signal notification) to tell the SPE when commands complete.

When SPEs are used in the Device-Extension Model, they usually run privileged software that is part of the operating system. As such, this code is trusted and may be given access to privileged registers for a physical device. For example, a secure file system may be treated as a device. The operating system's device driver can be written to use the SPE for encryption and decryption and for responding to disk-controller requests on all file reads and writes to this virtual device.