Hello,
This is what I understand:
In vl.c code:
-In qemu initialization will be create and initialize a table in which there
are I/O read/write function corresponding to I/O address.
-When emulated devices are inizialited they register IO function in this
table, in this way it can be called when an instruction want to access at
this address-device right?
-These function are called by cpu_write{b|w|l} in case of byte/word/long.

I suppose when there is an instruction op-code regards IO there is a
mechanism that calls these function. Can you say me where it does this?

Thank you

Ale