10-21-2013
Leaving aside the performance problems, there's a bigger fundamental difference between the two. There's things PCI can do and USB just can't. DMA as mentioned above is one of them.
This is because the PCI bus is an extension of the CPU's own hardwired bus, essentially. It has most of the same features as the CPU's own bus, and communication is extremely direct, to the point all hardware on PCI as well as system memory in general ends up inhabiting the same memory map. (More or less. It's gotten more complicated with each revision, but once configured, still handles itself in a manner like this.) Use one address and you're talking to system memory. Use another address and you're talking to video RAM. Use another address and you're talking to your network card's boot ROM. And such.
This turns out to be vital for things your computer absolutely needs to boot, like video. They can include little bits of machine code on the cards which give the BIOS a very basic ability to use them. The BIOS doesn't have to load a driver to see it, the code just kind of appears in memory where it needs to be thanks to the memory map. This is also why mac's require different kinds of video cards than PC's -- different kind of BIOS.
USB is extremely in-direct; it takes tons of messages back and forth to do anything, and wasn't designed as a CPU bus anyway. Things PCI does with one pulse would have to be emulated in software. Rigidly standard things like USB keyboards are handled in BIOS these days, USB media as well (though often badly). But you couldn't replace a PCI video card with a USB one and have your computer boot. The BIOS by nature is too dumb to load a driver, and sees no video as a result, and cannot boot.
Last edited by Corona688; 10-21-2013 at 05:42 PM..
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EHCI(4) BSD Kernel Interfaces Manual EHCI(4)
NAME
ehci -- USB Enhanced Host Controller driver
SYNOPSIS
device ehci
DESCRIPTION
The ehci driver provides support for the USB Enhanced Host Controller Interface, which is used by USB 2.0 controllers.
EHCI controllers are peculiar in that they can only handle the USB 2.0 protocol. This means that they normally have one or more companion
controllers (i.e., ohci(4) or uhci(4)) handling USB 1.x devices. Consequently each USB connector is electrically connected to two USB con-
trollers. The handling of this is totally automatic, but can be noticed since USB 1.x and USB 2.0 devices plugged in to the same connector
appear to connect to different USB busses.
SEE ALSO
ohci(4), uhci(4), usb(4)
HISTORY
The ehci device driver first appeared in FreeBSD 5.1.
BUGS
The driver is not finished and is quite buggy.
There is currently no support for isochronous transfers.
BSD March 20, 2005 BSD