Each device contains a SIMULATOR and a FRONTEND section. The 2 sections communicate via a shared memory and a via a pair of connected sockets

The CPU writes via multiple inb/inw to registers of the simulator-part

When the cpu is done, the frontend is called by the simulator via

sim_write(c->fd_io, &cmd, sizeof(cmd));

The frontend reads the data out of the shared memory to process it. The result is written back to the shared memory. Now, the frontend writes in the socket, which generates a SIGIO and the simulator part is called in his 'interrupt-handler'. The interrupt handler now creates a interrupt towards the cpu.