>>131
Instead of speculating.... actually, you're a troll. I fell for it.
In any case, here's the relevant sources:
The libc function that you read(1) calls:
http://plan9.bell-labs.com/sources/plan9/sys/src/libc/9sys/write.c
The syscall pwrite(2), syspwrite which calls write:
http://plan9.bell-labs.com/sources/plan9/sys/src/9/port/sysfile.c
First, the file descriptor (arg[0]) is turned into a channel, then the channel device's write function is called directly with the same arguments. There's nothing special the OS does directly, rather it's all handled by the device. The device writes the code in a suitable way, depending on its nature.
devtab is made at compile time with the mkdevc script.