UNIX for Dummies Questions & Answers

Does anyone know the difference between these two OBP commands?

dev device-path
Chooses the indicated device node, making it the current node.

" device-path" select-dev
Selects the specified device and make it the active node.



I am a bit confused about this...Thanks.

dev device-path

It looks like "dev" is another name for the "cd" command. In recent versions on the OpenBoot Command Reference Manual, "cd" is no longer mentioned and "dev" is used in its place. So you can do stuff like:
dev /zs
dev ..
and it will be just like you did a "dev /". I think that Sun got tired of explaining that they borrowed the cd command from unix where it means "change directory" but in OBP it doesn't mean directories on disk.

But "cd" and/or "dev" make a device the "current node". And the "device-end" command will discard the "current node", leaving no device selected as the current node.

For one thing, the current node will obviously decide what a command like "dev .." will do. Also devices have some data associated with them. The ".attributes" command will show this data for the current node. And there are usually some special commands for a device. The "words" command will show these. Be careful with this stuff. I only use it when I am following step-for-step some docs that I got from Sun.

" device-path" select-dev
Whoa, I had to read up on this one. I'll try to explain this... but it's going to be rough...

Computer have physical memory and virtual memory. A virtual address get translated into a phyisical address by a memory management unit. And programs just use virtual addresses. It may be a little surprising that this applies even to OBP itself. Not all computers bother to turn on the MMU prior to boot.

Also, not all physical addresses actually point to memory. OBP itself resides somewhere in the physical address space. Also most devices have a presence in the phyisical address space.

What select-dev does is figure out exactly where in physical memory a particular device resides. Subsequent memory mapping commands can then just use an offset which is automatically added to the starting phyisical address of the device.

This is used to map all or part of a device's memory in the virtual address space of OBP.

My advice is to stay away from this stuff. Unless you're writing a device driver I doubt that you will ever need to fiddle with OBP's virtual address space.