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Full Discussion: Could USB ever take over PCI
Special Forums Hardware Could USB ever take over PCI Post 302866263 by Corona688 on Monday 21st of October 2013 04:21:28 PM
Old 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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VGA(4)							   BSD Kernel Interfaces Manual 						    VGA(4)

NAME
vga -- generic video card interface SYNOPSIS
options VESA options VESA_DEBUG=N options VGA_ALT_SEQACCESS options VGA_NO_FONT_LOADING options VGA_NO_MODE_CHANGE options VGA_SLOW_IOACCESS options VGA_WIDTH90 device vga In /boot/device.hints: hint.vga.0.at="isa" DESCRIPTION
The vga driver is a generic video card driver which provides access to video cards. This driver is required for the console driver syscons(4). The console driver will call the vga driver to manipulate video hardware (changing video modes, loading font, etc). The vga driver supports the standard video cards: MDA, CGA, EGA and VGA. In addition, the driver can utilize VESA BIOS extensions if the video card supports them. VESA support can either be statically included in the kernel or can be loaded as a separate module. In order to statically link the VESA support to the kernel, the VESA option (see below) must be defined in the kernel configuration file. The vesa module can be dynamically loaded into the kernel using kldload(8). DRIVER CONFIGURATION
Kernel Configuration Options The following kernel configuration options (see config(8)) can be used to control the vga driver. These options provide compatibility with certain VGA cards. VGA_ALT_SEQACCESS You may want to try this option if the mouse pointer is not drawn correctly or the font does not seem to be loaded properly on the VGA card. However, it may cause flicker on some systems. VGA_SLOW_IOACCESS Older VGA cards may require this option for proper operation. It makes the driver perform byte-wide I/O to VGA registers and slow down a little. VGA_WIDTH90 This option enables 90 column modes: 90x25, 90x30, 90x43, 90x50, 90x60. These modes are not always supported by the video card and the display. It is highly likely that LCD display cannot work with these modes. The following options add optional features to the driver. VESA Add VESA BIOS support to the driver. If the VGA card has the VESA BIOS extension 1.2 or later, this option will utilize the VESA BIOS service to switch to high resolution modes. VESA_DEBUG=N Set the VESA support debug level to N. The default value is zero, which suppresses all debugging output. The following options will remove some features from the vga driver and save kernel memory. VGA_NO_FONT_LOADING The vga driver can load software font to EGA and VGA cards. This option removes this feature. Note that if you use this option and still wish to use the mouse on the console then you must also use the SC_ALT_MOUSE_IMAGE option. See syscons(4). VGA_NO_MODE_CHANGE This option prevents the driver from changing video modes. EXAMPLES
Your kernel configuration should normally have: device vga And you need the following line in /boot/device.hints. hint.vga.0.at="isa" The following lines should be included in the kernel configuration file in order to enable the VESA BIOS Extension support. options VESA device vga If you do not want VESA support included in the kernel, but want to use occasionally, do not add the VESA option. And load the vesa module as desired: kldload vesa SEE ALSO
vgl(3), syscons(4), config(8), kldload(8), kldunload(8) STANDARDS
Video Electronics Standards Association, VESA BIOS Extension (VBE). HISTORY
The vga driver first appeared in FreeBSD 3.1. AUTHORS
The vga driver was written by Soren Schmidt <sos@FreeBSD.org> and Kazutaka Yokota <yokota@FreeBSD.org>. This manual page was written by Kazutaka Yokota. BSD
June 30, 1999 BSD
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