Notes on the AGX Server : Xga configuration
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6. Xga configuration

This server now has tested support for XGA-2 compatible boards (aka. XGA-NI). The main issue for XGA-1 support is whether clock probing works. At this time probing for board configuration is limited and detailed configuration may need to be done manually.

By default the ISA POS register will be performed. If the XGA Instance number is specified the scope of probing will be narrowed a bit. To override or disable probing, a minimum of the Instance, COPbase, and MEMbase must be specified in the XF86Config device section for the XGA card. MCA probing is not supported.

Instance nn

XGA instance number (0-7).

IObase nnnn

The I/O address of the the XGA general control registers. The standard, and default, is 0x21i0, where i is the instance number.

MEMbase nnnn

The XGA display memory address (the address the XGA coprocessor uses for video memory). This is also the system memory address of the linear aperture on boards that support it.

POS register 4 bits 7-1 contains bits 31-25 of the XGA's display memory address. Bits 24-22 of of the display memory address contains the XGA instance number. Bit 0 of POS register 4 is not used by this server as the XGA's linear aperture is not used. However, the coprocessor must still be configured with this.

The AGX-01[456] chips have a fixed display memory address.

COPbase nnnnnn

Address of the graphics engine's memory mapped control registers.


0xC1C00 + (ext_mem_addr * 0x2000) + (instance * 0x80)

where ext_mem_addr is the high order 4-bits of POS register 2 (0-16 the server assumes zero).

The AGX-01[456] chips support 0xB1F00 (default) and 0xD1F00.

BIOSbase nnnnnn

Address of the XGA BIOS (not VGA BIOS). Can be specified as an alternate to COPbase.


0xC0000 + (ext_mem_addr * 0x2000)

where ext_mem_addr is the high order 4-bits of POS register 2 (0-16 -- the server assumes zero).

VGAbase nnnn

Can be used to override the default 0xA0000 address for the 64KB video memory address used by the server. The only values acceptable are 0xA0000 and 0xB0000. VGA text mode restore does not work under Linux if 0xB0000 is specified.

AGX-01[456] also default to 0xA0000.

POSbase nnnn

Can be used to specify an alternate POS register probe address base from the ISA default of 0x100. The VESA VXE standard for EISA is 0xzC80, where z is the slot number).

A value of zero will disable POS register probing (required for MCA).

DACspeed nnnn

Can be used to override the servers default maximum Pixel Clock for XGA-2 of 80Mhz. The limit can be raised as high as 90Mhz, or set to lower values.

An alternate way to determine the POS register values is with the setup/diag programs that should have been included with your video board, or possibly from jumper values.

The XGA-2 has programmable clocks up to 90MHz, however at 1024x768, 72MHz is generally the max that will produce a stable display with the CatsEye/XGA-2 used for testing (IBM coprocessor and INMOS RAMDAC/serializer). Higher clocks will often generate artifacts at the top and left edges of the screen. Such artifacts can sometimes be tuned out by increasing the vertical and horizontal blanking intervals or slightly changing the clock. At pixel clock rates above 80Mhz I have seen the chip lose sync after running for several minutes, so 80Mhz has been set as the default limit for XGA-2 pixel clocks. I don't have specs on actual limits, and as there are a number of different XGA chipsets, you should use the modes documented in your owner's manual as a guide to max refresh rates. No clocks or clockchip parm are required to specify use of programmable clocks for the XGA-2.

8bpp and 16bpp are supported for the XGA-2.

For XGA-1 cards the clocks must be specified as for the AGX chips, it is not known whether the clockprobing will work. Some XGA-1 chips may support 16bpp.

$XFree86: xc/programs/Xserver/hw/xfree86/doc/sgml/agx.sgml,v 1998/11/07 13:37:50 dawes Exp $

$XConsortium: agx.sgml /main/9 1996/10/19 18:03:50 kaleb $

Notes on the AGX Server : Xga configuration
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