Bochs VBE Extensions: Difference between revisions

The [[Bochs]] VGA BIOS supports, to an extent, the VBE specification. Since Bochs only emulates a VGA card down to the hardware level (and a Cirrus graphics card if you enable it, but that is not tied in with the Bochs VBE extensions), it emulates very simple graphics hardware that the VBE BIOS can drive. The advantage of this is that if you are running your OS in Bochs (or QEMU, which uses the Bochs VGA BIOS), you can use this emulated hardware to directly set video modes without using VBE (which would require real mode or v86).

The Bochs emulated graphics hardware (henceforth called BGA for Bochs Graphics Adaptor) is accessed via two 16-bit IO-ports. The first one is an index port, the second one a data port (comparable to how the VGA handles its sets of registers). Via these ports it is possible to enable or disable the VBE extensions, change the screen resolution and bit depth, and manage a larger virtual screen. There are five versions of the BGA (0xB0C0 through 0xB0C4), but if you use the latest version of Bochs you only need to concern yourself with the latest one (0xB0C4). QEMU (with the -std-vga command line argument) also use the latest version.

[TODO: if and when VGA attribute controller (AC) is supported]

Most likely, setting the display resolution and bit depth is all you need. To do so, disable the VBE extensions (see above), write the X resolution, Y resolution and BPP to their respective registers (VBE_DISPI_INDEX_XRES (1), VBE_DISPI_INDEX_YRES (2) and VBE_DISPI_INDEX_BPP (3)) and enable the VBE extensions. Since the BGA is not real hardware, X and Y resolutions can be set at will up to a maximum horizontal resolution (depending on your version of Bochs) of either 1024 or 1600 (VBE_DISPI_MAX_XRES) and a maximum vertical resolution of either 768 or 1200 (VBE_DISPI_MAX_YRES). It seems that both Bochs and QEMU can use about any resolutions with a few restrictions. In QEMU the X resolution must be divisible by 8. [TODO: Describe how Bochs handle different X resolutions. It can crash with segmentation fault on various X resolutions, so it is difficult to test]. The Y resolution can be any number from 1 to maximum in both Bochs and QEMU. The bit depth needs to be one of the following:

If you try to set an invalid resolution (greater than the maximum or an irregular X resolution) or bit depth, usually nothing happens. The current video mode is preserved. In some cases Bochs will crash with a segmentation fault if you set an invalid X resolution, so only use standard ones. [TODO: Check latest CVS version of Bochs and search their bug tracker]. You can always read the current resolution and bit depth from their respective registers. When you have set a resolution it is a good idea to read it back and check whether your new resolution is really set. You should do that before you enable the BGA to minimize the chance of your emulator crashing. [TODO: Check what happens when the first resolution you set is invalid.]

=== Memory layout of video modes ===

For 24 BPP, the order of the colour components is blue first, then green, then red.

[I'm about to add info on the other modes. --[[User:Walling|Walling]] 05:57, 26 October 2008 (UTC)]

When using a linear frame buffer, the BGA exposes all of the graphics memory in a linear fashion, starting at address 0xE0000000 (VBE_DISPI_LFB_PHYSICAL_ADDRESS). [TODO: check whether this also goes for 4 BPP modes] To enable the LFB, use the VBE_DISPI_LFB_ENABLED flag (0x40) when enabling the VBE extensions (so write a value of VBE_DISPI_INDEX_ENABLE | VBE_DISPI_LFB_ENABLED (0x41)).

QEMU does not pay attention to the VBE_DISPI_LFB_ENABLED flag. Both the linear frame buffer and memory bank are available at all times. Bochs pays attention to the flag. In LFB mode it ignores requests to change the bank and nothing happens when you write to the bank memory. In banked mode nothing happens when you write to the LFB memory.